Clouds and fogs play an important role in processing chemical species in the atmosphere. The scavenging of fine particles followed by direct deposition or incorporation into precipitation represent important mechanisms for cleansing the atmosphere. The uptake of soluble gases followed by reaction in cloud/fog droplets is increasingly recognized as an important source of secondary organic aerosol (SOA). Here we provide an overview of our research examining the processing of atmospheric organic matter by clouds and fogs. This includes field measurements of fogs and clouds in California and China and their impacts on carbonaceous fine particles as well as laboratory studies of the photochemical oxidation of organic matter in these collected cloud/fog waters. Field observations reveal efficient, but differential scavenging and deposition of carbonaceous fine particles from different source types. Laboraty photo-oxidation studies were conducted on authentic cloud water using ultraviolet light and H2O2 to produce hydroxyl radical. Organic matter composition was followed during aging using on-line ESI-ToF-MS, offline ion chromatography, and direct atomization of aged cloud water followed by evaporation of resulting droplets and on-line drop residue measurements using a high resolution Aerosol Mass Spectrometer (AMS). These experiments typically reveal increasing oxygenation of the cloud organic matter throughout the aging process. Production of carbonyls and carboxylic acids is observed, although production rates are slowed, especially by competition for oxidants in the complex, real cloud water matrix. The AMS measurements often reveal a change from net production of SOA to net loss of organic aerosol with increasing levels of oxidation. This transition from OA production to loss typically occured when the fraction of the AMS fragment at m/z 44 (CO2+) reached approximately 20 to 25% of the organic matter.

Cloud and fog processing of atmospheric organic matter

249th National Meeting (2015)

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