Application of ion chromatography to the Hong Kong rainfall monitoring program

Abstract Results from a rainwater monitoring program in Hong Kong from May 1994 to April 1995, with 60 sample collection days, are presented for Cl − , NO 3 − and SO 4 2− concentrations from suppressed and non-suppressed column ion chromatography. Replicate sampling was employed on a daily basis using both bulk (B) and wet only (W) samplers, and there was no statistically significant difference between the B-W datasets for Cl − , NO 3 − and SO 4 2− concentrations in rainwater. From the more complete bulk sample collection dataset ( n = 60), large changes in analyte concentrations were observed for samples collected on different days, with the ranges (in μequiv. 1 −1 ) 3–348 for Cl − , 3–132 for NO 3 − and 12–303 for SO 4 2− . Neglecting the February 1995 dataset for which n = 1, the volume-weighted monthly mean concentrations displayed narrower ranges, being 16–61, 7–42 and 27–87 respectively. The nature of the source of the analyte and its flux, type of weather system, season and atmospheric conditions, together with the rainfall amount and intensity are some of the factors which produce the changes in analyte concentrations. Below-cloud washout processes cause large changes in analyte concentration at the start of a single rain event, and contribute to the horizontal local variation in analyte concentrations. The ratio of SO 4 2− -NO 3 − in Hong Kong rainwater has decreased since 1992 and the reasons for this are discussed.