Publications

Carbonaceous aerosols (CAs) are major components of fine particulate matter (PM2.5) that dramatically influence the energy budget of Earth. However, accurate assessment of the climatic impacts of CAs is still challenging due to the large uncertainties remaining in the measurement of their optical properties. In this respect, a modified versatile aerosol concentration enrichment system integrated into optical instruments (VACES-OPTS) was set up to increase particle concentration and amplify signal-noise ratio during optical measurement. Based on the novel technique, this study was able to lower the detection limit of CAs by an order of magnitude under high temporal resolution (2 h) and small sampling flow (6 L min−1). Besides, stable and reliable optical data were obtained for absorption apportionment and source identification of black carbon (BC) and brown carbon (BrC). In the field application of the new system, high absorption coefficient of CAs in Shanghai, China was witnessed. Further analysis of the contribution of black carbon BC and BrC to light absorption revealed that BrC could account for over 15% of the total absorption at 370 nm. According to the potential source contribution function model (PSCF) classification, CAs with strong light absorption in urban Shanghai originated not only from highly polluted inland China but also from active marine ship emissions.
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•The VACES-OPTS was developed to obtain reliable optical data of BC and BrC.•Accurate and high temporal resolution observations of BC and BrC was realized.•BrC accounted for over 15% of the total absorption at 370 nm in urban Shanghai.•Significant optical impacts of ship emissions on a coastal megacity were found.

Microbially-produced ice nucleating proteins (INpro) are unique molecular structures with the highest known catalytic efficiency for ice formation. Airborne microorganisms utilize these proteins to enhance their survival by reducing their atmospheric residence times. INpro also have critical environmental effects including impacts on the atmospheric water cycle, through their role in cloud and precipitation formation, as well as frost damage on crops. INpro are ubiquitously present in the atmosphere where they are emitted from diverse terrestrial and marine environments. Even though bacterial genes encoding INpro have been discovered and sequenced decades ago, the details of how the INpro molecular structure and oligomerization foster their unique ice-nucleation activity remain elusive. Using machine-learning based software AlphaFold 2 and trRosetta, we obtained and analysed the first ab initio structural models of full length and truncated versions of bacterial INpro. The modeling revealed a novel beta-helix structure of the INpro central repeat domain responsible for ice nucleation activity. This domain consists of repeated stacks of two beta strands connected by two sharp turns. One beta-strand is decorated with a TxT amino acid sequence motif and the other strand has an SxL[T/I] motif. The core formed between the stacked beta helix-pairs is unusually polar and very distinct from previous INpro models. Using synchrotron radiation circular dichroism, we validated the β-strand content of the central repeat domain in the model. Combining the structural model with functional studies of purified recombinant INpro, electron microscopy and modeling, we further demonstrate that the formation of dimers and higher-order oligomers is key to INpro activity. Using computational docking of the new INpro model based on rigid-body algorithms we could reproduce a previously proposed homodimer structure of the INpro CRD with an interface along a highly conserved tyrosine ladder and show that the dimer model agrees with our functional data. The parallel dimer structure creates a surface where the TxT motif of one monomer aligns with the SxL[T/I] motif of the other monomer widening the surface that interacts with water molecules and therefore enhancing the ice nucleation activity. This work presents a major advance in understanding the molecular foundation for bacterial ice-nucleation activity.

The diversity of microbes and their transmission between ocean and atmosphere are poorly understood despite the implications for microbial global dispersion and biogeochemical processes. Here, we survey the genetic diversity of airborne and surface ocean bacterial communities sampled during springtime transects across the northwest Pacific and subtropical north Atlantic as part of the Tara Pacific Expedition. We find that microbial community composition is more variable in the atmosphere than in the surface ocean. Bacterial communities were more similar between the two surface oceans than between the ocean and the overlying atmosphere. Likewise, Pacific and Atlantic atmospheric microbial communities were more similar to each other than to those in the ocean beneath. Atmospheric community composition over the Atlantic was dominated by terrestrial and specifically, dust-associated bacteria, whereas over the Pacific there was a higher prevalence and differential abundance of marine bacteria. Our findings highlight regional differences in long-range microbial exchange and dispersal between land, ocean, and atmosphere.

This study investigates selected secondary atmospheric responses to the widely reported emission change attributed to COVID‐19 lockdowns in the highly polluted Indo‐Gangetic Plain (IGP) using ground‐based measurements of trace gases and particulate matter. We used a chemical box‐model to show that production of nighttime oxidant, NO3, was affected mainly by emission decrease (average nighttime production rates 1.2, 0.8 and 1.5 ppbv hr−1 before, during and relaxation of lockdown restrictions, respectively), while NO3 sinks were sensitive to both emission reduction and seasonal variations. We have also shown that the maximum potential mixing ratio of nitryl chloride, a photolytic chlorine radical source which has not been previously considered in the IGP, is as high as 5.5 ppbv at this inland site, resulting from strong nitrate radical production and a potentially large particulate chloride mass. This analysis suggests that air quality measurement campaigns and modeling explicitly consider heterogeneous nitrogen oxide and halogen chemistry.
Plain Language Summary
The Indo‐Gangetic Plain (IGP) is one of the most polluted regions on earth, with poor air quality affecting the majority of the Indian population. The atmospheric chemistry that transforms major regional emissions into harmful secondary pollutants is complex. Here, we quantify, for the first time, several important oxidative processes and show the potential for substantial oxidation of biogenic volatile organic compounds and the production of chlorine through unconventional chemistry in the IGP. We further show how these chemical cycles varied due to the emission reductions as a result of COVID‐19 lockdown, findings that will serve to define their sensitivity to future emission changes in the region.
Key Points
Atmospheric response in the Indo‐Gangetic Plain varied according to seasonal changes and emissions reductions due to COVID‐19 lockdown
NO3 production was mainly affected by emission changes, while NO3 sinks were sensitive to both emissions and seasonal changes
Nitryl chloride, a photolytic chlorine radical source not previously considered in the inland Indo‐Gangetic Plain, may be up to 5.5 ppbv

Background:Secondary organic aerosols (SOAs) formed from anthropogenic or biogenic gaseous precursors in the atmosphere substantially contribute to the ambient fine particulate matter [PM ≤2.5μm in aerodynamic diameter (PM2.5)] burden, which has been associated with adverse human health effects. However, there is only limited evidence on their differential toxicological impact.Objectives:We aimed to discriminate toxicological effects of aerosols generated by atmospheric aging on combustion soot particles (SPs) of gaseous biogenic (β-pinene) or anthropogenic (naphthalene) precursors in two different lung cell models exposed at the air–liquid interface (ALI).Methods:Mono- or cocultures of lung epithelial cells (A549) and endothelial cells (EA.hy926) were exposed at the ALI for 4 h to different aerosol concentrations of a photochemically aged mixture of primary combustion SP and β-pinene (SOAβPIN-SP) or naphthalene (SOANAP-SP). The internally mixed soot/SOA particles were comprehensively characterized in terms of their physical and chemical properties. We conducted toxicity tests to determine cytotoxicity, intracellular oxidative stress, primary and secondary genotoxicity, as well as inflammatory and angiogenic effects.Results:We observed considerable toxicity-related outcomes in cells treated with either SOA type. Greater adverse effects were measured for SOANAP-SP compared with SOAβPIN-SP in both cell models, whereas the nano-sized soot cores alone showed only minor effects. At the functional level, we found that SOANAP-SP augmented the secretion of malondialdehyde and interleukin-8 and may have induced the activation of endothelial cells in the coculture system. This activation was confirmed by comet assay, suggesting secondary genotoxicity and greater angiogenic potential. Chemical characterization of PM revealed distinct qualitative differences in the composition of the two secondary aerosol types.Discussion:In this study using A549 and EA.hy926 cells exposed at ALI, SOA compounds had greater toxicity than primary SPs. Photochemical aging of naphthalene was associated with the formation of more oxidized, more aromatic SOAs with a higher oxidative potential and toxicity compared with β-pinene. Thus, we conclude that the influence of atmospheric chemistry on the chemical PM composition plays a crucial role for the adverse health outcome of emissions.

The atmosphere plays an important role in transporting microorganisms on a global scale, yet the processes affecting the composition of the airborne microbiome, the aerobiome, are not fully outlined. Here we present the community compositions of bacteria and fungi obtained by DNA amplicon-sequencing of aerosol samples collected in a size-resolved manner during nine consecutive days in central Israel. The campaign captured dust events originating from the Sahara and the Arabian deserts, as well as days without dust (“clear days”). We found that the source of the aerosol was the main variable contributing to the composition of both fungal and bacterial communities. Significant differences were also observed between communities representing particles of different sizes. We show evidence for the significant transport of bacteria as cell-aggregates and/or via bacterial attachment to particles during dust events. Our findings further point to the mixing of local and transported bacterial communities, observed mostly in particles smaller than 0.6 μm in diameter, representing bacterial single cells. Fungal communities showed the highest dependence on the source of the aerosols, along with significant daily variability, and without significant mixing between sources, possibly due to their larger aerodynamic size and shorter atmospheric residence times. These results, obtained under highly varied atmospheric conditions, provide significant assurances to previously raised hypotheses and could set the course for future studies on aerobiome composition.

Sea spray aerosol (SSA) formation have a major role in the climate system, but measurements at a global-scale of this micro-scale process are highly challenging. We measured high-resolution temporal patterns of SSA number concentration over the Atlantic Ocean, Caribbean Sea, and the Pacific Ocean covering over 42,000 km. We discovered a ubiquitous 24-hour rhythm to the SSA number concentration, with concentrations increasing after sunrise, remaining higher during the day, and returning to predawn values after sunset. The presence of dominating continental aerosol transport can mask the SSA cycle. We did not find significant links between the diel cycle of SSA number concentration and diel variations of surface winds, atmospheric physical properties, radiation, pollution, nor oceanic physical properties. However, the daily mean sea surface temperature positively correlated with the magnitude of the day-to-nighttime increase in SSA concentration. Parallel diel patterns in particle sizes were also detected in near-surface waters attributed to variations in the size of particles smaller than ~1 µm. These variations may point to microbial day-tonight modulation of bubble-bursting dynamics as a possible cause of the SSA cycle.

Personal exposure PM samples aid in determining the sources and chemical composition of real-world exposures, particularly in settings with household air pollution. However, their use in toxicological research is limited, despite uncertainty regarding health effects in these settings and evidence of differential toxicity among PM2.5 sources and components. This study used women's PM2.5 exposure samples collected using personal exposure monitoring in rural villages in three Chinese provinces (Beijing, Shanxi, and Sichuan) during summer and winter. Water-soluble organic carbon, ions, elements, and organic tracers (e.g. levoglucosan and polycyclic aromatic hydrocarbons [PAHs]) were quantified in water and organic PM2.5 extracts. Human lung epithelial cells (A549) were exposed to the extracts. Cell death, reactive oxygen species (ROS), and gene expression were measured. Biomass burning contributions were higher in Sichuan samples than in Beijing or Shanxi. Some PM characteristics (total PAHs and coal combustion source contributions) and biological effects of organic extract exposures (cell death, ROS, and cytokine gene expression) shared a common trend of higher levels and effects in winter than in summer for Shanxi and Beijing but no seasonal differences in Sichuan. Modulation of phase I/AhR-related genes (cyp1a1 and cyp1b1) and phase II/oxidative stress-related genes (HO-1, SOD1/2, NQO-1, and catalase) was either low or insignificant, without clear trends between samples. No significant cell death or ROS production was observed for water extract treatments among all sites and seasons, even at possible higher concentrations tested. These results support organic components, particularly PAHs, as essential drivers of biological effects, which is consistent with some other evidence from ambient PM2.5.

Portable aethalometers are commonly used for online measurements of light-absorbing carbonaceous particles (LAC). However, they require strict calibration. In this study, the performance of a micro-aethalometer (MA200 with polytetrafluoroethylene filter) in charactering brown carbon aerosol (BrC) absorption was evaluated in comparison with reference materials and techniques that included bulk solution absorbance and Mie-theory based particle extinction retrieval via broadband cavity enhanced spectrometer (BBCES). Continuous-wavelength resolved (300–650 nm) imaginary refractive index (kBrC) was derived with these methods for various BrC proxies and standard materials representing a wide range of sources and absorbing abilities, including the strongly absorbing nigrosin, pahokee peat fluvic acid (PPFA), tar aerosol from wood pyrolysis, humic-like substance (HULIS) separated from wood smoldering burning emissions, and secondary organic aerosols (SOA) from photochemical oxidation of indole and naphthalene in the presence of NOx. The BrC and nigrosin optical results by bulk solution absorption are comparable with the properties retrieved from BBCES. The MA200 raw measurements provide reliable absorption Ångström exponent (AAE) but overestimate kBrC largely. The parameterized overestimates against reference methods depend on light absorption strength, so that the MA200 overestimates more for the less absorbing BrC. The correction factor for MA200 can be expressed well as an exponential function of kBrC or particle single scattering albedo (SSA), and also as a power-law function of the MA200 raw results derived BrC mass absorption efficiency (MAE). The ensemble correction factor regressed for all these BrC and nigrosin is 2.8 based on bulk absorption and 2.7 using BBCES result as reference. Simple radiative forcing (SRF) calculations for different scenarios using the correction for MA200, show consistent SRF when using the aethalometer results after the kBrC-dependent correction.

