ATMOSPHERIC GASES, PARTICULATES AND RAINFALL CONCENTRATIONS DURING SUMMER RAIN EVENTS

Concentrations of the predominant ions and of hydrogen peroxide in rai nwater were measured during two rain events in Hong Kong in August 199 5. The ambient nitrogen oxides, sulphur dioxide and ozone concentratio ns, as well as respirable suspended particulates, were monitored throu ghout the month, and the concentration and composition of total suspen ded particulates were also measured before and after the events. Event 1 heralded the commencement of a typhoon. The concentrations of all a nalytes, with the exception of hydrogen ion, decreased during event 1, due to exhaustive washout of alkaline particulate matter. The pH reac hed 3.8 at the end of the event, compared with 5.1 at the start. The s horter, 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+, SO42- and NO3- in rainwater were much hig her than in event 1, and all analyte concentrations exhibited minima a t maximum rainfall intensity. Contributions from in-cloud scavenging a nd below-cloud gas scavenging to rainwater acidity were both identifie d, and neutralization by suspended particulate matter also played an i mportant role. The rainwater acidity is attributed to mainly local sou rces in both events. Both events were characterized by tropical cyclon es in the China Seas, which when approaching Hong Kong, lead to a stag nation of local air pollutants. Strengthening winds at the time of eve nt 1 provided more ventilation. Hydrogen peroxide concentrations in ra inwater decreased rapidly during both events, and were higher in event 1 than in event 2 because SO2, and NOx levels were lower. Elevated co ncentrations of nitrogen (II) oxide were found to occur during periods of thunderstorm activity, in conjunction with low levels of ozone.