Characteristics of aerosol acidity in Hong Kong

Abstract

The ammonium-to-sulfate ratio ([NH4+]/[SO42−]) and the strong acidity have been generally used as parameters to describe the acidic nature of atmospheric aerosols. However, both parameters do not provide the in situ acidic characteristics of atmospheric aerosols, which are more relevant to the reactivity and the environmental impacts of the aerosols. In this study, the in situ free acid concentrations and the in situ pH of aerosols are investigated to understand the acidic characteristics of atmospheric aerosols in Hong Kong (HK). Over 182 datasets on 24 h Respirable Suspended Particles (RSP) samples collected in 2001 from seven air-quality-monitoring sites run by the Hong Kong Environmental Protection Department are analyzed. Simulations using the Aerosol Inorganic Model (AIM2) reveal that the in situ acidity, i.e., the free acid concentration ([H+]free), is only a minor fraction (∼23%) of the estimated strong acidity in the fine particles because of the presence of bisulfate ions. The acidity characteristics of fine particles are a function of mainly RH and ammonium to sulfate ratio. The in situ free acid concentration, the normalized water content ([H2O]AIM2/[SO42−]), and the dissociation of bisulfate to free acid in the aerosols decrease as the [NH4+]/[SO42−] ratio increases and the Relative Humidity (RH) decreases. The acidic fine mode particles have average molar [NH4+]/[SO42−] ratio of 1.42, strong acidity of 51 nmol m−3, in situ acidity of 11 nmol m−3, and in situ pH of 0.25 on average. Our findings suggest that even the more neutralized ([NH4+]/[SO42−] >1.5) particles, such as those found when HK is under the influence of continental air masses from the Chinese mainland, can have high in situ acidity and low pH when the RH is low. This study calls for more investigation of the acidity of aerosols in HK, incorporating the concepts of in situ acidity and pH.