Gas/particle partitioning of water-soluble organic aerosol in Atlanta
1School of Civil and Environmental Engineering, Georgia Institute of Technology, USA
2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, USA
3School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, USA
Abstract. Gas and particle-phase organic carbon compounds soluble in water (e.g., WSOC) were measured simultaneously in Atlanta throughout the summer of 2007 to investigate gas/particle partitioning of ambient secondary organic aerosol (SOA). Previous studies have established that, in the absence of biomass burning, particulate WSOC (WSOCp) is mainly from secondary organic aerosol (SOA) production. Comparisons between WSOCp, organic carbon (OC) and elemental carbon (EC) indicate that WSOCp was a nearly comprehensive measure of SOA in the Atlanta summertime. To study SOA formation mechanisms, WSOC gas-particle partitioning was investigated as a function of temperature, RH, NOx, O3, and organic aerosol mass concentration. Identifying a clear temperature effect on partitioning was confounded by other temperature-dependent processes, which likely included the emissions of biogenic SOA precursors and photochemical SOA formation. Relative humidity data indicated a linear dependence between partitioning and fine particle liquid water. Lower NOx concentrations were associated with greater partitioning to particles, but WSOC partitioning had no visible relation to O3 or fine particle OC mass concentration. There was, however, a relationship between WSOC partitioning and the WSOCp concentration, suggesting a compositional dependence between partitioning semi-volatile gases and the phase state of the aerosol. Combined, the overall results suggest that partitioning to liquid water, followed by heterogeneous reactions may represent the main process by which SOA is formed in urban Atlanta during summer.