Comparison of thermodynamic predictions for in situ pH in PM2.5

In situ aerosol pH is important in the study of atmospheric chemistry because many heterogeneous atmospheric chemical processes are pH dependent, but it is not easy to measure the in situ pH of PM2.5 due to the low liquid water content of aerosol particles. In situ aerosol pH is usually estimated by thermodynamic modeling. This study compared the in situ pH of PM2.5 in Hong Kong estimated by different thermodynamic methods such as AIM-II (Aerosol Inorganics Model-II) with the gas–aerosol partitioning calculation disabled, HCl/Cl−, HNO3/NO3− and NH3/NH4+ gas–aerosol equilibrium equations, and two thermodynamic gas–aerosol equilibrium models (ISORROPIA and SCAPE2). In the first method, actual measured particulate composition was used as input while only the total (gas+aerosol) composition was used in the other methods. AIM-II with the gas–aerosol partitioning calculation disabled gives the lowest in situ aerosol pH, 3.5, 2.8, 3.1, 2.7 and 4.3 unit (on average) less than that estimated by HCl/Cl−, HNO3/NO3− and NH3/NH4+ gas–aerosol equilibrium equations, and by SCAPE2 and ISORROPIA, respectively. The differences in estimated in situ pH are discussed in terms of the extent of the gas–aerosol equilibrium achieved, especially the NH3/NH4+ equilibrium. Although the estimated equilibrium concentrations deviated from the measured NH4+ in PM2.5 by only 20–35%, the estimated in situ pH is very sensitive to the deviation. It is possible that approaches based on the assumption of the gas–aerosol equilibrium may not be suitable for estimating in situ aerosol pH in the atmosphere.