Estimates of the Contribution of Below-cloud Scavenging to the Pollutant Loadings of Rain in Taipei, Taiwan

Abstract

In this study, we use a simple and semi-quantitative scheme to estimate the contribution (denoted as ƒ) of below-cloud scavenging of pollutants by raindrops, based on the precipitation chemistry collected in Taipei City and the clound chemistry observed at the peak of the Mt. Bamboo (around 1100 m MSL) in the winters of 1996-1998. The magnitudes of ƒ (nss (non-sea salt)-SO42-), ƒ(NO3-) and ƒ(NH4+) were estimated to range between 0.31-0.88, 0.25-0.88 and 0.54-96,respecitively. For instance, 54-96% of the NH4+ ions in rainwater arose by below-cloud scavenging, reflecting their origins from local sources. For most of the cases, at least 75% of the NO3- was accounted for by the same scavenging mechanism, whereas more than 25% of nss-SO42- resulted from in-cloud scavenging. In comparison with NH4+ and NO3-, relatively higher amounts of nss-SO42- obviously derived from long-range transport.

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