Water absorption by secondary organic aerosol and its effect an inorganic aerosol behavior

The hygroscopic nature of atmospheric aerosol has generally been associated with its inorganic fraction. In this study, a group contribution method is used to predict the water absorption of secondary organic aerosol (SOA). C ompared against growth measurements of mixed inorganic-organic particles, t his method appears to provide a first-order approximation in predicting SOA water absorption. The growth of common SOA species is predicted to be sign ificantly less than common atmospheric inorganic salts such as (NH4)(2)SO4 and NaCl. Using this group contribution method as a tool in predicting SOA water absorption, an integrated modeling approach is developed combining av ailable SOA and inorganic aerosol models to predict overall aerosol behavio r. The effect of SOA on water absorption and nitrate partitioning between t he gas and aerosol phases is determined. On average, it appears that SOA ac counts for approximately 7% of total aerosol water and increases aerosol ni trate concentrations by approximately 10%. At high relative humidity (great er than or equal to 85%) and low SOA mass fractions (<20% of total PM2.5), the role of SOA in nitrate partitioning and its contribution to total aeros ol water is negligible. However, the water absorption of SOA appears to be less sensitive to changes in relative humidity than that of inorganic speci es, and thus at low relative humidity (similar to 50%) and high SOA mass fr action concentrations (similar to 30% of total PM2.5), SOA is predicted to account for approximately 20% of total aerosol water and a 50% increase in aerosol nitrate concentrations. These findings could improve the results of modeling studies where aerosol nitrate has often been underpredicted.