Analysis of the pH of daily rainfall at a rural site in Hong Kong, 1989–1993

Daily bulk precipitation samples have been collected and their pH measured since April 1989 at a rural site, Kadoorie Agricultural Research Centre (KARC), in the central New Territories, Hong Kong. The frequency distribution of this population (N = 470) indicates a quasi-log normal distribution up until December, 1993. This is perhaps one of the longest daily pH-records available and it has climatological significance for the future. The annual median and the running, volume-weighted pH values (year end) are presented and they indicate collectively an annually oscillating pattern for which there is no obvious explanation at present. The annual volume-weighted pH values for 1990–1993 are 4.53, 4.30, 5.04 and 4.32 and the corresponding running volume-weighted pH values are 4.53, 4.39, 4.57 and 4.50. Most of the daily observations indicate acidity on the Sørensen scale as expected, and there appears to be no seasonal pattern to pH or amount of rainfall. Also, the volume of rainfall appears to exhibit no simple relationship with the acidity of the rainfall. On the other hand, there is, apparently, an anthropogenic component of acid in Hong Kong if the increase in free hydrogen ion concentration measured at the site is not entirely due to orographic reduction of alkaline dust present at relatively lower elevations in Hong Kong. Thus, there is some evidence that in contrast to the volume-weighted pH in the mid-4ʹs at Kadoorie, the pH of local background rainfall in the territory of Hong Kong may be ∼4.8, while that of regional background (unpolluted) rainfall is probably bracketed by pH ∼5–5.5.