In-Cloud Concentrations and Below-Cloud Scavenging Processes in Hong Kong, China

Abstract

Environmental Context.There are fewer studies of cloudwater than of precipitation because of the limited occurrence of cloud events, and technical and logistic sampling difficulties. In order to assess the importance of in-cloud and below-cloud processes, cloudwater and rainwater samples were collected at the highest mountain in Hong Kong and also near sea level. Abstract.Preliminary measurements of ionic concentrations in cloudwater collected at the highest mountain in Hong Kong show that the pH can be as low as 3.4, which is lower than that of rainwater collected in urban–residential Hong Kong. Sulfate is the most abundant anion, and hydrogen or ammonium ions are the most abundant cations in the cloudwater samples. Results are also presented for analyte concentrations in sequential samples taken during a rain event from a maritime airmass. The concentrations of sea salt species were high and exhibited an exponential decrease with concentration at the start of the event. The concentrations of other species also decreased to fairly constant, low values after ~10 mm rainfall. The fitting of the concentration–rainfall plots enabled in-cloud and below-cloud mechanisms of incorporation of the species to be distinguished.