Effect of nitrate and sulfate relative abundance in PM_2.5 on liquid water content explored through half-hourly observations of inorganic soluble aerosols at a polluted receptor site

Liquid water content (LWC) is the amount of liquid water on aerosols. It contributes to visibility degradation, provides a surface for gas condensation, and acts as a medium for heterogeneous gas/particle reactions. In this study, 520 half-hourly measurements of ionic chemical composition in PM_2.5 at a receptor site in Hong Kong are used to investigate the dependence of LWC on ionic chemical composition, particularly on the relative abundance of sulfate and nitrate. LWC was estimated using a thermodynamic model (AIM-III). Within this data set of PM_2.5 ionic compositions, LWC was highly correlated with the multivariate combination of sulfate and nitrate concentrations and RH (R² = 0.90). The empirical linear regression result indicates that LWC is more sensitive to nitrate mass than sulfate. During a nitrate episode, the highest LWC (80.6±17.9μgm^-3) was observed and the level was 70% higher than that during a sulfate episode despite a similar ionic PM_2.5 mass concentration. A series of sensitivity tests were conducted to study LWC change as a function of the relative nitrate and sulfate abundance, the trend of which is expected to shift to more nitrate in China as a result of SO₂ reduction and increase in NO_x emission. Starting from a base case that uses the average of measured PM_2.5 ionic chemical composition (63% SO₄^2-, 11% NO₃^-, 19% NH₄⁺, and 7% other ions) and an ionic equivalence ratio, [NH₄⁺]/(2[SO₄^2-]+[NO₃^-]), set constant to 0.72, the results show LWC would increase by 204% at RH=40% when 50% of the SO₄^2- is replaced by NO₃^- mass concentration. This is largely due to inhibition of (NH₄)₃H(SO₄)₂ crystallization while PM_2.5 ionic species persist in the aqueous phase. At RH=90%, LWC would increase by 12% when 50% of the SO₄^2- is replaced by NO₃^- mass concentration. The results of this study highlight the important implications to aerosol chemistry and visibility degradation associated with LWC as a result of a shift in PM_2.5 ionic chemical composition to more nitrate in atmospheric environments as is expected in many Chinese cities.