The Effect of Increased Organic Solubility on Predicted CCN Activity in the Anthropogenically-Influenced Marine Boundary Layer

Aerosol particles in the marine boundary layer (MBL) are transported from their source region and processed in the atmosphere, and are therefore often mixtures of inorganic (e.g. SO4 2−, NH4 , and NO3 −) and organic components (e.g. carboxylic acid, hydroxyl, and saturated aliphatic functional groups). Although the solubility of inorganic aerosol components is fairly well known, the large number of unknown organic compounds present in ambient aerosol, in combination with their wide variety of hygroscopic properties, makes CCN activity of mixtures difficult to constrain. Here we present predicted CCN concentrations using recent measurements of submicron aerosol organic functional groups from the stratocumulus topped MBL in the southeast Pacific Ocean. The particles included both hydrophobic (saturated aliphatic C-CH) and hydrophilic (carboxylic acid COOH and hydroxyl COH) groups, in addition to small amounts of amine C-NH2 and organosulfate COSO3 groups. These measurements have been coupled with simultaneous measurements of particle size, inorganic elements, and inorganic ions to predict CCN activity of the ambient aerosol using a multi-component Köhler model. The model calculations show that CCN concentrations at 0.3% supersaturation are sensitive to the solubility of the organic fraction for the particle sizes observed during VOCALS-REx and that the sensitivity is inversely correlated with particle size below 180 nm. Above 180 nm, predicted CCN are close to 100% for all compositions. For the smallest particles, a 1% increase in average solubility resulted in a 6% increase in the fraction of activated particles. Predicted CCN concentrations in the region of highest continental influence (less than 600 km from shore) are highest and most sensitive to composition because of smaller particle size. The coastal region also has smaller effective cloud drop radii based on satellite observations from the Geostationary Operational Environmental Satellite (GOES-10) as expected in an area with elevated CCN.