Widespread smoke from wildfires and biomass burning contributes to air pollution and the deterioration of air quality and human health. A common and major emission of biomass burning, often found in collected smoke particles, is spherical wood tar particles, also known as “tar balls”. However, the toxicity of wood tar particles and the mechanisms that govern their health impacts and the impact of their complicated chemical matrix are not fully elucidated. To address these questions, we generated wood tar material from wood pyrolysis and isolated two main subfractions: water-soluble and organic-soluble fractions. The chemical characteristics as well as the cytotoxicity, oxidative damage, and DNA damage mechanisms were investigated after exposure of A549 and BEAS-2B lung epithelial cells to wood tar. Our results suggest that both wood tar subfractions reduce cell viability in exposed lung cells; however, these fractions have different modes of action that are related to their physicochemical properties. Exposure to the water-soluble wood tar fraction increased total reactive oxygen species production in the cells, decreased mitochondrial membrane potential (MMP), and induced oxidative damage and cell death, probably through apoptosis. Exposure to the organic-soluble fraction increased superoxide anion production, with a sharp decrease in MMP. DNA damage is a significant process that may explain the course of toxicity of the organic-soluble fraction. For both subfractions, exposure caused cell cycle alterations in the G2/M phase that were induced by upregulation of p21 and p16. Collectively, both subfractions of wood tar are toxic. The water-soluble fraction contains chemicals (such as phenolic compounds) that induce a strong oxidative stress response and penetrate living cells more easily. The organic-soluble fraction contained more polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs and induced genotoxic processes, such as DNA damage.

Asian dust is an important source of atmospheric ice-nucleating particles (INPs). However, the freezing activity of airborne Asian dust, especially its sensitivity to particle size, is poorly understood. In this study we report the first INP measurement of size-resolved airborne mineral dust collected during East Asian dust events. The measured total INP concentrations in the immersion mode ranged from 10(-2) to 10(2) L-1 in dust events at temperatures between 25 and 5 degrees C. The average contributions of heat-sensitive INPs at three temperatures, -10, 15, and 20 degrees C, were 81 +/- 12 %, 70 +/- 15 %, and 38 +/- 21 %, respectively, suggesting that proteinaceous biological materials have a substantial effect on the ice nucleation properties of Asian airborne mineral dust at high temperatures. The dust particles which originated from China's northwest deserts are more efficient INPs compared to those from northern regions. In general, there was no significant difference in the ice nucleation properties between East Asian dust particles and other regions in the world. An explicit size dependence of both INP concentration and surface ice-active-site density was observed. The nucleation efficiency of dust particles increased with increasing particle size, while the INP concentration first increased rapidly and then leveled, due to the significant decrease in the number concentration of larger particles. A new set of parameterizations for INP activity based on size-resolved nucleation properties of Asian mineral dust particles were developed over an extended temperature range (35 to 6 degrees C). These size-dependent parameterizations require only particle size distribution as input and can be easily applied in models.

Nighttime oxidation of biogenic volatile organic compounds (BVOCs) by nitrate radicals (NO3·) represents one of the most important interactions between anthropogenic and natural emissions, leading to substantial secondary organic aerosol (SOA) formation. The direct climatic effect of such SOA cannot be quantified because its optical properties and atmospheric fate are poorly understood. In this study, we generated SOA from the NO3· oxidation of a series BVOCs including isoprene, monoterpenes, and sesquiterpenes. The SOA were subjected to comprehensive online and offline chemical composition analysis using high-resolution mass spectrometry and optical properties measurements using a novel broadband (315–650 nm) cavity-enhanced spectrometer, which covers the wavelength range needed to understand the potential contribution of the SOA to direct radiative forcing. The SOA contained a significant fraction of oxygenated organic nitrates (ONs), consisting of monomers and oligomers that are responsible for the detected light absorption in the 315–400 nm range. The SOA created from β-pinene and α-humulene was further photochemically aged in an oxidation flow reactor. The SOA has an atmospheric photochemical bleaching lifetime of >6.2 h, indicating that some of the ONs in the SOA may serve as atmosphere-stable nitrogen oxide sinks or reservoirs and will absorb and scatter incoming solar radiation during the daytime.

Anthropogenic pollution from marine microplastic particles is a growing concern, both as a source of toxic compounds, and because they can transport pathogens and other pollutants. Airborne microplastic particles were previously observed over terrestrial and coastal locations, but not in the remote ocean. Here, we collected ambient aerosol samples in the North Atlantic Ocean, including the remote marine atmosphere, during the Tara Pacific expedition in May-June 2016, and chemically characterized them using micro-Raman spectroscopy. We detected a range of airborne microplastics, including polystyrene, polyethylene, polypropylene, and poly-silicone compounds. Polyethylene and polypropylene were also found in seawater, suggesting local production of airborne microplastic particles. Terminal velocity estimations and back trajectory analysis support this conclusion. For technical reasons, only particles larger than 5 µm, at the upper end of a typical marine atmospheric size distribution, were analyzed, suggesting that our analyses underestimate the presence of airborne microplastic particles in the remote marine atmosphere.

The Negev Desert in Israel is susceptible to frequent atmospheric events of high dust loading which have been linked with negative human health outcomes, including cardiovascular and respiratory distress. Previous research suggests that the highest levels of dust over the region occur during an atmospheric pattern with a cyclone situated over the eastern Mediterranean. This Cyprus Low can bring unsettled weather and strong westerly winds over the Negev. However, while the overall pattern associated with dust events in the Negev Desert is generally well-understood, it remains unclear why days with seemingly similar weather patterns result in different levels of atmospheric dust. Thus, the goal of this study is to better differentiate the atmospheric patterns during dust events over the Negev. Using PM10 data collected in Be’er Sheva, Israel, from 2000 to 2015 in concert with 72-h HYSPLIT back trajectories at three different height levels (surface, 200 m, 500 m), we examine the source region, trajectory groups using a K-Means clustering procedure, and overall synoptic pattern during dust events. Further, we use sea-level pressure data across the region to determine how cyclone strength and location impact dust events in Be’er Sheva. We find that the highest levels of atmospheric dust in the Negev are associated with the Cyprus Low pattern, and air traversing Libya seems to play an especially important role, likely due to the country’s arid surface cover. Cyclone strength is also a critical factor, as lower sea-level pressure results in more severe dust events. A better understanding of the atmospheric features associated with dust events over the Negev Desert will hopefully aid in forecasting these occurrences across the region.

The composition of atmospheric aerosols is dynamic and influenced by their emission sources, organic and inorganic composition, transport pathways, chemical and physical processes, microorganisms' content and more. Characterization of such factors can improve the ability to evaluate air quality and health risks under different atmospheric scenarios. Here we investigate the microbial composition of the atmospheric particulate matter (<10 μm; PM10), sampled in Bolu, Turkey, and the linkage to the chemical composition changes, and different environmental factors. We show distinct differences between aerosol composition of different sources and air-mass transport patterns, sampled in July–August 2017 and in February 2018. The summer samples had a typical northern component air mass trajectories and higher local wind speed. They were characterized by high PM10 levels, marine and mineral dust tracers and high relative abundance of Ascomycota, suggesting long-range transport of the particles from remote sources. In contrast, samples collected in February were characterized by a dominant contribution of southern air masses, and low wind speed. They had low PM10 values, higher relative abundance of antibiotic resistance genes and anthropogenic ions related to local industries and farming, suggesting a dominance of local sources. With the microbiome analyses reported here for the first time for this region, we show good agreement between airborne microbial composition, aerosol mass load, chemistry, and meteorology. These results allow better air quality evaluation and prediction capabilities.

Marine aerosols play a significant role in the global radiative budget, in clouds' processes, and in the chemistry of the marine atmosphere. There is a critical need to better understand their production mechanisms, composition, chemical properties, and the contribution of ocean-derived biogenic matter to their mass and number concentration. Here we present an overview of a new dataset of in situ measurements of marine aerosols conducted over the 2.5-yr Tara Pacific Expedition over 110,000 km across the Atlantic and Pacific Oceans. Preliminary results are presented here to describe the new dataset that will be built using this novel set of measurements. It will characterize marine aerosols properties in detail and will open a new window to study the marine aerosol link to the water properties and environmental conditions.

Atmospheric brown carbon (BrC) is an important contributor to the radiative forcing of climate by organic aerosols. Because of the molecular diversity of BrC compounds and their dynamic transformations, it is challenging to predictively understand BrC optical properties. OH radical and O3 reactions, together with photolysis, lead to diminished light absorption and lower warming effects of biomass burning BrC. The effects of night-time aging on the optical properties of BrC aerosols are less known. To address this knowledge gap, night-time NO3 radical chemistry with tar aerosols from wood pyrolysis was investigated in a flow reactor. This study shows that the optical properties of BrC change because of transformations driven by reactions with the NO3 radical that form new absorbing species and lead to significant absorption enhancement over the ultraviolet-visible (UV-vis) range. The overnight aging increases the mass absorption coefficients of the BrC by a factor of 1.3-3.2 between 380 nm and 650 nm. Nitrated organic compounds, particularly nitroaromatics, were identified as the main products that contribute to the enhanced light absorption in the secondary BrC. Night-time aging of BrC aerosols represents an important source of secondary BrC and can have a pronounced effect on atmospheric chemistry and air pollution.

Atmospheric immersion freezing (IF), a heterogeneous ice nucleation process where an ice nucleating particle (INP) is immersed in supercooled water, is a dominant ice formation pathway impacting the hydrological cycle and climate. Implementation of IF derived from field and laboratory data in cloud and climate models is difficult due to the high variability in spatio-temporal scales, INP composition, and morphological complexity. We demonstrate that IF can be consistently described by a stochastic nucleation process accounting for uncertainties in the INP surface area. This approach accounts for time-dependent freezing, a wide range of surface areas and challenges phenomenological descriptions typically used to interpret IF. The results have an immediate impact on the current description, interpretation, and experiments of IF and its implementation in models. The findings are in accord with nucleation theory, and thus should hold for any supercooled liquid material that nucleates in contact with a substrate.

Ice-binding proteins (IBPs) are found in many organisms, such as fish and hexapods, plants, and bacteria that need to cope with low temperatures. Ice nucleation and thermal hysteresis are two attributes of IBPs. While ice nucleation is promoted by large proteins, known as ice nucleating proteins, the smaller IBPs, referred to as antifreeze proteins (AFPs), inhibit the growth of ice crystals by up to several degrees below the melting point, resulting in a thermal hysteresis (TH) gap between melting and ice growth. Recently, we showed that the nucleation capacity of two types of IBPs corresponds to their size, in agreement with classical nucleation theory. Here, we expand this finding to additional IBPs that we isolated from snow fleas (the arthropod Collembola), collected in northern Israel. Chemical analyses using circular dichroism and Fourier-transform infrared spectroscopy data suggest that these IBPs have a similar structure to a previously reported snow flea antifreeze protein. Further experiments reveal that the ice-shell purified proteins have hyperactive antifreeze properties, as determined by nanoliter osmometry, and also exhibit low ice-nucleation activity in accordance with their size.

The prediction of cloud ice formation in climate models remains a challenge, partly due to the complexity of ice-related processes. Mineral dust is a prominent aerosol in the troposphere and is an important contributor to ice nucleation in mixed-phase clouds, as dust can initiate ice heterogeneously at relatively low supercooling conditions. We characterized the ice nucleation properties of size-segregated mineral dust sampled during dust events in the eastern Mediterranean. The sampling site allowed us to compare the properties of airborne dust from several sources with diverse mineralogy that passed over different atmospheric paths. We focused on particles with six size classes determined by the Micro-Orifice Uniform Deposit Impactor (MOUDI) cutoff sizes: 5.6, 3.2, 1.8, 1.0, 0.6 and 0.3 μm. Ice nucleation experiments were conducted in the Weizmann Supercooled Droplets Observation on a Microarray (WISDOM) setup, whereby the particles are immersed in nanoliter droplets using a microfluidics technique. We observed that the activity of airborne particles depended on their size class; supermicron and submicron particles had different activities, possibly due to different composition. The concentrations of ice-nucleating particles and the density of active sites (ns) increased with the particle size and particle concentration. The supermicron particles in different dust events showed similar activity, which may indicate that freezing was dominated by common mineralogical components. Combining recent data of airborne mineral dust, we show that current predictions, which are based on surface-sampled natural dust or standard mineral dust, overestimate the activity of airborne dust, especially for the submicron class. Therefore, we suggest including information on particle size in order to increase the accuracy of ice formation modeling and thus weather and climate predictions.

