SMALL-SCALE SPATIAL VARIABILITY IN THE REPRESENTATIVE IONIC COMPOSITION OF RAINWATER WITHIN URBAN HONG-KONG

Data on the ionic composition of rainwater from six locations in Hong Kong, representing the 3 year period, 1990-1992 have been critically a nalysed and synthesised, employing the original and screened/validated weekly data sets. A significant observation common to all sampling si res (in the original data) is the presence of a free acid fraction whi ch cannot be accounted For by sulphuric and nitric acids. Such data se ts, among others, also possess an anion deficit, which fail to meet th e quality criterion for ion balance, It is tentatively suggested that the unaccounted free acidity could be from one or more of oxalic, phos phoric. formic and acetic acids. The observed ranges of skewness and k urtosis indicate considerable inter-site variability in the correspond ing distributions of ionic concentrations. Results indicate that the s outhernmost station, Hong Kong South is a strong contender for the cle anest site among the six considered. In terms of ions, NH4+ and SO42- have the lowest skewness and kurtosis and their distributions are also the most log-normal. However, most species including the H+ ion are p oorly, or hardly represented by the log-normal distribution. The north ernmost site, Tai Po and Kwai Chung have a significant anthropogenic C l- component which is highly correlated to a similar K+ component. The se are possibly emitted by biomass burning, or from chemical industria l pollution, possibly in the form of KCl. Analysis of variance (ANOVA) conducted on the four main acid-base ionic species indicates signific ant inter-site (spatial) variability of non-sea-salt SO4, non-sea salt Ca as well as NH4, but not of NO3. While the presence over Hong Kong of the sulphates and nitrates of NH4+ and Ca2+ is probable, the non-se a-salt SO24- is relatively the stronger contributor to free acidity as compared with NO3-. The overall statistical and chemical variability across the six sampling sites is significant and strongly suggests tha t the non-marine components of rainwater ions in Hong Kong have a cons iderable local origin, with contributions from washout and possibly, f rom rainout. (C) 1997 Elsevier Science Ltd.