Observations of new aerosol particle formation in a tropical urban atmosphere

Abstract

Particle number concentrations (PNC) and particle size distributions (PSD) in the size range of 5.6–560 nm were measured in Singapore during South West (SW) and North East (NE) monsoon periods. The field study was conducted from 27 July 2008 to 15 August 2008 and from 21 January 2009 to 22 February 2009. A distinct peak of PNC in the afternoon was observed in addition to morning and evening rush hour peaks during the SW monsoon period. Concurrent measurements of PSD, SO2, Black Carbon (BC) and proxy H2SO4 concentrations revealed that the afternoon peaks observed during the SW monsoon period were likely due to new particle formation. These nucleation events were frequently observed during the SW monsoon period, but were rarely seen during the NE monsoon period. The effect of meteorological parameters viz. Temperature (T), Relative Humidity (RH), Incoming Solar radiation (SR) on the rate and intensity of nucleation was examined. Strong nucleation events were observed in the presence of high H2SO4 concentrations at high T, high SR, and low RH. The newly formed particles did not show any signs of growth during the nucleation events. New particle formation (NPF) events appear to be mainly induced by SO2 emissions from the local point sources (e.g. petroleum refineries), so when winds blew from that direction nucleation events were prominent. Local bush fires were observed during the course of air sampling due to a prolonged dry spell in the months of January and February 2009. During the occurrence of the local smoke haze induced by bush fires, nucleation events were suppressed.