Polycyclic aromatic hydrocarbons (PAHs) are toxic organic trace components in atmospheric aerosols that have impacts on climate and human health. They are bound to airborne particles and transported over long distances. Observations of their distribution, transport pathways, and degradation are crucial for risk assessment and mitigation. Such estimates would benefit from online detection of PAHs along with analysis of the carrying particles to identify the source. Typically, laser desorption/ionization (LDI) in a bipolar mass spectrometer reveals the inorganic constituents and provides limited molecular information. In contrast, two-step ionization approaches produce detailed PAH mass spectra from individual particles but without the source-specific inorganic composition. Here we report a new technique that yields the single-particle PAH composition along with both positive and negative inorganic ions via LDI. Thus, the complete particle characterization and source apportionment from conventional bipolar LDI-analysis becomes possible, combined with a detailed PAH spectrum for the same particle. The key idea of the method is spatiotemporal matching of the ionization laser pulse to the transient component distribution in the particle plume after laser desorption. The technique is robust and field-deployable with only slightly higher costs and complexity compared to two-step approaches. We demonstrate its capability to reveal the PAH-distribution on different particle types in combustion aerosols and ambient air.

Criegee intermediates (CI) from ozonolysis of biogenic volatile organic compounds (BVOC) have been suggested to be important atmospheric oxidants. However, due to their low atmospheric concentrations, possible high reactivity with water vapor, and unconstrained thermal unimolecular decay rates, their impact on atmospheric oxidation of trace species such as SO2 and NO2 remains uncertain. In this study, we investigate the formation of secondary sulfate aerosols (SSA) in nocturnal power plant plumes in the Southeastern US. These plumes have large mixing ratios of SO2 and NO that make reaction with CI competitive with other pathways, such as thermal unimolecular decay and water vapor reaction. The background into which these plumes are emitted has high levels of BVOC and O-3, whose reaction produces a large source of CI. Observed nighttime power plant plume intercepts had measurable sulfate aerosol, ranging from 0.7-1.2% of the total plume sulfur (SO2 + sulfate) on a molar basis. In the absence of photochemical OH oxidation, these observed sulfate levels can be compared to calculated CI + SO2 production. We present a plume dispersion model that simulates the chemical evolution of these nighttime plumes and compare the results to observed sulfate aerosol. Thermal unimolecular decay of CI is the largest uncertainty. In the absence of thermal unimolecular CI decay, CI reactions with SO2 in the dark account for up to 41% of the total observed sulfate aerosol, with the remainder attributable to reaction of SO2 with secondary OH and direct emission. Conversely, with a thermal unimolecular decay rate for all CI of 200 s(-1), equivalent to the highest measured rate, CI reactions with SO2 accounted for only S.7% of the total SSA. A second uncertainty is the rate coefficients for larger, and as yet unmeasured, CI species. The most important CI in the modeled scenario is the C, compound, CH2OO, which accounts for up to 50% of the CIs produced from isoprene. C-4 CIs may contribute up to 40% of the CIs produced and are expected to have substantially slower thermal unimolecular decay rates and water vapor reaction rate coefficients. Therefore, the model results may be a lower limit to the CI contribution to SSA. Calculated nighttime (10 h) total SO2 oxidation was 1.8%, of which 1.1% was due to CI + SO2, and the remainder to secondary OH + SO2. This compares to daytime (14 h) SO2 oxidation rates of 4% due to photochemical OH + SO2 reaction.

Several types of natural molecules interact specifically with ice crystals. Small antifreeze proteins (AFPs) adsorb to particular facets of ice crystals, thus inhibiting their growth, whereas larger ice-nucleating proteins (INPs) can trigger the formation of new ice crystals at temperatures much higher than the homogeneous ice nucleation temperature of pure water. It has been proposed that both types of proteins interact similarly with ice and that, in principle, they may be able to exhibit both functions. Here we investigated two naturally occurring antifreeze proteins, one from fish, type-III AFP, and one from beetles, TmAFP. We show that in addition to ice growth inhibition, both can also trigger ice nucleation above the homogeneous freezing temperature, providing unambiguous experimental proof for their contrasting behavior. Our analysis suggests that the predominant difference between AFPs and INPs is their molecular size, which is a very good predictor of their ice nucleation temperature.

The second phase of the Fifth International Ice Nucleation Workshop (FIN-02) involved the gathering of a large number of researchers at the Karlsruhe Institute of Technology's Aerosol Interactions and Dynamics of the Atmosphere (AIDA) facility to promote characterization and understanding of ice nucleation measurements made by a variety of methods used worldwide. Compared to the previous workshop in 2007, participation was doubled, reflecting a vibrant research area. Experimental methods involved sampling of aerosol particles by direct processing ice nucleation measuring systems from the same volume of air in separate experiments using different ice nucleating particle (INP) types, and collections of aerosol particle samples onto filters or into liquid for sharing amongst measurement techniques that post-process these samples. In this manner, any errors introduced by differences in generation methods when samples are shared across laboratories were mitigated. Furthermore, as much as possible, aerosol particle size distribution was controlled so that the size limitations of different methods were minimized. The results presented here use data from the workshop to assess the comparability of immersion freezing measurement methods activating INPs in bulk suspensions, methods that activate INPs in condensation and/or immersion freezing modes as single particles on a substrate, continuous flow diffusion chambers (CFDCs) directly sampling and processing particles well above water saturation to maximize immersion and subsequent freezing of aerosol particles, and expansion cloud chamber simulations in which liquid cloud droplets were first activated on aerosol particles prior to freezing. The AIDA expansion chamber measurements are expected to be the closest representation to INP activation in atmospheric cloud parcels in these comparisons, due to exposing particles freely to adiabatic cooling.The different particle types used as INPs included the minerals illite NX and potassium feldspar (K-feldspar), two natural soil dusts representative of arable sandy loam (Argentina) and highly erodible sandy dryland (Tunisia) soils, respectively, and a bacterial INP (Snomax (R)). Considered together, the agreement among post-processed immersion freezing measurements of the numbers and fractions of particles active at different temperatures following bulk collection of particles into liquid was excellent, with possible temperature uncertainties inferred to be a key factor in determining INP uncertainties. Collection onto filters for rinsing versus directly into liquid in impingers made little difference. For methods that activated collected single particles on a substrate at a controlled humidity at or above water saturation, agreement with immersion freezing methods was good in most cases, but was biased low in a few others for reasons that have not been resolved, but could relate to water vapor competition effects. Amongst CFDC-style instruments, various factors requiring (variable) higher supersaturations to achieve equivalent immersion freezing activation dominate the uncertainty between these measurements, and for comparison with bulk immersion freezing methods. When operated above water saturation to include assessment of immersion freezing, CFDC measurements often measured at or above the upper bound of immersion freezing device measurements, but often underestimated INP concentration in comparison to an immersion freezing method that first activates all particles into liquid droplets prior to cooling (the PIMCA-PINC device, or Portable Immersion Mode Cooling chAmber-Portable Ice Nucleation Chamber), and typically slightly underestimated INP number concentrations in comparison to cloud parcel expansions in the AIDA chamber; this can be largely mitigated when it is possible to raise the relative humidity to sufficiently high values in the CFDCs, although this is not always possible operationally.Correspondence of measurements of INPs among direct sampling and post-processing systems varied depending on the INP type. Agreement was best for Snomax (R) particles in the temperature regime colder than 10 degrees C, where their ice nucleation activity is nearly maximized and changes very little with temperature. At temperatures warmer than -10 degrees C, Snomax (R) INP measurements (all via freezing of suspensions) demonstrated discrepancies consistent with previous reports of the instability of its protein aggregates that appear to make it less suitable as a calibration INP at these temperatures. For Argentinian soil dust particles, there was excellent agreement across all measurement methods; measures ranged within 1 order of magnitude for INP number concentrations, active fractions and calculated active site densities over a 25 to 30 degrees C range and 5 to 8 orders of corresponding magnitude change in number concentrations. This was also the case for all temperatures warmer than -25 degrees C in Tunisian dust experiments. In contrast, discrepancies in measurements of INP concentrations or active site densities that exceeded 2 orders of magnitude across a broad range of temperature measurements found at temperatures warmer than- 25 degrees C in a previous study were replicated for illite NX. Discrepancies also exceeded 2 orders of magnitude at temperatures of -20 to -25 degrees C for potassium feldspar (K-feldspar), but these coincided with the range of temperatures at which INP concentrations increase rapidly at approximately an order of magnitude per 2 degrees C cooling for K-feldspar.These few discrepancies did not outweigh the overall positive outcomes of the workshop activity, nor the future utility of this data set or future similar efforts for resolving remaining measurement issues. Measurements of the same materials were repeatable over the time of the workshop and demonstrated strong consistency with prior studies, as reflected by agreement of data broadly with parameterizations of different specific or general (e.g., soil dust) aerosol types. The divergent measurements of the INP activity of illite NX by direct versus post-processing methods were not repeated for other particle types, and the Snomax (R) data demonstrated that, at least for a biological INP type, there is no expected measurement bias between bulk collection and direct immediately processed freezing methods to as warm as -10 degrees C. Since particle size ranges were limited for this workshop, it can be expected that for atmospheric populations of INPs, measurement discrepancies will appear due to the different capabilities of methods for sampling the full aerosol size distribution, or due to limitations on achieving sufficient water supersaturations to fully capture immersion freezing in direct processing instruments. Overall, this workshop presents an improved picture of present capabilities for measuring INPs than in past workshops, and provides direction toward addressing remaining measurement issues.

Exposure to ambient fine particulate matter (PM
2.5) is a leading risk factor for the global burden of disease. However, uncertainty remains about PM
2.5 sources. We use a global chemical transport model (GEOS-Chem) simulation for 2014, constrained by satellite-based estimates of PM
2.5 to interpret globally dispersed PM
2.5 mass and composition measurements from the ground-based surface particulate matter network (SPARTAN). Measured site mean PM
2.5 composition varies substantially for secondary inorganic aerosols (2.4-19.7 μg/m
3), mineral dust (1.9-14.7 μg/m
3), residual/organic matter (2.1-40.2 μg/m
3), and black carbon (1.0-7.3 μg/m
3). Interpretation of these measurements with the GEOS-Chem model yields insight into sources affecting each site. Globally, combustion sectors such as residential energy use (7.9 μg/m
3), industry (6.5 μg/m
3), and power generation (5.6 μg/m
3) are leading sources of outdoor global population-weighted PM
2.5 concentrations. Global population-weighted organic mass is driven by the residential energy sector (64%) whereas population-weighted secondary inorganic concentrations arise primarily from industry (33%) and power generation (32%). Simulation-measurement biases for ammonium nitrate and dust identify uncertainty in agricultural and crustal sources. Interpretation of initial PM
2.5 mass and composition measurements from SPARTAN with the GEOS-Chem model constrained by satellite-based PM
2.5 provides insight into sources and processes that influence the global spatial variation in PM
2.5 composition.

Obesity and exposure to particular matter (PM) have become two leading global threats to public health. However, the exact mechanisms and tissue-specificity of their health effects are largely unknown. Here we investigate whether a metabolic challenge (early nutritional obesity) synergistically interacts with an environmental challenge (PM exposure) to alter genes representing key response pathways, in a tissue-specific manner. Mice subjected to 7 weeks obesogenic nutrition were exposed every other day during the final week and a half to aqueous extracts of PM collected in the city of London (UK). The expression of 61 selected genes representing key response pathways were investigated in lung, liver, white and brown adipose tissues. Principal component analysis (PCA) revealed distinct patterns of expression changes between the 4 tissues, particularly in the lungs and the liver. Surprisingly, the lung responded to the nutrition challenge. The response of these organs to the PM challenge displayed opposite patterns for some key genes, in particular, those related to the Nrf2 pathway. While the contribution to the variance in gene expression changes in mice exposed to the combined challenge were largely similar among the tissues in PCA1, PCA2 exhibited predominant contribution of inflammatory and oxidative stress responses to the variance in the lungs, and a greater contribution of autophagy genes and MAP kinases in adipose tissues. Possible involvement of alterations in DNA methylation was demonstrated by cell-type-specific responses to a methylation inhibitor. Correspondingly, the DNA methyltransferase Dnmt3a2 increased in the lungs but decreased in the liver, demonstrating potential tissue-differential synergism between nutritional and PM exposure. The results suggest that urban PM, containing dissolved metals, interacts with obesogenic nutrition to regulate diverse response pathways including inflammation and oxidative stress, in a tissue-specific manner. Tissue-differential effects on DNA methylation may underlie tissue-specific responses to key stress-response genes such as catalase and Nrf2. Obesogenic nutrition and air pollution activate inflammation and other stress response pathway in a tissue specific manner, potentially reflecting tissue-specific regulation of DNA methyltransferases.

