Articles | Volume 17, issue 9
Atmos. Chem. Phys., 17, 5703–5719, 2017
Atmos. Chem. Phys., 17, 5703–5719, 2017

Research article 08 May 2017

Research article | 08 May 2017

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 D. Froyd2,3, James M. Roberts2, Patrick R. Veres2,3, Patrick L. Hayes3,4,5, Jose L. Jimenez3,5, Athanasios Nenes1,6,7,8, and Rodney J. Weber1 Hongyu Guo et al.
  • 1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
  • 2Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, CO, 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 Technology, Atlanta, GA, USA
  • 7Foundation for Research and Technology, Hellas, Greece
  • 8National Observatory of Athens, Athens, Greece
  • anow at: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA

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 into 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.

Short summary
Fine particle pH is linked to many environmental impacts by affecting particle concentration and composition. Predicted Pasadena, CA (CalNex campaign), PM1 pH is 1.9 and PM2.5 pH 2.7, the latter higher due to sea salts. The model predicted gas–particle partitionings of HNO3–NO3, NH3–NH4+, and HCl–Cl are in good agreement, verifying the model predictions. A summary of contrasting locations in the US and eastern Mediterranean shows fine particles are generally highly acidic, with pH below 3.
Final-revised paper