Investigating the pH of atmospheric fine particles and implications for atmospheric chemistry
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Particle acidity is a critical but poorly understood quantity that affects many aerosol processes and properties, including aerosol composition and toxicity. In this study, particle pH and water (which affects pH) are predicted using a thermodynamic model and measurements of RH, T, and inorganic gas and particle species. The method was first developed during the SOAS field campaign conducted in the southeastern U.S. in summer (pH = 0.94 ± 0.59), and then extended to aircraft observations in the northeastern U.S. in winter (WINTER study; pH = 0.77 ± 0.96) and ground observations in the coastal southwestern U.S. in early summer (CalNex study; PM1 pH = 1.9 ± 0.5 and PM2.5 pH = 2.7 ± 0.3). All studies have consistently found highly acidic PM1 with pH generally below 3. The results are supported by reproducing particle water and gas-particle partitioning of inorganic NH4+, NO3-, and Cl-. Nonvolatile cations may increase pH with particle size above 1µm depending on mixing state but have little effect on PM1 pH. Ion balance or molar ratio, are not accurate pH proxy and highly sensitive to observational uncertainties. Impacts of low particle pH were investigated, including the effects on aerosol nitrate trends and the role of acidity in heterogeneous chemistry. We found that PM2.5 remained highly acidic despite a ~70% sulfate reduction in the southeastern U.S. in the last 15 years, due to buffering by semivolatile NH3; that the bias in molar ratio predictions in past studies is linearly correlated to nonvolatile cations but not organics, challenging the organic film postulation that exclusively limits the gas-particle transfer of NH3; that recently proposed rapid SO2 oxidation by NO2 during China haze events may not be a significant source of sulfate due to relatively low pH (~4); and lastly that pH is also not highly sensitive to NH3, a 10-fold increase in NH3 only increases pH by one unit in various locations and seasons, which has implications for use of NH3 controls to reduce PM2.5 concentrations.