Atmospheric photooxidation of isoprene forms isoprene epoxydiols (IEPOX) and hydroxymethel-methyl-α-lactone (HMML) via hydroperoxyl radical (HO
2) channel and NO/NO
2 channel, respectively. Reactive uptake of these epoxides onto particles produces isoprene secondary organic aerosols (iSOA). Currently, there is little information regarding these two epoxides during iSOA formation in polluted regions. In this study, iSOA tracers from IEPOX and HMML were measured from summer to fall in the heavily polluted Pearl River Delta (PRD) region. The total concentration of the iSOA tracers ranged from 5.77 to 466 ng m
−3. Isoprene SOA tracers correlated well with sulfate (p < 0.01), indicating that the abundant sulfate in the PRD plays an important role in iSOA formation. A kinetic model of IEPOX loss showed that 58% of IEPOX could undergo ring-opening reactions under the polluted PRD conditions in summer. This leads to high levels of IEPOX-derived SOA tracers in the PRD. High temperature in the PRD (>22 °C) suppresses the production of HMML, likely as a result of fast decomposition of HMML's precursor under high temperatures. Thus, the HMML-derived tracers had lower levels than the IEPOX-derived SOA tracers during the whole campaign. The ratios of the IEPOX-derived tracers to the HMML-derived SOA tracers in summer were ~3 times higher than those in fall. This seasonal trend may be explained by the relative high isoprene/NO
x ratio, temperature, and fast heterogeneous reaction of IEPOX in summer. Our study shows that in highly polluted regions like PRD, reduction in SO
2 emission can significantly reduce iSOA formation.

Fine particulate matter (PM2.5) air pollution poses a major risk to human health worldwide, and absorbed chemicals play a key role in determining the toxicity of PM2.5. After inhalation and entry into the lungs, PM2.5 components induce pro-inflammatory cytokines (e.g., interleukin (1)-1 beta) in pulmonary cells. To test whether PM2.5 components induce IL-1 beta through signing pathways that include the toll-like receptor 4 (TLR4)/nuclear factor-kappa-gene binding (NF-kappa B), nucleotide-binding domain and leucine-rich repeat protein 3 (NLRP3), we exposed the mouse macrophage cell-line RAW264.7 to both water and organic extracts of PM2.5 sampled over a 1-year period in Beijing, China. Varying degrees of oxidative stress and inflammatory responses were induced following exposure, while organic extracts of PM2.5 collected during the heating season induced more significant responses. This response is attributed to high concentrations of polycyclic aromatic hydrocarbons (PAHs) originating from coal combustion and biomass burning for domestic heating. The inhibition of signaling molecules suggested that increased IL-1 beta was associated with the TLR4/NF-kappa B pathway and NLRP3 inflammasome activation, with a slightly difference between water and organic extracts exposure groups, which was likely the result of different chemical components. Our study elucidated a potentially important mechanism by which PM2.5 components could trigger pulmonary inflammation, thus improving our understanding of the deleterious effects of this important and prevalent form of air pollution. (C) 2018 Elsevier Ltd. All rights reserved.

The wavelength-dependence of the complex refractive indices (RI) in the visible spectral range of secondary organic aerosols (SOA) are rarely studied, and the evolution of the RI with atmospheric aging is largely unknown. In this study, we applied a novel white light-broadband cavity enhanced spectroscopy to measure the changes in the RI (400650 nm) of beta-pinene and p-xylene SOA produced and aged in an oxidation flow reactor, simulating daytime aging under NOx-free conditions. It was found that these SOA are not absorbing in the visible range, and that the real part of the RI, n, shows a slight spectral dependence in the visible range. With increased OH exposure, n first increased and then decreased, possibly due to an increase in aerosol density and chemical mean polarizability for SOA produced at low OH exposures, and a decrease in chemical mean polarizability for SOA produced at high OH exposures, respectively. A simple radiative forcing calculation suggests that atmospheric aging can introduce more than 40% uncertainty due to the changes in the RI for aged SOA.

Lag Ba'Omer, a nationwide bonfire festival in Israel, was chosen as a case study to investigate the influence of a major biomass burning event on the light absorption properties of atmospheric brown carbon (BrC). The chemical composition and optical properties of BrC chromophores were investigated using a high performance liquid chromatography (HPLC) platform coupled to photo diode array (PDA) and high resolution mass spectrometry (HRMS) detectors. Substantial increase of BrC light absorption coefficient was observed during the night-long, biomass burning event. Most chromophores observed during the event were attributed to nitroaromatic compounds (NAC), comprising 28 elemental formulas of at least 63 structural isomers. The NAC, in combination, accounted for 50-80% of the total visible light absorption (> 400 nm) by solvent extractable BrC. The results highlight that NAC, in particular nitrophenols, are important light absorption contributors of biomass burning organic aerosol (BBOA), suggesting that night time chemistry of center dot NO3 and N2O5 with particles may play a significant role in atmospheric transformations of BrC. Nitrophenols and related compounds were especially important chromophores of BBOA. The absorption spectra of the BrC chromophores are influenced by the extraction solvent and solution pH, implying that the aerosol acidity is an important factor controlling the light absorption properties of BrC.

Yinon Rudich responded: It would be beneficial to reach out to other scientific communities and adopt new methods and technologies to atmospheric chemistry. For example, biologists have developed very sensitive tools and assays, that can help address questions relevant to the Anthropocene.

Y. Rudich remarked: About comparisons between biogenic and anthropogenic SOA, in terms of potency to have a biological effect, chamber studies suggest that anthropogenic SOA is more toxic and leads to more mortality than BSOA.1,2 Can you bridge the gap between your conclusions and these studies?1 W. Y. Tuet et al., Atmos. Chem. Phys., 2017, 17, 839–853.2 V. Verma et al., Environ. Sci. Technol., 2017, 51, 3128–3137

Microorganisms carried by dust storms are transported through the atmosphere and may affect human health and the functionality of microbial communities in various environments. Characterizing the dust-borne microbiome in dust storms of different origins or that followed different trajectories provides valuable data to improve our understanding of global health and environmental impacts. We present a comparative study on the diversity of dust-borne bacterial communities in dust storms from three distinct origins (North Africa, Syria and Saudi Arabia) and compare them with local bacterial communities sampled on clear days, all collected at a single location: Rehovot, Israel. Storms from different dust origins exhibited distinct bacterial communities, with signature bacterial taxa. Dust storms were characterized by a lower abundance of selected antibiotic resistance genes (ARGs) compared with ambient dust, asserting that the origin of these genes is local and possibly anthropogenic. With the progression of the storm, the storm-borne bacterial community showed increasing resemblance to ambient dust, suggesting mixing with local dust. These results show, for the first time, that dust storms from different sources display distinct bacterial communities, suggesting possible diverse effects on the environment and public health.

Exposure to ambient particulate matter (PM), including PM from resuspension of soils and dusts, increases the risk for respiratory diseases. However, the exact mechanism of PM-mediated damage to the lungs remains unclear. Due to recent increases in the frequency of dust storms in many areas, we examined the cytotoxic effects of soil-dust samples collected in an arid zone in Israel on rat lung macrophages. The desert soil contains soil crusts and low levels of toxic metal content. Exposure of cells to water extracts from the dust samples caused significant reduction in the concentration of live cells and overall cell viability. The dust samples induced cell death through apoptosis, mitochondrial dysfunction, and increased mitochondrial lipid peroxidation. The dust samples generated more reactive oxygen species (ROS) compared to control-treated samples and National Institute of Standards and Technology San Joaquin Valley standard reference material. To assess whether the oxidative imbalance induced by dust extract also interferes with the antioxidant defense, we evaluated phase II detoxifying and antioxidant enzymes, which are Nrf2 classical targets. The Nrf2 transcription factor is a master regulator of cellular adaptation to stress. The dust extracts produced a significant increase in phase II detoxifying genes. This work suggests that the health-related injury observed in rat lung cells exposed to dust extracts is associated with ROS generation, mitochondrial dysfunction, mitochondrial lipid peroxidation, and cellular antioxidant imbalance. Damage to lung mitochondria may be an important mechanism by which dust-containing bacterial material induces lung injury upon inhalation.

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines.

Anthropogenic activities are responsible for the emission of gaseous and particulate pollutants that modify atmospheric composition. Such changes are, in turn, responsible for the degradation of air quality at the regional/local scale as well as for changes of climate. Air pollution and climate change are two intimately connected environmental issues. However, these two environmental challenges are still viewed as separate issues, which are dealt with by different science communities and within different policy frameworks. Indeed, many mitigation options offer the possibility to both improve air quality and mitigate climate change but, at the same time, mitigation options that may provide benefits to one aspect, are worsening the situation in the other. Therefore, coordinated actions taking into account the air quality-climate linkages are required. These actions need to be based on strong scientific grounds, as recognised by the European Commission that in the past few years has promoted consultation processes among the science community, the policy makers and the relevant stakeholders. Here, the main fields in which such coordinated actions are needed are examined from a policy perspective. (C) 2016 Elsevier Ltd. All rights reserved.

Atmospheric aerosols play an important part in the Earth's energy budget by scattering and absorbing incoming solar and outgoing terrestrial radiation. To quantify the effective radiative forcing due to aerosol-radiation interactions, researchers must obtain a detailed understanding of the spectrally dependent intensive and extensive optical properties of different aerosol types. Our new approach retrieves the optical coefficients and the single-scattering albedo of the total aerosol population over 300 to 650 nm wavelength, using extinction measurements from a broadband cavity-enhanced spectrometer at 315 to 345 nm and 390 to 420 nm, extinction and absorption measurements at 404 nm from a photoacoustic cell coupled to a cavity ring-down spectrometer, and scattering measurements from a three-wavelength integrating nephelometer. By combining these measurements with aerosol size distribution data, we retrieved the time-and wavelength-dependent effective complex refractive index of the aerosols. Retrieval simulations and laboratory measurements of brown carbon proxies showed low absolute errors and good agreement with expected and reported values. Finally, we implemented this new broadband method to achieve continuous spectral-and time-dependent monitoring of ambient aerosol population, including, for the first time, extinction measurements using cavity-enhanced spectrometry in the 315 to 345 nm UV range, in which significant light absorption may occur.

We evaluated the impact of Saharan dust storms on the local airborne microbiome in a city in the Eastern Mediterranean area. Samples of particles with diameter less than 10 mum were collected during two spring seasons on both dusty and nondusty days. DNA was extracted, and partial 16S rRNA gene amplicons were sequenced using the Illumina platform. Bioinformatic analysis showed the effect of dust events on the diversity of the atmospheric microbiome. The relative abundance of desert soil-associated bacteria increased during dust events, while the relative abundance of anthropogenic-influenced taxa decreased. Quantitative polymerase chain reaction measurements of selected clinically significant antibiotic resistance genes (ARGs) showed that their relative abundance decreased during dust events. The ARG profiles on dust-free days were similar to those in aerosol collected in a poultry house, suggesting a strong agricultural influence on the local ambient profiles. We conclude that dust storms enrich the ambient airborne microbiome with new soil-derived bacteria that disappear as the dust settles, suggesting that the bacteria are transported attached to the dust particles. Dust storms do not seem to be an important vector for transport of probed ARGs.

Outdoor aerosol research commonly uses particulate matter sampled on filters. This procedure enables various characterizations of the collected particles to be performed in parallel. The purpose of the method presented here is to obtain a highly accurate and reliable analysis of the endotoxin and DNA content of bio-aerosols extracted from filters. The extraction of high molecular weight organic molecules, such as lipopolysaccharides, from sampled filters involves shaking the sample in a pyrogen-free water-based medium. The subsequent analysis is based on an enzymatic reaction that can be detected using a turbidimetric measurement. As a result of the high organic content on the sampled filters, the extraction of DNA from the samples is performed using a commercial DNA extraction kit that was originally designed for soils and modified to improve the DNA yield. The detection and quantification of specific microbial species using quantitative polymerase chain reaction (q-PCR) analysis are described and compared with other available methods.

Exposure to particulate matter (PM) pollution in cities and urban canyons can be harmful to the exposed population. However, the underlying mechanisms that lead to health effects are not yet elucidated. It is postulated that exposure to repeated, small, environmentally relevant concentrations can affect lung homeostasis. This study examines the impact of repeated exposures to urban PM on mouse lungs with focus on inflammatory and oxidative stress parameters. Aqueous extracts from collected urban PM were administered to mice by 5 repeated intra-tracheal instillations (IT). Multiple exposures, led to an increase in cytokine levels in both bronchoalveolar lavage fluid and in the blood serum, indicating a systemic reaction. Lung mRNA levels of antioxidant/phase II detoxifying enzymes decreased by exposure to the PM extract, but not when metals were removed by chelation. Finally, disruption of lung tissue oxidant-inflammatory/defense balance was evidenced by increased levels of lipid and protein oxidation. Unlike response to a single IT exposure to the same dose and source of extract, multiple exposures result in lung oxidative damage and a systemic inflammatory reaction. These could be attributed to compromised capacity to activate the protective Nrf2 tissue defense system. It is suggested that water-soluble metals present in urban PM, potentially from break and tire wear, may constitute major drivers of the pulmonary and systemic responses to multiple exposure to urban PM.

