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 electrical conductivity of Hong Kong rainwater has been carried out employing a large population data set representing the three-year period, 1990–1992. No relationship is found between the sample volume and the pH of rainwater, similar to a previous result for daily rainfall in rural Hong Kong. Post-depositional acid-base reactions apparently narrow down and smoothen the field pH-distribution to the laboratory pH-distribution, shifting the modal pH from 4.4 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-high 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 μS cm−1. Both these distributions show long-drawn tails. There are strong indications that the ion ratio Σ−/Σ+ 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 only 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 practically ineffective. Similar “blind spots” in quality assurance could exist for many other rainwater data sets from around the world. The overall results suggest that for the combined validity of the ion and conductivity ratios defined in this paper as QA parameters, the aqueous solution (precipitation water) must be “infinitely” dilute, typically with ionic strength, I 10−4M and the analytical methods employed must be accurate enough at such low concentrations.