Fine particle pH and gas-particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign
Hongyu Guo1, Jiumeng Liu1,a, Karl Froyd2,3, James M. Robert2, Patrick R. Veres2,3, Patrick L. Hayes3,4,5, Jose L. Jimenez3,5, Athanasios Nenes1,6,7,8, and Rodney J. Weber11School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA , USA 2Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA 3Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, CO , USA 4Department of Chemistry, Université de Montéal, Montréal, Québec H3T 1J4, Canada 5Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO, USA 6School of Chemical and Biomolecular Engineering, Georgia Institute of Te chnology, Atlanta, GA , USA 7Foundation for Research and Technology, Hellas , Greece 8National Observatory of Athens, Greece aNow at Atmospheric Sciences and G lobal Change Division, Pacific Northwest National Laboratory, Richland, WA, USA
Received: 23 Dec 2016 – Accepted: 02 Jan 2017 – Published: 05 Jan 2017
Abstract. pH is a fundamental aerosol property that affects ambient particle concentration and composition, linking pH to all aerosol environmental impacts. Here, PM1 and PM2.5 pH are calculated based on data from measurements during the California Research at the Nexus of Air Quality and Climate Change (CalNex) study from 15 May to 15 June 2010 in Pasadena CA. Particle pH and water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to measured gas-particle partitioning of inorganic nitrate, ammonium and chloride. The study mean ± standard deviation PM1 pH was 1.9 ± 0.5 for the SO42−-NO3−-NH4+-HNO3-NH3 system. For PM2.5, internal mixing of sea salt components (SO42−-NO3−-NH4+-Na+-Cl−-K+-HNO3-NH3-HCl system) raised the bulk pH to 2.7 ± 0.3 and improved predicted nitric acid partitioning with PM2.5 components. The results show little effect of sea salt on PM1 pH, but significant effects on PM2.5 pH. A mean PM1 pH of 1.9 at Pasadena was approximately one unit higher than what we have reported in the southeastern US, despite similar temperature, relative humidity and sulfate ranges and is due to higher total nitrate concentrations (nitric acid plus nitrate) relative to sulfate, a situation where particle water is affected by semi-volatile nitrate concentrations. Under these conditions nitric acid partitioning can further promote nitrate formation by increasing aerosol water, which raises pH by dilution, further increasing nitric acid partitioning and resulting in a significant increase in fine particle nitrate and pH. This study provides insights on the complex interactions between particle pH and nitrate in a summertime coastal environment and a contrast to recently reported pH in the eastern US in summer and winter and the eastern Mediterranean. All studies have consistently found highly acidic PM1 with pH generally below 3.
Guo, H., Liu, J., Froyd, K., Robert, J. M., Veres, P. R., Hayes, P. L., Jimenez, J. L., Nenes, A., and Weber, R. J.: Fine particle pH and gas-particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1158, in review, 2017.