Characterization of aerosol acidity at a high mountain site in central eastern China

Abstract

Aerosol acidity plays an important role in the formation of secondary organic aerosols. In the present study, strong and aerosol acidity properties of PM2.5 were evaluated based on the highly time-resolved measurements of PM2.5 ionic compositions obtained at the highest mountain site in central eastern China in spring and summer of 2007. Overall, PM2.5 was weakly acidic at Mt. Tai with 57.2% and 81.3% of the observations being acidic aerosols in spring and summer, respectively. Strong and aerosol acidities showed higher levels in summer (mean ± stand deviation: 142.65 ± 115.23 and 35.27 ± 30.88 nmol m−3) and lower concentrations in spring (64.82 ± 75.07 and 25.25 ± 32.23 nmol m−3). Aerosol pH exhibited an opposite seasonal trend with less acidic aerosols in summer compared to the aerosols in spring due to high water content in the particles in summer. Strong acidity showed a well-defined diurnal profile with a broad peak during the daytime, while aerosol acidity was at relatively low level in the daytime. The effects of ambient RH and atmospheric aging on the acidities of PM2.5 were examined. Aerosol water content facilitated the release of free H+ in the aerosol droplet via hydrolysis processes of bisulphate and acidic aerosols were often associated with more processed air masses. Several cases with formation and accumulation of secondary organic aerosols occurring were investigated. The results indicated that the increase of secondary organic aerosols was probably due to effects of acidity promotion and aqueous phase formation. This is the first attempt to investigate aerosol acidity based on high resolution measurements of aerosol ions in central eastern China.