Analysis of gas-aerosol partitioning in the Arctic: Comparison of size-resolved equilibrium model results with field data

Size-resolved aerosol mass and gas concentrations were measured during the European Arctic Aerosol Study (EAAS). A thermodynamic equilibrium model was applied to the data in order to (1) test whether gas-aerosol equilibrium a ppeared to be obtained by NH3, HNO3, HCl, HCOOH, and CH3COOH, and (2) test the sensitivity of gas-aerosol equilibrium to several nonvolatile organic a cids that were measured. Model results indicated that all submicrometer, ac cumulation-mode aerosols appeared to be near equilibrium with NH3. Supermic rometer, coarse-mode aerosols of recent marine origin appeared to be out of equilibrium with HNO3 and closer to equilibrium with HCl, while continenta lly influenced aerosols sometimes appeared to be near equilibrium with both HNO3 and HCl. However, the observed gas-aerosol partitioning of HCOOH and CH3COOH could not be explained by effective Henry's law partitioning, consi stent with other studies. Nonvolatile organic acids measured were methanesu lfonate, oxalate, succinate, and glutarate. Sensitivity tests indicated tha t methanesulfonate retained similar to 30% of NH4+ under marine conditions but had <3% impact on other species and under other conditions. Whereas oxa lic acid was predicted to be similar to 15-30% dissociated in the aerosol s olution, succinic and glutaric acids were predicted to be <10% dissociated, limiting their ability to influence gas aerosol partitioning. Together, th e three dicarboxylic acids were responsible for retaining 0-2% of predicted NH4+ and displacing 0-6% of predicted Cl- and NO3-. Model results were sen sitive to the assumed mixing state of the aerosols, as well as the degree o f aerosol size resolution represented by the model.