Simple Framework to Quantify the Contributions from Different Factors Influencing Aerosol pH Based on NHx Phase-Partitioning Equilibrium.

While aerosol pH is among the most important parameters in atmospheric chemistry, it can be challenging to have a priori knowledge of the factors that are most strongly influencing the pH in a specific environment. In this study, we present a calculation method to more intuitively quantify the relationship between aerosol pH and its influencing factors, including gaseous NH3 concentration, particle properties, relative humidity, temperature, and nonvolatile cations, based on the NHx phase-partitioning equilibrium used in the E-AIM thermodynamic model. The applications of this calculation framework include (1) expressing the pH values directly as the function of influencing factors, (2) quantitatively studying the contribution of different factors to pH value changes, and (3) decomposing the standard deviation of pH values to find the dominant influencing factors on total pH fluctuations. This calculation framework provides a direct, quantitative, and intuitive approach to interpret pH values and differences. The relationship derived from pH and phase partitioning of semivolatile NHx can be extended to other phase-partitioning pairs as well. Our method provides a new way to quantitatively study pH and allows the pH studies conducted in different locations and meteorological conditions to be more easily compared and interpreted.