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 analysed and synthesised, employing the original and screened/validated weekly data sets. A significant observation common to all sampling sires (in the original data) is the presence of a free acid fraction which cannot be accounted For by sulphuric and nitric acids. Such data sets, among others, also possess an anion deficit, which fail to meet the quality criterion for ion balance, It is tentatively suggested that the unaccounted free acidity could be from one or more of oxalic, phosphoric. formic and acetic acids. The observed ranges of skewness and kurtosis indicate considerable inter-site variability in the corresponding distributions of ionic concentrations. Results indicate that the southernmost station, Hong Kong South is a strong contender for the cleanest 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 poorly, or hardly represented by the log-normal distribution. The northernmost site, Tai Po and Kwai Chung have a significant anthropogenic Cl- component which is highly correlated to a similar K+ component. These are possibly emitted by biomass burning, or from chemical industrial pollution, possibly in the form of KCl. Analysis of variance (ANOVA) conducted on the four main acid-base ionic species indicates significant 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-sea-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 that the non-marine components of rainwater ions in Hong Kong have a considerable local origin, with contributions from washout and possibly, from rainout. (C) 1997 Elsevier Science Ltd.