Research
- Organic aerosols and their atmospheric chemistry
- Heterogeneous processes of tropospheric aerosols
- Optical properties of atmospheric aerosols
- Atmospheric chemical kinetics and photochemistry
- Interactions of aerosols with the climate system
- Environmental analytical chemistry
- SMOCC
Atmospheric chemistry
The Earth's atmosphere is a large photochemical reactor. Atmospheric chemistry involves a complex set of chemical reactions and chemical cycles that can often lead to exciting and unexpected phenomena; witness the unique combination of dynamical forces that lead to a wintertime polar vortex over Antarctica, with the concomitant formation of polar stratospheric clouds that serve as sites for heterogeneous chemical reactions involving chlorine compounds resulting from anthropogenic chlorofluorocarbons - all leading to the near total depletion of stratospheric ozone over the South Pole each spring; or witness the chemical cycles that release chlorine and bromine atoms from sea salt particles; or look at the exciting connection between the chemistry of suspended particles, cloud properties and the climate system. The chemical constituents of the atmosphere do not go through their life cycles independently; the cycles of the various species are linked together in a complex way. Thus, a perturbation of one component can lead to significant, and nonlinear, changes to other components and to feedbacks that can amplify or damp the original perturbation.
Aerosols are particles suspended in the atmosphere. They arise directly from emissions of particles and from the conversion of certain gases to particles in the atmosphere. The recent IPCC assessment identified the effects of aerosols on climate, through modification of cloud properties, as the largest uncertainty in our understanding of the Earth's climate system. For many years it was thought that atmospheric aerosols did not interact in any appreciable way with the cycles of trace gases. We now know that particles in the air affect climate and interact chemically in heretofore unrecognized ways with atmospheric gases. Aerosols reflect solar radiation back to space, and, in so doing, cool the Earth. Aerosols are also the nuclei around which clouds droplets form . Clouds are one of the most important elements of our climate system, so the effect of increasing global aerosol levels on the Earth's cloudiness is a key problem in climate studies. At elevated levels aerosols limit visibility and are a human health hazard. There is a growing body of epidemiological data that suggests that increasing levels of aerosols may cause a significant increase in human mortality.
In a recent article, Andreae and Crutzen have summarized the state of our knowledge: "We have a tantalizing glimpse into the complexity and potential importance of these processes but not even enough information to include them in a meaningful way into models of atmospheric chemistry.
Motivated by this gap, our research focuses on the chemistry of aerosols and on understanding their climatic and environmental roles. Our studies use a wide verity of laboratory based experiments in which we study chemical kinetics of gas-surface/liquid interactions, identify reaction mechanisms and measure chemical and physical parameters for use in atmospheric models. We also have a field measurements program that focuses on characterization of the chemical composition of atmospheric particles. Finally, our group has a strong analytical program for development of tools for environmental measurements and monitoring.
Optical properties of aerosols
Atmospheric aerosols affect Earth's climate both directly and indirectly. The direct effect of aerosols on climate is by absorbing and/or scattering the incoming solar radiation and outgoing terrestrial radiation. This interaction strongly modifies Earth's radiation budget and hence the climate on regional and global scales. Much attention has been devoted to purely scattering aerosols, such as sulphate aerosols, mostly due to their "cooling effect". More recently, considerable attention has been directed to absorbing aerosols such as soot, dust, organics and mixed aerosols that contain absorbing species and inclusions. Absorbing aerosols can heat the atmosphere and affect atmospheric circulation and cloud formation (i.e., the semi-direct effect). There is a growing need to understand and measure atmospheric aerosol optical properties in order to better constrain their direct and semi-direct climatic effects.
Cavity ring down spectroscopy was developed by O'Keefe and Deacan (1988). Typically, it consists of two highly reflective plano-concave mirrors set opposite to one another. The placement of the mirrors is dependent on the cavity stability conditions. A pulsed or continuous laser beam is coupled into the cavity from one side and performs multiple reflections inside the cavity. A photomultiplier (PMT) is placed at the other side of the cavity and measures the exponential decay of the emerging light intensity. The intensity (I) decay is a result of losses inside the cavity and due to the mirrors:
[1]
The time constant for an empty cavity, τ0, is:
[2]
where L is the length of the cavity (distance between the two mirrors), C is the speed of light, and R is the reflectivity of the mirrors. When the cavity is filled with an absorbing or scattering medium, the molecules or particles further reduce the intensity on each pass. This process results in a ring down trace with a shorter time constant due to additional terms in the ring down expression, and the time constant is described by
[3]
where αext is the extinction coefficient of the molecules or particles inside the cavity, and d is the actual distance in the cavity filled with absorbing molecule. The extinction coefficient can be extracted from the difference between the time constant of the empty and the filled cavity:
[4]
The extinction coefficient (αext) of homogeneous spheres (aerosols) is described by:
[5]
where Qext is the extinction efficiency of the particles, N is the particle number density, and D is the particle diameter. By selecting a monodisperse aerosol population and measuring the particle number density (N), the extinction efficiency (Qext) can be determined. For a fixed wavelength, Qext can be measured as a function of the size parameter by performing measurements on a series of monodisperse particles of different sizes. The size parameter, x, is the ratio of the particle size (D) to the laser's wavelength (λ) and is given by (x = πD/λ). Having Qext as a function of size parameter enables a retrieval of the particle refractive index
We are using a newly built cavity ring down aerosol spectrometer (CRD-AS) for studying the optical properties of aerosols and how these properties change with chemical reactions. Using this new experimental set up we can measure the complex index of refraction of the aerosols and calculate various parameters needed in climate models.
SMOCC
Smoke Aerosols, Clouds, Rainfall and Climate: Aerosols from Biomass Burning Perturb Global and Regional Climate. More information
Student Information
Postdoctoral fellowships and Ph.D. student positions in atmospheric chemistry:
Applications for post doc positions are invited from candidates with a recent Ph.D. in physical chemistry, atmospheric chemistry or related fields. The position is for one to two years. Ph.D. student fellowships are also available for students with an undergraduate degree in physics or chemistry. Successful applicants will be involved in research projects that involve studying the chemical processes or organic aerosols .Other possible projects include laser applications for studying aerosol optical properties. For application forms, please see http://www.weizmann.ac.il/feinberg/postdoc_fell.shtml for a post doc fellowship; or http://www.weizmann.ac.il/feinberg/PhD/ for a student fellowship.
For additional details please contact:
Yinon Rudich
Department of Environmental Sciences
Weizmann Institute of Science
Rehovot, 76100, ISRAEL
yinon.rudich@weizmann.ac.il
Tel: 972-8-9344237; Fax: 972-8-9344124