Impact of biomass burning on rainwater acidity and composition in Singapore

The Indonesian forest fires that took place from August through October 199 7 released large amounts of gaseous and particulate pollutants into the atm osphere. The particulate emissions produced a plume that was easily visible by satellite and significantly affected regional air quality in Southeast Asia. This prolonged haze episode provided an unprecedented opportunity to examine the effects of biomass burning on regional atmospheric chemistry. W e undertook a comprehensive field study to assess the influence of biomass burning impacted air masses on precipitation chemistry in Singapore. Major inorganic and organic ions were determined in 104 rain samples collected us ing an automated wet-only sampler from July through December 1997. Mean pH values ranged from 3.79 to 6.20 with a volume-weighted mean of 4.35. There was a substantially large number of rain events with elevated concentration s of these ions during the biomass burning period. The relatively high conc entrations of SO42-, NO3-, and NH4+ observed during the burning period are attributed to a long residence time of air masses, leading to progressive g as to particle conversion of biomass burning emission components. The decre ase in pH of precipitation in response to the increased concentrations of a cids is only marginal, which is ascribed to neutralization of acidity by NH 3 and CaCO3.