Analysis of Hong Kong daily bulk and wet deposition data from 1994 to 1995

Results from a 1-year daily rainwater sampling program, employing both wet and bulk deposition samplers with replicate samples, from 1994 to 1995 in Hong Kong are presented and analysed. Analyte concentrations were found to vary over a wide range of several orders of magnitude, with [H+] for example, from 0.16 to 208.9 mu eq dm(-3). Diurnal pH values less than 3.83 were measured on five occasions. A significant correlation between pH and lognormal windspeed has been found. This is taken to indicate the minor importance of long-range transport in determining rainwater acidity, since local pollutant emissions accumulate and react under conditions of atmospheric stability in the sub-tropical climate. The H+ wet deposition flux onto a polythene surface was 90 meq m(-2) yr(-1) during 1994-1995 at City University. Dry deposition exerts a neutralizing influence upon the acidity from this wet deposition. Although paired t-tests indicated significant differences between the bulk versus wet deposition datasets for cations, but not anions, the dataset means consequently showed such large standard deviations that t-tests indicated no significant differences. In rainwater, the charges from SO42- and NO3- anions seldom balance the proton charges, implying that they are also derived from solubilization of primary and secondary airborne Ca2+, Mg2+ and NH4+ particulate matter in rainwater. Use of the [SO42-]/[NO3-] ratios in rainwater in fingerprinting pollutant origins has drawbacks, but is generally indicative of a predominantly regional contribution of these secondary pollutants to rainwater. Bulk deposition pH in Hong Kong would be in the region of 4.1 if basic Ca2+ compounds alone did not neutralize acidity. The regional rainout pH, inferred after exhaustive below-cloud scavenging, is about 5. The temporal trends in Hong Kong rainwater acidity are blurred. (C) 1999 Elsevier Science Ltd. All rights reserved.