Investigating the Partitioning of Nitric and Organic Acids in Los Angeles During 2010 CalNex Campaign
JIUMENG LIU, Xiaolu Zhang, Eric Parker, Rodney Weber, et al.
Georgia Institute of Technology
Abstract Number: 368
Working Group: Recent Campaigns in the North American West Coast
Last modified: April 4, 2011
A major step leading to the formation of secondary organic aerosols (SOA) is the partitioning between gas and particle phases of semi-volatile organic compounds (SVOCs). A range of gas and particle phase organic and inorganic compounds were measured in Los Angeles during the CalNex field campaign, May - June 2010, using high-time resolution instrumentation. Using these data we investigate the partitioning behavior of formic acid by comparing it to: the partitioning of nitric acid, the overall partitioning of water-soluble components (WSOC) and trends in oxalic acid. As expected, nitric acid partitioning was correlated with Relative Humidity (RH), indicating the major role of dissolution into aerosol water to form nitrate aerosol. Conversely, formic acid partitioning appeared to be related to liquid water only under conditions of high RH and low photochemical activity. During these periods the extent of formic acid partitioning to water was greatly under-predicted based on bulk aerosol pH determined from observed nitric acid partitioning. Highest concentrations of particle phase formic acid occurred during mid-afternoon and tracked peaks in photochemically-formed gas phase formic acid, although the particle/gas partitioning ratios were significantly lower than periods of high RH. In contrast to formate, other secondary organic aerosol components (e.g., oxalate and WSOC) had peak particle phase concentrations that occurred slightly later in the day relative to the gas phase components, which may in part be attributed to differences in volatility. As a contrast to Los Angeles, similar measurements were made in Atlanta. There, formic acid partitioning was consistently associated with elevated RH (aerosol liquid water content), similar to what has been observed for WSOC (Hennigan et al., 2008), and the diurnal trends of particle phase nitrate, formate, oxalate, and WSOC were completely different than those of Los Angeles.