Cooperative Institute for Research in Environmental Sciences

A winning proposal for the Innovative Research Program, 2005:

Collaborative Studies of Atmospheric Aerosols

Investigators: M. A. Tolbert, R. Garland, A.R. Ravishankara, E. Lovejoy and T. Baynard

Objective: The objective of this study is to determine the water uptake and optical properties of complex aerosol compositions.

Background and importance

The interaction of solar and terrestrial radiation with atmospheric particles strongly impacts the global and local environment. For example it is well known that tropospheric aerosols play an important role in the global climate system. Tropospheric aerosols directly affect climate by scattering and absorbing incoming solar radiation (aerosol direct effect), leading to a cooling at the surface. Indirectly, aerosols impact climate by serving as cloud condensation nuclei (aerosol indirect effect). While the indirect effect of aerosols on clouds is thought to be a negative forcing (surface cooling), there are considerable uncertainties in the magnitude of the effect. According to the IPCC, aerosol effects represent some of the most uncertain aspects of the climate system (IPCC, 2001). In addition to their importance globally, aerosol optical problems impact us on a local scale. For example, tropospheric aerosols are a major cause of visibility reduction. The Boulder/Denver area experiences poor visibility year round, with violations of the state visibility standard one in three days each winter. By far the largest contribution to this visibility reduction is particles. A key factor in both the climate and visibility issues is how the ambient particles take up water and grow to sizes where they can most optimally interact with the radiation field.

While past research has determined how simple model aerosol systems respond to light, emerging research indicates that the true atmospheric system is considerably more complex. A key finding in the last decade is that tropospheric sulfate aerosols, once thought to be pure, are essentially all internal mixtures of many components, with organics comprising 50% or more of the particle mass (Murphy et al., 98). Organics have strong biogenic and anthropogenic sources and appear to be ubiquitous in the tropospheric aerosol. The effect of the organics on the ability of the sulfate particles to take up water and grow into clouds is not well established and represents a significant weakness in our understanding of the aerosol - cloud - climate loop. The uptake of water by the complex mixtures is also important for determining the direct effect of aerosols on climate. Finally, water uptake by the complex aerosols allows them to grow to sizes where they effectively scatter light and reduce visibility. The proposed studies will use a novel technique to probe the water uptake and optical properties of complex aerosols containing both organic and inorganic components.

What makes this innovative? While research in the Tolbert and Ravishankara groups has been focused on similar issues over the last 20 years, so far there has yet to be an actual collaborative effort. The proposed collaboration will bring together campus and NOAA expertise on atmospheric aerosols and enable a larger objective to be met than would be possible separately. We are proposing an innovative laboratory study to measure water uptake and optical properties by complex aerosols using cavity ring down spectroscopy. This is a very new technique for the study of atmospheric aerosols in the field, and has great potential for advancing fundamental knowledge of atmospheric aerosol properties through laboratory work.

Research plan

We propose to generate internally mixed organic/inorganic aerosols using established techniques. After formation, the aerosols will then be passed through a differential mobility analyzer (DMA) to size select out one particular size from 0.1 to 1 micrometer in diameter for detailed analysis of the relative humidity dependence. Very few laboratory studies have been performed on size selected aerosol samples, but is critical to relate laboratory results to the atmosphere.

After a monodisperse sample of the aerosol of interest is generated, the particles will be monitored using cavity ring down (CRD) spectroscopy. CRD spectroscopy has been used extensively for the analysis of atmospheric gases. Very recently, the technique has been applied to measurements of atmospheric aerosols (Thompson et al., 2003, Pettersson et al., 2004). In brief, the CRD system consists of a 90 cm cavity enclosed by two highly reflective mirrors. Laser light at 532 nm is coupled into the cavity and the ring down signal is measured through the output mirror using a photomultiplier tube. The background ring down decay constant represents light losses due to the mirrors and Rayleigh scattering by gases in the cell. Ring down times are shorter when aerosols are in the cell because of additional losses due to particles scattering and absorbing the light. The difference in the decay constant is then used to determine the total extinction due to particles. The extinction is a function of the particle number density, size distribution, and refractive index. The proposed experiments will directly measure the relative humidity dependence of aerosol optical properties, which significantly influences aerosol climate forcing and visibility.

Expected outcome and impact: We expect to determine the optical response of internally mixed organic/inorganic aerosols of various compositions to growth and evaporation in response to changes in relative humidity. Because of the extreme sensitivity and flexibility of the novel technique to be used, we will be able to study a wide range of organic species encompassing both biogenic and anthropogenic sources. By understanding how different aerosol compositions take up water and respond to light, we will provide essential data for the evaluation of the climatic and visibility impacts of natural and anthropogenic organic emissions.

References

Intergovernmental Panel on Climate Change (IPCC), Climate Change 2001: The Scientific Basis, edited by J. H. Houghton et al., Cambridge Univ. Press., New York, 2001.

Murphy, D. M., D. S. Thomson, A. M. Middlebrook and M. E. Schein. "In situ single-particle characterization at Cape Grim." Journal of Geophysical Research, 103, 1664-1669, 98.

Pettersson, A., E. R. Lovejoy, C. A. Brock, S. S. Brown and A. R. Ravishankara. "Measurement of aerosol optical extinction at 532nm with pulsed cavity ring down spectroscopy." Journal of Aerosol Science, 35, 8: 995-1011, 2004.

Thompson, J. E., H. D. Nasajpour, B. W. Smith, and J. D. Winefordner, "Atmospheric aerosol measurements by cavity ringdown turbidimetry," Aerosol Science and Technology, 37, 221-230, 2003.