HR: 0830h
AN: A51F-0756    [PDF]
TI: Field Observations of the Processing of Organic Aerosol Particles and Trace Gases by Fogs and Clouds
AU: * Collett, J L
EM: collett@lamar.colostate.edu
AF: Colorado State University, Atmospheric Science Department, Fort Collins, CO 80523 United States
AU: Herckes, P
EM: herckes@lamar.colostate.edu
AF: Colorado State University, Atmospheric Science Department, Fort Collins, CO 80523 United States
AB: In many environments, organic compounds account for a significant fraction of fine particle mass. Because the lifetimes of accumulation mode aerosol particles are governed largely by interactions with clouds, it is important to understand how organic aerosol particles are processed by clouds and fogs. Recently we have examined the organic composition of clouds and fogs in a variety of environments as well as how these fogs and clouds process organic aerosol particles and soluble organic trace gases. The investigations, conducted in Europe, North America, Central America, and the Pacific region, have included studies of polluted radiation fogs, orographic clouds in clean and polluted environments, and marine stratocumulus. Our results show that organic matter is a significant component of fog and cloud droplets. In polluted California radiation fogs, we observed concentrations of total organic carbon (TOC) ranging from 2 to 40 ppmC, with significantly lower concentrations measured in marine and continental clouds. An average of approximately 80 percent of organic matter was found in solution, while the remainder appears to be suspended material inside cloud and fog drops. Ultrafiltration measurements indicate that as much as half of the dissolved organic carbon is present in very large molecules with molecular weights in excess of 500 Daltons. Field measurements made using a two-stage cloud water collector reveal that organic matter tends to be enriched in smaller cloud or fog droplets. Consequently, removal of organic compounds by precipitating clouds or by direct cloud/fog drop deposition will be slowed due to the fact that small drops are incorporated less efficiently into precipitation and removed less efficiently by sedimentation or inertial impaction. Despite this trend, we have observed that sedimentation of droplets from long-lived radiation fogs provides a very effective mechanism for cleansing the atmosphere of carbonaceous aerosol particles, with organic carbon removed more efficiently than elemental carbon. Efforts to characterize organic matter in clouds and fogs reveal that the most abundant species are typically low molecular weight carboxylic acids and aldehydes. These species have been observed collectively to account for roughly 20-30 percent of the total organic carbon in some fogs and clouds. Dicarboxylic acids, frequently used as model compounds for organic CCN, typically account for only about 1 percent of the organic carbon, with oxalic acid the most important contributor. Measurements by GC/MS, HPLC, and H-NMR reveal that many other organic compounds are present, including aerosol source markers and compound families frequently detected in aerosol particles including n-alkanes, n-alkanoic acids, and polycyclic aromatic hydrocarbons. These latter compounds were detected in both dissolved and undissolved forms in droplets, with dissolved concentrations often higher than their solubilities in pure aqueous solutions, suggesting a possible role of surface organic films. Although more than 100 organic species have been quantified in many samples, the majority of the organic carbon mass remains unspeciated. Given the importance of high molecular weight material, future efforts will focus in part on further characterization of these compounds, including possible contributions from humic like substances.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0322 Constituent sources and sinks
DE: 0345 Pollution--urban and regional (0305)
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting