A 3-year weekly data set on the ionic composition of rainwater from 6 sampling sites in Hong Kong representing the 1990–92 period was resolved employing factor analysis. Two-to-three factors explaining cumulative variances of 78–93% of the total in the 9 ionic parameters studied are utilised in the interpretation. The procedure is repeated on the screened data sets obtained on the basis, among other constraints, of a cut-off ion balance of ± 15%.
The results indicate that: (i) the ‘mixed’ (residential-industrial) area, Central-Western has a 7-membered ‘degenerate’ first factor which fails to resolve into smaller groups; (ii) the quasi-rural site, Junk Bay has a partially neutralised acid (PNA) and a pure sea salt (SS) component; (iii) two other mainly residential areas, Hong Kong South – whose factors resemble those for industrial Kwun Tong – and Tai Po have a third factor containing potassium, and additional chloride, respectively. The latter factor, V/W, for Tai Po believed to be due to burning vegetation/waste incineration is similar only to that for the industrial Kwai Chung.
On the other hand, the process of data screening has no effect on the factorial sequence or make-up, except in the cases of the two industrial locations, where the order of the resolved factors in two of their sets are swapped among two different factors; and, in the case of one of them, a variable (ion) shifts its loading to a different factor upon screening.
Finally, stepwise multiple regression corroborates our earlier provisional conclusion that the free acidity of rainwater in Hong Kong is also contributed by acids other than H2SO4 and HNO3, possibly by oxalic, formic and acetic acids, among some others.