Partitioning of the organic aerosol component between fog droplets and interstitial air

Limited information is available on the nature of organic compounds in the tropospheric aerosol and their effect on aerosol hygroscopic properties and cloud condensation nuclei (CCN) ability. Here we analyze samples of liquid droplets and interstitial aerosol, concurrently collected during fog episo des, to determine how the organic compounds are partitioned between the two reservoirs. By comparing the nature and concentration of different organic carbon classes found in the two reservoirs, we find that fog acts as an ef ficient separator for carbon (C) species on the basis of their chemical pro perties, with polar water soluble species representing the greater part of total C within fog droplets, and water insoluble C species preferentially f ound in the interstitial reservoir. Water-soluble organic species are scave nged by fog droplets to a comparable extent to major inorganic ions and are therefore expected to play an important role in the droplet nucleation pro cess. The main classes of water soluble organic carbon (WSOC) identified in fog water and interstitial aerosol by the techniques traditionally used in aerosol analysis are aliphatic dicarboxylic acids, sugars, aliphatic alcoh ols, and aliphatic carboxylic acids. However, such species, similar to 120 individual compounds, only account for a few percent (<5% on average) of to tal WSOC. A pew class of water soluble macromolecular compounds (MMC), dete cted in aerosol samples from different areas of the globe, are found to con stitute a large fraction (similar to 40% on average) of WSOC in the fog sys tem (fog droplets plus interstitial aerosol) and represent the main class o f water soluble species identified. More than 50% of WSOC still remains und etermined.