Characterization of diurnal variations of PM_2.5 acidity using an open thermodynamic system: A case study of...

Aerosol acidity has significant implications for atmospheric processing, and high time-resolution measurements can provide critical insights into those processes. This paper reports diurnal variations of aerosol acidity characterized using an open thermodynamic system in Guangzhou, China. Hourly measurements of PM_2.5-associated ionic species and related parameters were carried out during June-September 2013 followed by application of the Extended Aerosol Inorganic Model in open mode to estimate aerosol pH. The model-estimated aerosol pH was 2.4 ± 0.3, and the pH diurnal profile exhibited peaks in the early morning (6 a.m.) and troughs in the afternoon (2 p.m.) that appeared to be constrained by liquid water content (LWC) and free H⁺. A linear regression model was developed to predict aerosol pH, which performed strongly with 4 variables during daytime (NH₄⁺, Na⁺, SO₄^2- and RH; R² = 0.95) and 3 during nighttime (NH₄⁺, SO₄^2- and RH; R² = 0.91). The effect of aerosol acidity on the partitioning of HNO₃, HCl and NH₃ was studied based on theoretical considerations and measurement data. The fractions in particulate phase for acidic compounds correlated strongly with pH (R² = 0.64-0.69) while that for NH₃, interestingly, was weak (R² = 0.17). Analytical expressions were developed to explain these observations and it was concluded that the partitioning of HCl and HNO₃ was more sensitive to pH compared to that of NH₃. These results are significant in terms of potential atmospheric depletion rates of HCl and HNO₃ in the region and stress the need for future studies in this direction.