Acidity and sampling artifacts of PM(2.5) in Hong Kong.

摘要

Sulfate in atmospheric particles has been found to be correlated with excess mortality rate and it is well known that sulfate particles are acidic. This thesis investigates the aerosol acidity, which is likely a better causal agent of health effects, in PM2.5 in Hong Kong. The strong acidity and the concentrations of ammonium, sodium, sulfate, nitrate and chloride in PM2.5 in Hong Kong, using a Harvard honeycomb denuder/filter-pack system were measured at the different sites (HKUST, TST and Ho Man Tin) in Hong Kong. Overall, the strong acidity of PM2.5 in HK is higher than that reported elsewhere (maximum of about 180 nmol/m3 in this study). Using back trajectory and mixing height analysis, we estimated that 40% of the measured sulfate and ammonium of PM2.5 in Hong Kong is from non-local sources.; The sampling of acidity and semi-volatile species such as ammonium, nitrate and chloride is complicated by artifacts. Various reactions between gas and particles and among the collected particles led to sampling errors. We examined the role of aerosol composition in the sampling artifacts of aerosols. The results show that the analysis of the artifact formation can be categorized into two regimes; as ammonium rich (AR) with the molar ratio [NH4 +]/[SO42-] ≥ 1.5 and as ammonium poor (AP) samples with the molar ratio [NH4+]/[SO 42-] 1.5. The AR samples were characterized by high nitrate concentrations and low in-situ free acid; the evaporation of HNO3 and HCl with concomitant evaporation of NH3, was more important in the sampling artifact of nitrate and chloride. The AP samples were characterized by low nitrate concentrations and high in-situ free acid; the evaporation of HNO3 or HCl alone was the important parameter in controlling the extent of sampling loss. We have also developed a methodology to estimate the contribution of each artifact reaction in the sampling artifacts of nitrate, chloride, ammonium and acidity.; We extended our analysis to a much larger database, that is the 24-hr Respirable Suspended Particles (RSP: PM10) data set from seven air-quality-monitoring sites run by the Hong Kong Environmental Protection Department in 2001, to estimate the aerosol acidity of PM2.5, which is not regularly monitored. (Abstract shortened by UMI.)