Large Variability in Ambient Fine Particle pH in Fairbanks Winter

JAMES CAMPBELL, Michael Battaglia, Kayane Dingilian, Meeta Cesler-Maloney, William Simpson, Ellis Robinson, Peter F. DeCarlo, Brice Temime-Roussel, Barbara D'Anna, Jack Dibb, Athanasios Nenes, Rodney J. Weber, Jingqiu Mao, University of Alaska Fairbanks

     Abstract Number: 293
     Working Group: Aerosols Spanning Spatial Scales: Measurement Networks to Models and Satellites

Abstract
Fairbanks, Alaska is a subarctic city with fine-particle (PM2.5) concentrations that often exceed air quality regulations in winter due to poor dispersion caused by strong temperature inversions coupled with large local emissions and unique chemistry occurring under cold and dark conditions. The average temperature during the study was -18°C but reached -35°C during a pollution event. Many of the processes that contribute to severe air pollution in this climate are not yet well understood. Particle pH is a major concern because pH affects many secondary processes that can increase the overall PM mass concentration, alter its chemical composition, and contribute to its toxicity. Here, we use a comprehensive dataset of aerosol particle and gas measurements from the Alaska Layered Pollution And Chemical Analysis (ALPACA) campaign, combined with a thermodynamic model (ISORROPIA II), to examine pH in ambient PM2.5. We find that pH shows a bimodal distribution, where it is either relatively high (3 to 5) or low (-1 to 1). This pH bifurcation is largely due to the volatility of ammonium (the main base), but not sulfate (the main acid). The volatility of ammonia drops with decreasing temperature and raises the pH at which NH3(g)/NH4+(aq) buffers, whereas the buffering pH range of HSO4-(aq)/SO42-(aq) is low regardless of temperature. This makes particle pH in Fairbanks highly sensitive to the total ammonium to total sulfate ratio (TA/TS) to a degree not observed at more moderate temperatures. Frequent periods of sufficiently high pH in Fairbanks to support aqueous formation of hydroxymethanesulfonate, which requires a pH of 4 to 5, is predicted to result from this behavior, in agreement with observations. Overall, these findings show that aerosol particle pH in very cold environments behaves in a unique way that modulates particle composition and affects air quality.