Estimation of water uptake by organic compounds in submicron aerosols measured during the Southeastern Aerosol and Visibility Study

In situ measurements of size-dependent water uptake by atmospheric particle s made with a tandem differential mobility analyzer (TDMA) and size-resolve d chemical composition of aerosol samples collected with cascade impactors in the Smoky Mountains have been examined in order to ascertain the influen ce of organic carbon compounds on aerosol hygroscopicity. Particles were dr ied to similar to 5% relative humidity (RH) before entering the TDMA, leadi ng us to believe that salts of ammonium and sulfate were in crystalline sta tes for relative humidities below their expected deliquescent points. TDMA- measured water content was found to be in excess of the sulfate-associated water modeled using laboratory data for binary aqueous solutions and the me thod of Zdanovskii-Stokes-Robinson for multicomponent solutions over a wide range of humidities (RH=5-85%), Furthermore, excess water was observed to increase in proportion to the organic fraction of mass associated with each examined size in the range 0.05 to 0.4 mu m These data are used to obtain an empirical relationship between the amount of water associated with parti culate organics and relative humidity. This analysis shows that organic-ass ociated water content is considerably less than that of sulfate compounds, on a volume basis, for high RH, but comparable or greater for low RH. These results are consistent with laboratory data for water absorption by a rang e of organics vis-a-vis ammonium sails of sulfate.