Interaction of biogenic volatile organic compounds (VOCs) with Anthropogenic VOC (AVOC) affects the physicochemical properties of secondary organic aerosol (SOA). We investigated cloud droplet activation (CCN activity), droplet growth kinetics, and hygroscopicity of mixed anthropogenic and biogenic SOA (ABSOA) compared to pure biogenic SOA (BSOA) and pure anthropogenic SOA (ASOA). Selected monoterpenes and aromatics were used as representative precursors of BSOA and ASOA, respectively.We found that BSOA, ASOA, and ABSOA had similar CCN activity despite the higher oxygen to carbon ratio (O/C) of ASOA compared to BSOA and ABSOA. For individual reaction systems, CCN activity increased with the degree of oxidation. Yet, when considering all different types of SOA together, the hygroscopicity parameter, kappa(CCN), did not correlate with O/C. Droplet growth kinetics of BSOA, ASOA, and ABSOA were comparable to that of (NH4)(2)SO4, which indicates that there was no delay in the water uptake for these SOA in supersaturated conditions.In contrast to CCN activity, the hygroscopicity parameter from a hygroscopic tandem differential mobility analyzer (HTDMA) measurement, kappa(HTDMA) of ASOA was distinctively higher (0.09-0.10) than that of BSOA (0.030-0.06), which was attributed to the higher degree of oxida-tion of ASOA. The ASOA components in mixed ABSOA enhanced aerosol hygroscopicity. Changing the ASOA fraction by adding biogenic VOC (BVOC) to ASOA or vice versa (AVOC to BSOA) changed the hygroscopicity of aerosol, in line with the change in the degree of oxidation of aerosol. However, the hygroscopicity of ABSOA cannot be described by a simple linear combination of pure BSOA and ASOA systems. This indicates that additional processes, possibly oligomerization, affected the hygroscopicity.Closure analysis of CCN and HTDMA data showed kappa(HTDMA) was lower than kappa(CCN) by 30-70 %. Better closure was achieved for ASOA compared to BSOA. This discrepancy can be attributed to several reasons. ASOA seemed to have higher solubility in subsaturated conditions and/or higher surface tension at the activation point than that of BSOA.

New measurements of water diffusion in secondary organic aerosol (SOA) material produced by oxidation of alpha-pinene and in a number of organic/inorganic model mixtures (3-methylbutane-1,2,3-tricarboxylic acid (3-MBTCA), levoglucosan, levoglucosan/NH4HSO4, raffinose) are presented. These indicate that water diffusion coefficients are determined by several properties of the aerosol substance and cannot be inferred from the glass transition temperature or bouncing properties. Our results suggest that water diffusion in SOA particles is faster than often assumed and imposes no significant kinetic limitation on water uptake and release at temperatures above 220 K. The fast diffusion of water suggests that heterogeneous ice nucleation on a glassy core is very unlikely in these systems. At temperatures below 220 K, model simulations of SOA particles suggest that heterogeneous ice nucleation may occur in the immersion mode on glassy cores which remain embedded in a liquid shell when experiencing fast updraft velocities. The particles absorb significant quantities of water during these updrafts which plasticize their outer layers such that these layers equilibrate readily with the gas phase humidity before the homogeneous ice nucleation threshold is reached. Glass formation is thus unlikely to restrict homogeneous ice nucleation. Only under most extreme conditions near the very high tropical tropopause may the homogeneous ice nucleation rate coefficient be reduced as a consequence of slow condensed-phase water diffusion. Since the differences between the behavior limited or non limited by diffusion are small even at the very high tropical tropopause, condensed-phase water diffusivity is unlikely to have significant consequences on the direct climatic effects of SOA particles under tropospheric conditions.

The literature on atmospheric particulate matter (PM), or atmospheric aerosol, has increased enormously over the last 2 decades and amounts now to some 1500-2000 papers per year in the refereed literature. This is in part due to the enormous advances in measurement technologies, which have allowed for an increasingly accurate understanding of the chemical composition and of the physical properties of atmospheric particles and of their processes in the atmosphere. The growing scientific interest in atmospheric aerosol particles is due to their high importance for environmental policy. In fact, particulate matter constitutes one of the most challenging problems both for air quality and for climate change policies. In this context, this paper reviews the most recent results within the atmospheric aerosol sciences and the policy needs, which have driven much of the increase in monitoring and mechanistic research over the last 2 decades. The synthesis reveals many new processes and developments in the science underpinning climate-aerosol interactions and effects of PM on human health and the environment. However, while airborne particulate matter is responsible for globally important influences on premature human mortality, we still do not know the relative importance of the different chemical components of PM for these effects. Likewise, the magnitude of the overall effects of PM on climate remains highly uncertain. Despite the uncertainty there are many things that could be done to mitigate local and global problems of atmospheric PM. Recent analyses have shown that reducing black carbon (BC) emissions, using known control measures, would reduce global warming and delay the time when anthropogenic effects on global temperature would exceed 2 degrees C. Likewise, cost-effective control measures on ammonia, an important agricultural precursor gas for secondary inorganic aerosols (SIA), would reduce regional eutrophication and PM concentrations in large areas of Europe

The Amazon basin is a hot spot of anthropogenically-driven biomass burning, accounting for approximately 15% of total global fire emissions. It is essential to accurately measure these fires for robust regional and global modeling of key environmental processes. Here we have explored the link between spatio-temporal variability patterns in the Amazon basin's fires and the resulting smoke loading using 11 years (2002-2012) of data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Aerosol Robotic Network (AERONET) observations. Focusing on the peak burning season (July-October), our analysis shows strong inter-annual correlation between aerosol optical depth (AOD) and two MODIS fire products: fire radiative power (FRP) and fire pixel counts (FC). Among these two fire products, the FC better indicates the amount of smoke in the basin, as represented in remotely sensed AOD data. This fire product is significantly correlated both with regional AOD retrievals from MODIS and with point AOD measurements from the AERONET stations, pointing to spatial homogenization of the smoke over the basin on a seasonal time scale. However, MODIS AODs are found better than AERONET AODs observation for linking between smoke and fire. Furthermore, MODIS AOD measurements are strongly correlated with number of fires similar to 1(0)-2(0) to the east, most likely due to westward advection of smoke by the wind. These results can be rationalized by the regional topography and the wind regimes. Our analysis can improve data assimilation of satellite and ground-based observations into regional and global model studies, thus improving the assessment of the environmental and climatic impacts of frequency and distribution variability of the Amazon basin's fires. We also provide the optimal spatial and temporal scales for ground-based observations, which could be used for such applications. (C) 2015 Elsevier Ltd. All rights reserved.

Background: Over the last decade, there is growing evidence that exposure to air pollution may be associated with increased risk for congenital malformations. Objectives: To evaluate the possible association between exposures to air pollution during pregnancy and congenital malformations among infants born following spontaneously conceived (SC) pregnancies and assisted reproductive technology (ART) pregnancies. Methods: This is an historical cohort study comprising 216,730 infants: 207,825 SC infants and 8905 ART conceived infants, during the periods 1997-2004. Air pollution data including sulfur dioxide (SO2), particulate matter

To evaluate the health impacts of particulate matter and develop effective pollutant abatement strategies, one needs to know the source contributions to the observed concentrations. The most common approach involves the collection of ambient air samples on filters, laboratory analyses to quantify the chemical composition, and application of receptor modeling methods. This approach is expensive and time consuming and limits the ability to monitor the temporal and spatial impacts from different pollutant sources. An alternative method for apportioning the sources of ambient PM is the application of microscopic chemical imaging (MCI). The MCI method involves measuring individual particle's fluorescence and source attribution is based on the individual particle analysis coupled with identification from a source library. Using this approach, the apportionment of ambient PM can be performed in near real time, which allows for the generation of temporal and spatial maps of pollutant source impacts in an urban area. (C) 2013 Elsevier B.V. All rights reserved.

Using shipboard and satellite measurements we explore the environmental factors affecting the number concentration of aerosols with diameter 100<D<1000 nm over a cluster of three mesoscale (similar to 10-100 km) eddies in the North Atlantic. Strongest sensitivity to environmental conditions was found in the 400<D<1000nm size range. In this size range particle concentrations were closely linked to the surface wind speed, indicating in situ production of sea spray aerosols by wind-driven processes. Particle concentrations were also affected by mesoscale variability in oceanic conditions at the vicinity of an anticyclonic eddy. In addition, a distinct aerosol population possibly produced at a distance of similar to 1000-2000 km from the study area was identified. The results highlight the importance of oceanic and atmospheric mesoscale processes in determining the characteristics of aerosols over the marine environment.

The chemical and physical properties of secondary organic aerosol (SOA) formed by the photochemical degradation of biogenic and anthropogenic volatile organic compounds (VOC) are as yet still poorly constrained. The evolution of the complex refractive index (RI) of SOA, formed from purely biogenic VOC and mixtures of biogenic and anthropogenic VOC, was studied over a diurnal cycle in the SAPHIR photochemical outdoor chamber in Julich, Germany. The correlation of RI with SOA chemical and physical properties such as oxidation level and volatility was examined. The RI was retrieved by a newly developed broadband cavity-enhanced spectrometer for aerosol optical extinction measurements in the UV spectral region (360 to 420 nm). Chemical composition and volatility of the particles were monitored by a high-resolution time-of-flight aerosol mass spectrometer, and a volatility tandem differential mobility analyzer. SOA was formed by ozonolysis of either (i) a mixture of biogenic VOC (alpha-pinene and limonene), (ii) biogenic VOC mixture with subsequent addition of an anthropogenic VOC (p-xylene-d(10)), or (iii) a mixture of biogenic and anthropogenic VOC. The SOA aged by ozone/OH reactions up to 29.5 h was found to be non-absorbing in all cases. The SOA with p-xylene-d(10) showed an increase of the scattering component of the RI correlated with an increase of the O / C ratio and with an increase in the SOA density. There was a greater increase in the scattering component of the RI when the SOA was produced from the mixture of biogenic VOCs and anthropogenic VOC than from the sequential addition of the VOCs after approximately the same ageing time. The increase of the scattering component was inversely correlated with the SOA volatility. Two RI retrievals determined for the pure biogenic SOA showed a constant RI for up to 5 h of ageing. Mass spectral characterization shows the three types of the SOA formed in this study have a significant amount of semivolatile components.

Atmospheric absorption by brown carbon aerosol may play an important role in global radiative forcing. Brown carbon arises from both primary and secondary sources, but the mechanisms and reactions of the latter are highly uncertain. One proposed mechanism is the reaction of ammonia or amino acids with carbonyl products in secondary organic aerosol (SOA). We generated SOA in situ by reacting biogenic alkenes (alpha-pinene, limonene, and alpha-humulene) with excess ozone, humidifying the resulting aerosol, and reacting the humidified aerosol with gaseous ammonia. We determined the complex refractive indices (RI) in the 360-420 nm range for these aerosols using broadband cavity enhanced spectroscopy (BBCES). The average real part (n) of the measured spectral range of the NH3-aged alpha-pinene SOA increased from n = 1.50 (+/- 0.01) for the unreacted SOA to n = 1.57 (+/- 0.01) after 1.5 h of exposure to 1.9 ppm NH3, whereas the imaginary component (k) remained below k <0.001 ((+0.002)(-0.001)) For the limonene and alpha-humulene SOA the real part did not change significantly, and we observed a small change in the imaginary component of the RI. The imaginary component increased from k = 0.000 to an average k = 0.029 (+/- 0.021) for alpha-humulene SOA, and from k <0.001((+0.002)(-0.001)) to an average k = 0.032 (+/- 0.019) for limonene SOA after 1.5 h of exposure to 1.3 and 1.9 ppm of NH3, respectively. Collected filter samples of the aged and unreacted alpha-pinene SOA and limonene SOA were analyzed off-line by nanospray desorption electrospray ionization high resolution mass spectrometry (nano-DESI/HR-MS), and in situ using a Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS), confirming that the SOA reacted and that various nitrogen-containing reaction products formed. If we assume that NH3 aging reactions scale linearly with time and concentration, which will not necessarily be the case in the atmosphere, then a 1.5 h reaction with 1 ppm NH3 in the laboratory is e

Alkyl aminium sulfates have been postulated to constitute important components of nucleation and accumulation mode atmospheric aerosols. In this study we present laboratory data on the thermochemical, cloud condensation nuclei (CCN) activity, and optical properties of selected aminium sulfate compounds of atmospheric relevance (monomethyl aminium sulfate (MMAS), dimethyaminium sulfate (DMAS), trimethylaminium sulfate, monoethylaminium sulfate (MEAS), diethylaminium sulfate (DEAS), and triethylaminium sulfate (TEAS)). We found that the vapor pressure of these aminium salts is 1-3 orders of magnitude lower than that of ammonium sulfate and as such they can contribute to new aerosols and secondary aerosols formation. We infer that these species have very high CCN activity, with hygroscopicity parameter that is similar to that ammonium sulfate. Finally, between 360 and 420 nm, these aminium sulfate salts scatter light less efficiently than ammonium sulfate, and do not absorb light. These derived parameters can contribute to the better understanding and characterization of the role that these compounds play in atmospheric chemical reactions, gas-solid partitioning and their possible contribution to the microphysical and radiative effects of atmospheric aerosols.

