AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
The Influence of Aerosol Water in the Organic Phase on the Mass, Properties and Source Apportionment of Organic Aerosol in a Source-oriented Model
SHANTANU JATHAR, Abdullah Mahmud, James F. Pankow, Michael Kleeman, UC Davis
Abstract Number: 520 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Ambient organic aerosol (OA) is hygroscopic meaning that water partitions into the organic phase to a degree determined by the organic chemical composition and the ambient relative humidity. In addition to contributing to the OA mass, the absorbed water also alters the mean molecular weight of OA and the activity coefficients of OA’s constituent compounds, which in turn influences OA’s gas-particle partitioning. Classical OA treatment in aerosol transport models does not account for water uptake and its subsequent impacts.
In this work, we use the UCD/CIT source-oriented air quality model to simulate water uptake by OA on different internal mixtures or source-oriented particle types and assess its influence on the mass, properties and source-apportionment of OA. The model is run for a summer (July 2002) and winter (January 2003) month over two distinct regions in California: the South Coast Air Basin (dominated by motor vehicle emissions) and the San Joaquin Valley ( dominated by agricultural and wood burning emissions). The internally mixed OA calculations predict very little water in the organic phase (< 0.3 µg m$^(-3)) in the summer but modest amounts of water in the organic phase during the winter (> 1 µg m$^(-3)). The water in the organic phase is sufficient to substantially change the mean molecular weight of the OA-water mixture, which ends up increasing the partitioning coefficient of the organic constituents or pushes gas-particle equilibrium towards the particle phase. Hence, in winter, the absorbed water contributes significantly to OA enhancement which improves model performance. Tests of water uptake into the organic phase on source-oriented aerosols are ongoing. We anticipate that the source-oriented calculations will reveal that particles with a more hygroscopic OA (emitted from wood smoke or biomass burning) absorb more water in their organic phase than particles with less hygroscopic OA (emitted from gasoline or diesel combustion). This would have a strong influence not only on the water mass absorbed by OA but also on the properties and source apportionment of OA.