Atmospheric Gases, Particulates and Rainfall Concentrations During Summer Rain Events

Abstract

Abstract Concentrations of the predominant ions and of hydrogen peroxide in rainwater were measured during two rain events in Hong Kong in August 1995. The ambient nitrogen oxides, sulphur dioxide and ozone concentrations, as well as respirable suspended particulates, were monitored throughout the month, and the concentration and composition of total suspended particulates were also measured before and after the events. Event 1 heralded the commencement of a typhoon. The concentrations of all analytes, with the exception of hydrogen ion, decreased during event 1, due to exhaustive washout of alkaline particulate matter. The pH reached 3.8 at the end of the event, compared with 5.1 at the start. The shorter, more intense event 2 followed a sharp build-up of atmospheric pollutants and rainwater pH varied in a narrower range, between 3.4–3.7. The concentrations of H+, SO4 2- and NO3 − in rainwater were much higher than in event 1, and all analyte concentrations exhibited minima at maximum rainfall intensity. Contributions from in-cloud scavenging and below-cloud gas scavenging to rainwater acidity were both identified, and neutralization by suspended particulate matter also played an important role. The rainwater acidity is attributed to mainly local sources in both events. Both events were characterized by tropical cyclones in the China Seas, which when approaching Hong Kong, lead to a stagnation of local air pollutants. Strengthening winds at the time of event 1 provided more ventilation. Hydrogen peroxide concentrations in rainwater decreased rapidly during both events, and were higher in event 1 than in event 2 because SO2, and NOx levels were lower. Elevated concentrations of nitrogen (II) oxide were found to occur during periods of thunderstonn activity, in conjunction with low levels of ozone.