Increased susceptibility to allergies has been documented in the Western world in recent decades. However, a comprehensive understanding of its causes is not yet available. It is therefore essential to understand trends and mechanisms of allergy-inducing agents, such as fungal conidia. In this study, we investigated the hypothesis that environmental conditions linked to global atmospheric changes can affect the allergenicity of Aspergillus fumigatus, a common allergenic fungal species in indoor and outdoor environments and in airborne particulate matter. We show that fungi grown under present-day CO2 levels (392ppm) exhibit 8.5 and 3.5 fold higher allergenicity compared to fungi grown at preindustrial (280ppm) and double (560ppm) CO2 levels, respectively. A corresponding trend is observed in the expression of genes encoding for known allergenic proteins and in the major allergen Asp f1 concentrations, possibly due to physiological changes such as respiration rates and the nitrogen content of the fungus, influenced by the CO2 concentrations. Increased carbon and nitrogen levels in the growth medium also lead to a significant increase in the allergenicity. We propose that climatic changes such as increasing atmospheric CO2 levels and changes in the fungal growth medium may impact the ability of allergenic fungi such as A. fumigatus to induce allergies.

Close examination of the principle of global per-capita allocation of the earth's ability to absorb greenhouse gas

Credible climate change predictions require reliable fundamental scientific knowledge of the underlying processes. Despite extensive observational data accumulated to date, atmospheric aerosols still pose key uncertainties in the understanding of Earth's radiative balance due to direct interaction with radiation and because they modify clouds' properties. Specifically, major gaps exist in the understanding of the physicochemical pathways that lead to aerosol growth in the atmosphere and to changes in their properties while in the atmosphere. Traditionally, the driving forces for particle growth are attributed to condensation of low vapor pressure species following atmospheric oxidation of volatile compounds by gaseous oxidants. The current study presents experimental evidence of an unaccounted-for new photoinduced pathway for particle growth. We show that heterogeneous reactions activated by light can lead to fast uptake of noncondensable Volatile Organic Compounds (VOCs) at the surface of particles when only traces of a photosensitizer are present in the seed aerosol. Under such conditions, size and mass increase; changes in the chemical composition of the aerosol are also observed upon exposure to volatile organic compounds such as terpenes and near-UV irradiation. Experimentally determined growth rate values match field observations, suggesting that this photochemical process can provide a new, unaccounted-for pathway for atmospheric particle growth and should be considered by models.

Climate change will induce extended heat waves to parts of the vegetation more frequently. High temperatures may act as stress (thermal stress) on plants changing emissions of biogenic volatile organic compounds (BVOCs). As BVOCs impact the atmospheric oxidation cycle and aerosol formation, it is important to explore possible alterations of BVOC emissions under high temperature conditions. Applying heat to European beech, Palestine oak, Scots pine, and Norway spruce in a laboratory setup either caused the well-known exponential increases of BVOC emissions or induced irreversible changes of BVOC emissions. Considering only irreversible changes of BVOC emissions as stress impacts, we found that high temperatures decreased the de novo emissions of monoterpenes, sesquiterpenes and phenolic BVOC. This behaviour was independent of the tree species and whether the de novo emissions were constitutive or induced by biotic stress. In contrast, application of thermal stress to conifers amplified the release of monoterpenes stored in resin ducts of conifers and induced emissions of green leaf volatiles. In particular during insect attack on conifers, the plants showed de novo emissions of sesquiterpenes and phenolic BVOCs, which exceeded constitutive monoterpene emissions from pools. The heat-induced decrease of de novo emissions was larger than the increased monoterpene release caused by damage of resin ducts. For insect-infested conifers the net effect of thermal stress on BVOC emissions could be an overall decrease. Global change-induced heat waves may put hard thermal stress on plants. If so, we project that BVOC emissions increase is more than predicted by models only in areas predominantly covered with conifers that do not emit high amounts of sesquiterpenes and phenolic BVOCs. Otherwise overall effects of high temperature stress will be lower increases of BVOC emissions than predicted by algorithms that do not consider stress impacts.

The correct version of Fig. 9 is shown in this document

Simultaneously retrieving the complex refractive indices of the core and shell of coated aerosol particles given the measured extinction efficiency as a function of particle dimensions (core diameter and coated diameter) is much more difficult than retrieving the complex refractive index of homogeneous aerosol particles. Not only must the minimization be performed over a four-parameter space, making it less efficient, but in addition the absolute value of the difference between the measured extinction and the calculated extinction does not have an easily distinguished global minimum. Rather, there are a number of local minima to which almost all conventional retrieval algorithms converge. In this work, we develop a new (to our knowledge) retrieval algorithm that employs the numerical method known as simulated annealing with an innovative "temperature" schedule. This study is limited only to spherical particles with a concentric shell and to cases in which the diameter of both the core and the coated particle are known. We find that when the top ranking particle sizes according to their information content are combined from separate experiments to make up the particle size distribution, the simulated annealing retrieval algorithm is quite robust and by far superior to a greedy random perturbation approach often used. (c) 2011 Optical Society of America

In-situ chemical composition measurements of ambient aerosols have been used for characterizing the evolution of submicron aerosols from a large anthropogenic biomass burning (BB) event in Israel. A high resolution Time of Flight Aerosol Mass Spectrometer (HR-RES-TOF-AMS) was used to follow the chemical evolution of BB aerosols during a night-long, extensive nationwide wood burning event and during the following day. While these types of extensive BB events are not common in this region, burning of agricultural waste is a common practice. The aging process of the BB aerosols was followed through their chemical, physical and optical properties. Mass spectrometric analysis of the aerosol organic component showed that aerosol aging is characterized by shifting from less oxidized fresh BB aerosols to more oxidized aerosols. Evidence for aerosol aging during the day following the BB event was indicated by an increase in the organic mass, its oxidation state, the total aerosol concentration, and a shift in the modal particle diameter. The effective broadband refractive index (EBRI) was derived using a white light optical particle counter (WELAS). The average EBRI for a mixed population of aerosols dominated by open fires was m = 1.53(+/- 0.03) + 0.07i(+/- 0.03), during the smoldering phase of the fires we found the EBRI to be m = 1.54(+/- 0.01) + 0.04i(+/- 0.01) compared to m = 1.49(+/- 0.01) + 0.02i(+/- 0.01) of the aged aerosols during the following day. This change indicates a decrease in the overall aerosol absorption and scattering. Elevated levels of particulate Polycyclic Aromatic Hydrocarbons (PAHs) were detected during the entire event, which suggest possible implications for human health during such extensive event.

Humic-like substances (HULIS) represent an important fraction of particulate organic matter in the atmosphere. Understanding the water uptake by HULIS and the associated morphology evolution will improve the assessment of their ability to act as cloud condensation nuclei as well as their light scattering properties. The water uptake properties of Suwannee River Fulvic Acid and of tannic acid used as proxy for atmospheric HULIS, were investigated using X-ray absorption spectroscopy in combination with a scanning transmission X-ray microscope. For both compounds, continuous water uptake was observed, whereby in fulvic acid phase separation occurred, resulting in an inhomogeneous organic matrix. Within the inhomogeneous mixture, different regions with different amounts of water uptake could be differentiated based on their spectral signatures in near-edge X-ray absorption fine structure (NEXAFS) spectra, thus based on carbon functional group signatures, indicating that carboxyl-poor compounds separated from carboxyl-rich compounds upon water uptake. The differentiation into fractions with high/low water uptake ability is further refined by considering phenols, aromatic groups, and O-alkylic groups.

This study focuses on the retrieval of the normalized mass absorption cross section ( MAC) of soot using theoretical calculations that incorporate new measurements of the optical properties of organic carbon (OC) intrinsic to fresh diesel soot. Intrinsic OC was extracted by water and an organic solvent, and the complex refractive index of the extracted OC was derived at 532 and 355-nm wavelengths using cavity ring-down aerosol spectrometry. The extracted OC was found to absorb weakly in the visible wavelengths and moderately at blue wavelengths. The mass ratio of OC and elemental carbon (EC) in the collected particles was evaluated using a thermo-optical method. The measured EC/OC ratio in the soot exhibited substantial variability from measurement to measurement, ranging between 2 and 5. To test the sensitivity of the MAC to this variability, three different EC/OC ratios (2:1, 1:1, and 1:2) were chosen as representative. Particle size and spherule morphology were estimated using scanning electron microscopy, and the soot was found to be primarily in the form of aggregates with a dominant aggregate diameter mode in the range 200-250 nm. The measured refractive index of the extracted OC was used with a variety of theoretical models to calculate the MAC of internally mixed diesel soot at 532 and 355 nm. We conclude that Rayleigh-Debye-Gans theory on clusters of coated spherules and T-matrix of a solid EC spheroid coated by intrinsic OC are both consistent with previous measurements; however, Rayleigh-Debye-Gans theory provides a more realistic physical model for the calculation

Through long-range transport of dust, the North-African desert supplies essential minerals to the Amazon rain forest. Since North African dust reaches South America mostly during the Northern Hemisphere winter, the dust sources active during winter are the main contributors to the forest. Given that the Bod,l, depression area in southwestern Chad is the main winter dust source, a close link is expected between the Bod,l, emission patterns and volumes and the mineral supply flux to the Amazon. Until now, the particular link between the Bod,l, and the Amazon forest was based on sparse satellite measurements and modeling studies. In this study, we combine a detailed analysis of space-borne and ground data with reanalysis model data and surface measurements taken in the central Amazon during the Amazonian Aerosol Characterization Experiment (AMAZE-08) in order to explore the validity and the nature of the proposed link between the Bod,l, depression and the Amazon forest. This case study follows the dust events of 11-16 and 18-27 February 2008, from the emission in the Bod,l, over West Africa (most likely with contribution from other dust sources in the region) the crossing of the Atlantic Ocean, to the observed effects above the Amazon canopy about 10 days after the emission. The dust was lifted by surface winds stronger than 14 m s(-1), usually starting early in the morning. The lofted dust, mixed with biomass burning aerosols over Nigeria, was transported over the Atlantic Ocean, and arrived over the South American continent. The top of the aerosol layer reached above 3 km, and the bottom merged with the boundary layer. The arrival of the dusty air parcel over the Amazon forest increased the average concentration of aerosol crustal elements by an order of magnitude.

A workshop was held in the framework of the ACCENT (Atmospheric Composition Change - a European Network) Joint Research Programme on "Aerosols" and the Programme on "Access to Laboratory Data". The aim of the workshop was to hold "Gordon Conference" type discussion covering accommodation and reactive uptake of water vapour and trace pollutant gases on condensed phase atmospheric materials. The scope was to review and define the current state of knowledge of accommodation coefficients for water vapour on water droplet and ice surfaces, and uptake of trace gas species on a variety of different surfaces characteristic of the atmospheric condensed phase particulate matter and cloud droplets. Twenty-six scientists participated in this meeting through presentations, discussions and the development of a consensus review. In this review we present an analysis of the state of knowledge on the thermal and mass accommodation coefficient for water vapour on aqueous droplets and ice and a survey of current state-of the-art of reactive uptake of trace gases on a range of liquid and solid atmospheric droplets and particles. The review recommends consistent definitions of the various parameters that are needed for quantitative representation of the range of gas/condensed surface kinetic processes important for the atmosphere and identifies topics that require additional research.

Scientific findings from the last decades have clearly highlighted the need for a more comprehensive approach to atmospheric change processes. In fact, observation of atmospheric composition variables has been an important activity of atmospheric research that has developed instrumental tools (advanced analytical techniques) and platforms (instrumented passenger aircrafts, ground-based in situ and remote sensing stations, earth observation satellite instruments) providing essential information on the composition of the atmosphere. The variability of the atmospheric system and the extreme complexity of the atmospheric cycles for short-lived gaseous and aerosol species have led to the development of complex models to interpret observations, test our theoretical understanding of atmospheric chemistry and predict future atmospheric composition. The validation of numerical models requires accurate information concerning the variability of atmospheric composition for targeted species via comparison with observations and measurements.In this paper, we provide an overview of recent advances in instrumentation and methodologies for measuring atmospheric composition changes from space, aircraft and the surface as well as recent improvements in laboratory techniques that permitted scientific advance in the field of atmospheric chemistry. Emphasis is given to the most promising and innovative technologies that will become operational in the near future to improve knowledge of atmospheric composition. Our current observation capacity, however, is not satisfactory to understand and predict future atmospheric composition changes, in relation to predicted climate warming. Based on the limitation of the current European observing system, we address the major gaps in a second part of the paper to explain why further developments in current observation strategies are still needed to strengthen and optimise an observing system not only capable of responding to the requirements of atmospheric services but also to newly open scientific questions. (C) 2009 Elsevier Ltd. All rights reserved.

