ORGANICS ALTER HYGROSCOPIC BEHAVIOR OF ATMOSPHERIC PARTICLES

The optical and chemical properties of atmospheric particles and their ability to act as cloud condensation nuclei (CCN) depend strongly upo n their affinity for water. Laboratory experiments have shown that wat er soluble substances such as ammonium sulfate, ammonium nitrate, and sodium chloride, which are major inorganic components of atmospheric p articles, absorb water in an amount proportional to water vapor pressu re. Analogous information about the interactions between water and org anics, which are another major component of atmospheric particles, is lacking. Here we analyze concurrent observations of particle chemical composition and water content from a continental nonurban (Grand Canyo n) and an urban (Los Angeles) location to determine whether the water content of atmospheric particles is influenced by the presence of orga nics. By comparing the observed water content with the water content e xpected to be associated with the inorganic fraction, we find that the aggregate hygroscopic properties of inorganic particles are altered s ubstantially when organics are also present. Furthermore, the alterati ons can be positive or negative. For the nonurban location, organics e nhance water absorption by inorganics. In the relative humidity (RH) r ange of 80-88%, organics account for 25-40% of the total water uptake, on average. For the urban location, on the other hand, the net effect of organics is to diminish water absorption of the inorganics by 25-3 5% in the RH range of 83-93%.