Acidity and concentrations of ionic species of PM2.5 in Hong Kong

The strong acidity and the concentrations of ammonium, sodium, sulfate, nitrate and chloride in PM2.5 in Hong Kong were measured using a Harvard honeycomb denuder/filter-pack system. The role of atmospheric mixing height in the variation in concentrations of the species in PM2.5 was investigated and the contribution of long-range transport to the ambient aerosols in Hong Kong was quantitatively determined. The average measured strong acidity in the spring, summer, autumn and winter of 2000, and the spring of 2001 was 49(23-92), 27(11-52), 97(71-145), 45(26-57), and 103(78-167)nmol/m3, respectively. Simulations using the aerosol inorganic model (AIM2) revealed that the in situ acidity of PM2.5 was only a minor fraction (23-39%) of the measured strong acidity because of the presence of bisulfate ions. The in situ acidity was correlated with a large difference between the ambient relative humidity and the estimated deliquescence relative humidity of the aerosols. In equivalent concentrations, acidity, ammonium and sulfate accounted for 16% (9-21%), 31% (26-39%) and 42% (40-45%) of the water-soluble inorganic species, respectively. Analysis of air mass trajectories showed that long-range transport of aerosols through the East/South China Sea (marine) and mainland China (continental) was possible. When Hong Kong was under the influence marine air masses, the measured concentrations of sulfate, ammonium and nitrate showed a strong correlation with the atmospheric mixing height. Continental long-range transport of aerosols was found to increase the concentrations of sulfate and ammonium by 49-383% and 33-302%, respectively. In total, about 40% of the sulfate and ammonium of PM2.5 in Hong Kong measured in this study was from continental air masses. However, the influence of the continental air masses on nitrate concentration and acidity was not obvious. © 2003 Elsevier Science Ltd. All rights reserved.