By emission of volatile organic compounds (VOC) which
on oxidation form secondary organic aerosols (SOA) the
vegetation is coupled to atmosphere and climate. We
investigated new particle formation from tree emissions in a
new setup: a plant chamber housing the trees coupled to a
reaction chamber for oxidizing the plant emissions and for
forming SOA (J¸lich Plant Aerosol Atmosphere Chamber,
JPAC).
Boreal, temperate Midlatitude, and Mediterranean tree
species were studied and α-pinene was used as reference
compound to characterize the specifics of JPAC and to study
humidity and OH dependence of new particle formation. The
strength and the pattern of the tree emissions were varied by
increasing the temperature for the plants, thus mimicking the
higher frequency of occurrence of warmer days in a future
climate.
Under the experimental conditions OH radicals were
essential for inducing new particle formation, although O3 (≤
80 ppb) was always present and a part of the monoterpenes
and the sesquiterpenes reacted already with ozone before OH
was generated. Formation rates of 3 nm particles were linearly
related to the carbon mixing ratios of the VOC, as were the
maximum observed volume and the condensational growth
rates. The threshold of new particle formation was lower for
the tree emissions than for α-pinene.
Hygroscopic growth factors and activation to cloud
droplets were measured to characterize climate relevant
properties of the resulting aerosols. Overall the hygroscopic
properties of the biogenic SOA from the highly mixed tree
emissions are similar to those found for individual
monoterpenes and sesquiterpenes. However, changes of
emission pattern from species to species or induced by heat
stress is reflected in the hygroscopic properties of the resulting
SOA. The biogenic SOA revealed close to ideal Kˆhler
behavior with significantly reduced surface tension at
activation compared to pure water.

The major uncertainties associated with the direct impact of aerosols on climate call for fast and accurate characterization of their optical properties. Cavity ring down (CRD) spectroscopy provides highly sensitive measurement of aerosols' extinction coefficients from which the complex refractive index (RI) of the aerosol may be retrieved accurately for spherical particles of known size and number density, thus it is possible to calculate the single scattering albedo and other atmospherically relevant optical parameters. We present a CRD system employing continuous wave (CW) single mode laser. The single mode laser and the high repetition rate obtained significantly improve the sensitivity and reliability of the system, compared to a pulsed laser CRD setup. The detection limit of the CW-CRD system is between 6.67 x 10(-10) cm(-1) for an empty cavity and 3.63 x 10(-9) cm(-1) for 1000 particles per cm(3) inside the cavity, at a 400 Hz sampling and averaging of 2000 shots for one sample measurement taken in 5 s. For typical pulsed-CRD, the detection limit for an empty cavity is less than 3.8 x 10(-1) cm(-1) for 1000 shots averaged over 100 s at 10 Hz. The system was tested for stability, accuracy, and RI retrievals for scattering and absorbing laboratory-generated aerosols. Specifically, the retrieved extinction remains very stable for long measurement times (1 h) with an order of magnitude change in aerosol number concentration. In addition, the optical cross section (sigma(ext)) of a 400 nm polystyrene latex sphere (PSL) was determined within 2% error compared to the calculated value based on Mie theory. The complex RI of PSL, nigrosin, and ammonium sulfate (AS) aerosols were determined by measuring the extinction efficiency (Q(ext)) as a function of the size parameter ((pi D)/lambda) and found to be in very good agreement with literature values. A mismatch in the retrieved RI of Suwannee River fulvic acid (SRFA) compared to a previous study was observed and is a

A new approach to retrieve the effective broadband refractive indices (n(broad,eff)) of aerosol particles by a white light aerosol spectrometer (WELAS) optical particle counter (OPC) is presented. Using a tandem differential mobility analyzer (DMA)-OPC system, the n(broad,eff) are obtained for both laboratory and field applications. This method was tested in the laboratory using substances with a wide range of optical properties. With the obtained n(broad,eff), WELAS aerosol size distributions can be corrected. Therefore, this method can be used for instrument calibration in both laboratory and field measurements. The retrieved effective broadband refractive indices for the scattering aerosols were: ammonium sulfate ((NH(4))(2)SO(4), AS) n(broad,eff) = 1.52(+/-0.01) + i0.0, glutaric acid (HOOC(CH(2))(3)COOH, GA) n(broad,eff) = 1.45(+/- 0.01) + i0.0, and sodium chloride (NaCl) n(broad,eff) = 1.49(+/-0.02) + i0.0, all within 4% of literature values. A lightly absorbing substance, Suwannee river fulvic acid (SRFA), was also measured, and its retrieved n(broad,eff) is like that of a pure scatterer, with a value of n(broad,eff) = 1.53(+/-0.01) + i0.0. The retrieved real part of n(broad,eff) is in accordance with literature values and the imaginary part with the behavior of the white light spectrum of the WELAS, which is not sensitive below a wavelength of 380 nm, where SRFA mainly absorbs. For absorbing substances, nigrosine and various mixtures of nigrosine with AS and GA were measured. For nigrosine, n(broad,eff) = 1.64(+/-0.04) + i0.20(+/-0.03) was retrieved, in very good agreement with values found in the literature. The n(broad,eff). retrieved by this method for the mixtures was in accordance with the complex refractive index expected. The n(broad,eff) retrieved by this method would be similar to the values obtained using the solar spectrum.

Surfactants often found in tropospheric aerosols, can affect the onset and development of clouds. Due to high dilution during droplet growth, the effects of surfactants on cloud microphysical processes have been mostly neglected. However, while cloud growth by coalescence conserves the combined volume of all cloud droplets, it reduces the combined surface area. This could lead to enrichment of water-insoluble surfactants (WIS) and to reduced surface tension of droplets forming in warm processes. Measurements of individual raindrops reveal the presence of water insoluble surfactants. Our field and laboratory studies as well as simple theoretical arguments suggest that by causing varying and size-dependent surface tension, WIS can affect cloud microphysics.

Observations indicate that the amount of solar radiation reaching the surface of the Earth has varied significantly over decadal timescales during the past half century. A workshop of the Israel Science Foundation evaluated the observational evidence for global dimming and brightening (GDB), its possible causes, and its implications for the Earth’s climate, hydrology, agriculture, and land use. The workshop also identified gaps in our knowledge and made recommendations to fill these gaps. Speakers at the workshop reviewed current understanding of GDB. Surface observations indicate that widespread global dimming (reduced solar radiation) at a rate of about 2–3 watts per square meter per decade occurred between the late 1950s and the late 1980s over mid-latitude land surfaces of the Northern Hemisphere. These records also show a reversal of this trend over many land areas in the past two decades—brightening of the order of 2 watts per square meter per decade. Satellite estimates of solar radiation reaching the surface and aerosol trends, only available since the early 1980s, are generally consistent with these findings. Some analyses of the surface observations indicate that the rates of dimming and brightening are highest in major population centers and fall off significantly in areas with lower population densities....

Atmospheric aerosols absorb and reflect solar radiation causing surface cooling and heating of the atmosphere. The interaction between aerosols and radiation depends on their complex index of refraction, which is related to the particles' chemical composition. The contribution of light absorbing organic compounds, such as HUmic- LIke Substances ( HULIS) to aerosol scattering and absorption is among the largest uncertainties in assessing the direct effect of aerosols on climate. Using a Cavity Ring Down Aerosol Spectrometer ( CRD- AS), the complex index of refraction of aerosols containing HULIS extracted from pollution, smoke, and rural continental aerosols, and molecular weight- fractionated fulvic acid was measured at 390 nm and 532 nm. The imaginary part of the refractive index ( absorption) substantially increases towards the UV range with increasing molecular weight and aromaticity. At both wavelengths, HULIS extracted from pollution and smoke particles absorb more than HULIS from the rural aerosol. Sensitivity calculations for a pollution- type aerosol containing ammonium sulfate, organic carbon ( HULIS), and soot suggests that accounting for absorption by HULIS leads in most cases to a significant decrease in the single scattering albedo and to a signi. cant increase in aerosol radiative forcing effciency, towards more atmospheric absorption and heating. This indicates that HULIS in biomass smoke and pollution aerosols, in addition to black carbon, can contribute significantly to light absorption in the ultraviolet and visible spectral regions.

Surface-active organics such as humic-like substances (HULIS) are abundant in aerosol particles and can lower the surface tension of cloud droplets forming on secondary organic and biomass burning aerosols. How fast is the diffusion of these species, relative to the time scale of cloud droplet growth? Here we report surface tension measurements of solutions containing HULIS extracted from smoke and pollution aerosol particles as well those of molecular weight-fractionated aquatic fulvic acids. Diffusion coefficients are estimated based on the Gibbs adsorption isotherms. The results suggest that HULIS diffusion to the surface of forming droplets is typically more rapid than the time scale of droplet growth so that cloud microphysical properties are affected.

The hygroscopic growth (HG) of humic-like substances (HULIS) extracted from smoke and pollution aerosol particles and of Suwannee River fulvic acid (SRFA, bulk and fractions of different molecular weight) was measured by humidity tandem differential mobility analyzer (H-TDMA). By characterizing physical and chemical parameters such as molecular weight, elemental composition, and surface tension, we test the effect of these parameters on particle interactions with water vapor. For molecular weight-fractionated SRFA fractions, the growth factor at 90% relative humidity was generally inversely proportional to the molecular weight. HULIS extracts from ambient particles are more hygroscopic than all the SRFA fractions and exhibit different hygroscopic properties depending on their origin and residence time in the atmosphere. The results point out some dissimilarities between SRFA and aerosol- derived HULIS. The cloud condensation nuclei (CCN) behavior of the studied materials was predicted on the basis of hygroscopic growth using a recently introduced approach of Kreidenweis et al. (2005) and compared to CCN activity measurements on the same samples (Dinar et al., 2006). It is found that the computational approach (Kreidenweis et al., 2005) works reasonably well for SRFA fractions but is limited in use for the HULIS extracts from aerosol particles. The difficulties arise from uncertainties associated with HG measurements at high relative humidity, which leads to large errors in the predicted CCN activity.

Cavity ring down aerosol extinction measurements are combined with size distribution measurements to provide a multiparameter basis for the retrieval of the aerosol complex refractive index. We show that two distinct size distributions of small particles (<300 nm) suffice to obtain robust convergence of the Mie theory fit of the extinction function. Experiments are performed both for purely scattering and for absorbing aerosol. Thus this method provides a perspective to use cavity ring down aerosol spectroscopy in field and laboratory measurements that often suffer from low particle concentrations and a lack of large particles.

Aerosols and clouds play central roles in atmospheric chemistry and physics, climate, air pollution, and public health. The mechanistic understanding and predictability of aerosol and cloud properties, interactions, transformations, and effects are, however, still very limited. This is due not only to the limited availability of measurement data, but also to the limited applicability and compatibility of model formalisms used for the analysis, interpretation, and description of heterogeneous and multiphase processes. To support the investigation and elucidation of atmospheric aerosol and cloud surface chemistry and gas-particle interactions, we present a comprehensive kinetic model framework with consistent and unambiguous terminology and universally applicable rate equations and parameters. It enables a detailed description of mass transport and chemical reactions at the gas-particle interface, and it allows linking aerosol and cloud surface processes with gas phase and particle bulk processes in systems with multiple chemical components and competing physicochemical processes. The key elements and essential aspects of the presented framework are: a simple and descriptive double-layer surface model (sorption layer and quasi-static layer); straightforward flux-based mass balance and rate equations; clear separation of mass transport and chemical reactions; well-defined and consistent rate parameters (uptake and accommodation coefficients, reaction and transport rate coefficients); clear distinction between gas phase, gas-surface, and surface-bulk transport (gas phase diffusion, surface and bulk accommodation); clear distinction between gas-surface, surface layer, and surface-bulk reactions (Langmuir-Hinshelwood and Eley-Rideal mechanisms); mechanistic description of concentration and time dependences (transient and steady-state conditions); flexible addition of unlimited numbers of chemical species and physicochemical processes; optional aggregation or resolution

About 40 million tons of dust are transported annually from the Sahara to the Amazon basin. Saharan dust has been proposed to be the main mineral source that fertilizes the Amazon basin, generating a dependence of the health and productivity of the rain forest on dust supply from the Sahara. Here we show that about half of the annual dust supply to the Amazon basin is emitted from a single source: the Bodele depression located northeast of Lake Chad, approximately 0.5% of the size of the Amazon or 0.2% of the Sahara. Placed in a narrow path between two mountain chains that direct and accelerate the surface winds over the depression, the Bodele emits dust on 40% of the winter days, averaging more than 0.7 million tons of dust per day.

