A CRITICAL DATA-ANALYSIS AND INTERPRETATION OF THE PH, ION LOADINGS AND ELECTRICAL-CONDUCTIVITY OF RAINWATER FROM THE TERRITORY OF HONG-KONG

A detailed analysis and interpretation of the pH, ion loadings and ele ctrical conductivity of Hong Kong rainwater has been carried out emplo ying a large population data set representing the three-year period, 1 990-1992. No relationship is found between the sample volume and the p H of rainwater, similar to a previous result for daily rainfall in rur al Hong Kong. Post-depositional acid-base reactions apparently narrow down and smoothen the field pH-distribution to the laboratory pH-distr ibution, shifting the modal pH from 4.4 to similar to 4.2, suggesting the possibility of the continuing oxidation of locally washed out SO2 in some of the samples. The distribution of the pH shows moderate-to-h igh free acidity levels in Hong Kong rainwater. The ionic strength (I) and specific electrical conductivity (K) stretch over some two orders of magnitude, with their respective modes within the classes, 0.2-0.3 mM and 15-20 mu S cm(-1). Both these distributions show long-drawn ta ils, There are strong indications that the ion ratio Sigma(-)/Sigma(+) for the coastal-urban Hong Kong is a more practical quality assurance (QA) parameter than the conductivity ratio, K(calculated)/K(measured) , as long as the ionic strength exceeds 0.6 mM; the latter ratio is on ly applicable to the lowest ionic strength class, 01-0.2 mM. However, this implies that between 0.2 and 0.6 mM, both the QA parameters and p ractically ineffective. Similar ''blind spots'' in quality assurance c ould exist for many other rainwater data sets from around the world. T he overall results suggest that for the combined validity of the ion a nd conductivity ratios defined in this paper as QA parameters, the aqu eous solution (precipitation water) must be ''infinitely'' dilute, typ ically with ionic strength, I less than or equal to 10(-4) M and the a nalytical methods employed must be accurate enough at such low concent rations.