Levoglucosan is a tracer for biomass burning sources in atmospheric aerosol particles. Therefore, much effort has been recently put into developing methods for its quantification. This review describes and compares both established and emerging analytical methods for levoglucosan quantification in ambient aerosol samples, with the special needs of the environmental analytical chemist in mind.

A class of organic molecules extracted from atmospheric aerosol particles and isolated from fog and cloud water has been termed HUmic-LIke Substances (HULIS) due to a certain resemblance to terrestrial and aquatic humic and fulvic acids. In light of the interest that this class of atmospheric compounds currently attracts, we comprehensively review HULIS properties, as well as laboratory and field investigations concerning their formation and characterization in atmospheric samples. While sharing some important features such as polyacidic nature, accumulating evidence suggests that atmospheric HULIS differ substantially from terrestrial and aquatic humic substances. Major differences between HULIS and humic substances, including smaller average molecular weight, lower aromatic moiety content, greater surface activity, better droplet activation ability, as well as others, are highlighted. Several alternatives are proposed that may explain such differences: (1) the possibility that mono- and di-carboxylic acids and mineral acids abundant in the atmosphere prevent the formation of large humic "supramolecular associations"; (2) that large humic macromolecules are destroyed in the atmosphere by UV radiation, O-3, and OH- radicals; 3) that "HULIS" actually consists of a complex, unresolved mixture of relatively small molecules rather than macromolecular entities; and (4) that HULIS formed via abiotic and short-lived oxidative reaction pathways differ substantially from humic substances formed over long time periods via biologically-mediated reactions. It should also be recalled that the vast majority of studies of HULIS relate to the water soluble fraction, which would include only the fulvic acid fraction of humic substances, and exclude the humic acid (base-soluble) and humin (insoluble) fractions of humic substances. A significant effort towards adopting standard extraction and characterization methods is required to develop a better and meaningful comparison between

Clouds developing in a polluted environment tend to have more numerous but smaller droplets. This property may lead to suppression of precipitation and longer cloud lifetime. Absorption of incoming solar radiation by aerosols, however, can reduce the cloud cover. The net aerosol effect on clouds is currently the largest uncertainty in evaluating climate forcing. Using large statistics of 1-km resolution MODIS (Moderate Resolution Imaging Spectro-radiometer) satellite data, we study the aerosol effect on shallow water clouds, separately in four regions of the Atlantic Ocean, for June through August 2002: marine aerosol (30 degrees S-20 degrees S), smoke (20 degrees S-5 degrees N), mineral dust (5 degrees N-25 degrees N), and pollution aerosols (30 degrees N160 degrees N). All four aerosol types affect the cloud droplet size. We also find that the coverage of shallow clouds increases in all of the cases by 0.2-0.4 from clean to polluted, smoky, or dusty conditions. Covariability analysis with meteorological parameters associates most of this change to aerosol, for each of the four regions and 3 months studied. In our opinion, there is low probability that the net aerosol effect can be explained by coincidental, unresolved, changes in meteorological conditions that also accumulate aerosol, or errors in the data, although further in situ measurements and model developments are needed to fully understand the processes. The radiative effect at the top of the atmosphere incurred by the aerosol effect on the shallow clouds and solar radiation is -11 +/- 3 W/m(2) for the 3 months studied; 2/3 of it is due to the aerosol-induced cloud changes, and 1/3 is due to aerosol direct radiative effect.

Prior to the massive use of new oxygenated solvents, data on their multiphase reactivity must be obtained to assess their environmental fate and impact on water and air quality. For this, the kinetics and mechanisms of the photochemical and photocatalytic degradation of selected oxygenated solvents by common tropospheric oxidants (such as OH and ozone) must be characterized. We studied the oxidation kinetics of new oxygenated solvents as pure organic liquids and in an aqueous medium by ozone and by the OH radical, respectively. The studied chemicals are all unsaturated compounds, having none, one, or two ether groups. The results indicate that the OH reaction proceeds at the diffusion limit by addition to the double bond. The reactive uptake coefficients associated with the reaction initiated by ozone are of the order of 10(-3). The reactions of compounds with two double bonds are very fast and probably occur at the surface. This kinetic information demonstrates that organic solvents in an organic medium or in an aqueous droplet will be oxidized rapidly by these oxidation reactions. These reactions, however, are not significant sinks for ozone and OH radicals.

Real-time measurements of ammonia, nitric acid, hydrochloric acid, sulfur dioxide and the water-soluble inorganic aerosol species, ammonium, nitrate, chloride, and sulfate were performed at a pasture site in the Amazon Basin (Rondonia, Brazil). The measurements were made during the late dry season (biomass burning), the transition period, and the onset of the wet season (clean conditions) using a wet-annular denuder (WAD) in combination with a Steam-Jet Aerosol Collector (SJAC). Measurements were conducted from 12 September to 14 November 2002 within the framework of LBA-SMOCC (Large-Scale Biosphere Atmosphere Experiment in Amazonia-Smoke Aerosols, Clouds, Rainfall, and Climate: Aerosols From Biomass Burning Perturb Global and Regional Climate). Real-time data were combined with measurements of sodium, potassium, calcium, magnesium, and low-molecular weight (LMW) polar organic acids determined on 12-, 24-, and 48-hours integrated filter samples. The contribution of inorganic species to the fine particulate mass (D-p 90%) fine-mode NH4NO3 and NH4Cl are predicted to be formed in the aqueous aerosol phase. Probably, Cl- was driven out of the aerosol phase largely by reaction of pyrogenic KCl with HNO3 and H2SO4. As shown by an updated version of the equilibrium simplified aerosol model (EQSAM(2)), which incorporates mineral aerosol species and lumped LMW polar organic acids, daytime aerosol NH4+ was mainly balanced by organ

This paper introduces the capability to study simultaneously changes in the density, the chemical composition, the mobility diameter, the aerodynamic diameter, and the layer thickness of multi-layered aerosol particles as they are being altered by heterogeneous chemical reactions. A vaporization-condensation method is used to generate aerosol particles composed of oleic acid outer layers of 2 to 30 nm on 101-nm polystyrene latex cores. The layer density is modified by reaction of oleic acid with ozone for variable exposure times. For increasing ozone exposure, the mobility diameter decreases while the vacuum aerodynamic diameter increases, which, for spherical particles, implies that particle density increases. The aerosol particles are confirmed as spherical based upon the small divergence of the particle beam in the aerosol mass spectrometer. The particle and layer densities are calculated by two independent methods, namely one based on the measured aerodynamic and mobility diameters and the other based on the measured mobility diameter and particle mass. The uncertainty estimates for density calculated by the second method are two to three times greater than those of the first method. Both methods indicate that the layer density increases from 0.89 to 1.12 g(.)cm(-3) with increasing ozone exposure. Aerosol mass spectrometry shows that, concomitant with the increase in the layer density, the oxygen content of the reacted layer increases. Even after all of the oleic acid has reacted, the layer density and the oxygen content continue to increase slowly with prolonged ozone exposure, a finding which indicates continued chemical reactions of the organic products either with ozone or with themselves. The results of this paper provide new insights into the complex changes occurring for atmospheric particles during the aging processes caused by gas-phase oxidants.

Changes in aerosol composition associated with a cold front passage were examined during a field experiment in Tel Aviv, Israel (2-15 Dec, 2000). In addition to monitoring aerosol scattering and optical thickness, aerosol samples were collected for detailed chemical analyses. Data were compared to simultaneous measurements made at Sde Boker, a semi-remote site in the Negev Desert, to help determine what changes were due to local pollution as opposed to regional phenomena. During the pre-frontal period (2-7 Dec) both sites were influenced by air masses containing a relatively high content of sulphate and dust, originating from neighbouring regions of the Middle East. A steady build-up of local pollution was then observed in Tel Aviv due to vehicular emissions/industrial activities, as indicated by increasing concentrations of black carbon, organic carbon, V, Cu, Ni, Zn, Br, Pb, NO3- and PAHs. Identification of a number of organic biomass burning tracers (e.g., levoglucosan) indicates that smoke also contributed to the pollution build-up in Tel Aviv, while a range of sugars/sugar alcohols point to a microbial/bioaerosol component. Locally emitted pollutants tended to exhibit higher nighttime concentrations due to trapping of pollution under a nocturnal inversion. Fine aerosol iodine was the only element exhibiting higher daytime concentrations, hinting at a photochemical source. Post-frontal measurements (12-15 Dec) revealed a significant decrease in all pollutants due to dispersal of the haze by the cold front 66 (8-9 Dec), with the air initially being dominated by marine aerosol. Concentrations of pollutants then began to increase, with backward trajectories indicating a possible contribution from Eastern Europe. Overall, the study identified a range of useful tracers for monitoring the contribution of different sources to the aerosol over Israel. Crown Copyright (C) 2003 Published by Elsevier Ltd. All rights reserved.

[1] We investigated nucleation scavenging of aerosols by cloud drops during the passage of a shallow cold front at a mountain station in northern Israel. The chemical composition and size of the aerosols were measured during and following the passage of the front. Analysis of the air mass trajectories show that, prior to the frontal passage, the air originated from the north, bringing with it pollution particles from sources in Eastern Europe. Following the frontal passage, the air originated from the east, bringing with it some mineral dust particles. The results show that sulfate, nitrate, and ammonium were the dominant compounds in the particles. Of the total sulfate-containing particles, 65% nucleated cloud drops. We found nucleation scavenging of aerosols to be correlated with the size of the aerosols. Aerosols smaller than 0.14 mum were not significantly affected by nucleation scavenging, while the number concentration of particles larger than 0.14 mm decreased in correspondence to the increase in droplet concentrations. During the time that the cloud covered the measuring site, 80% of the particles in the size range 0.3-1 mum were scavenged. The concentrations of the particles with diameter smaller than 1 mm returned to their original values after the cloud dissipated.

Rate coefficients for the reaction of OH radical with eleven C-3-C-6 hydroxyalkyl nitrates and with two C-4 hydroxy nitrates containing a double bond were determined at atmospheric pressure and 296 +/- 2 K. The rate coefficients were measured in a photochemical reactor by the relative rate technique, employing solid-phase microextraction (SPME) coupled to gas chromatography (GC) for detection of the organic reactants. Hydroxyalkyl nitrates react faster than alkyl nitrates with the OH radical. The rate coefficients increase with increasing chain length and separation between the hydroxy and the nitrooxy groups. By including different loss processes such as photolysis, gas-phase reactions, and solubility, the tropospheric lifetime of C-3-C-6 hydroxyalkyl nitrates is estimated to range between 0.5 and 4.5 days. Due to their higher reactivity and solubility, hydroxyalkyl nitrates have a shorter atmospheric lifetime than alkyl nitrates.

Applications of new retrieval methods to old satellite data allowed us to study the effects of smoke from the Kuwait oil fires in 1991 on clouds and precipitation. The properties of smoke-affected and smoke-free clouds were compared on the background of the dust-laden desert atmosphere. Several effects were observed: (1) clouds typically developed at the top of the smoke plume, probably because of solar heating and induced convection by the strongly absorbing aerosols; (2) large salt particles from the burning mix of oil and brines formed giant cloud condensation nuclei (CCN) close to the source, which initiated coalescence in the highly polluted clouds; (3) farther away from the smoke source, the giant CCN were deposited, and the extremely high concentrations of medium and small CCN dominated cloud development by strongly suppressing drop coalescence and growth with altitude; and (4) the smaller cloud droplets in the smoke-affected clouds froze at colder temperatures and suppressed both the water and ice precipitation forming processes. These observations imply that over land the smoke particles are not washed out efficiently and can be transported to long distances, extending the observed effects to large areas. The absorption of solar radiation by the smoke induces convection above the smoke plumes and consequently leads to formation of clouds with roots at the top of the smoke layer. This process dominates over the semidirect effect of cloud evaporation due to the smoke-induced enhanced solar heating, at least in the case of the Kuwait fires.

[1] High concentrations of small atmospheric aerosols are known to reduce the size of cloud droplets, increase cloud albedo and suppress precipitation formation. In contrast, cloud simulations suggest that even low concentrations of large soluble aerosols should promote droplets' growth and rainfall. Until now, though, no observational evidence of such microphysical effects in natural circumstance over land has been presented. By using NOAA-AVHRR retrievals on cases where salt-dust from the Aral Sea interacts with clouds we show that large salt-containing dust particles increase cloud drops to sizes that promote precipitation. These findings are in line with the findings of the microphysical models and recent results from hygroscopic cloud seeding experiments for rain enhancement.