Measurement of major organic acids in rainwater in Southeast Asia during burning and non-burning periods

Rainwater samples were collected in Singapore from August to December 1997 using a wet-only collector. Major organic acids were analyzed in all collec ted samples, using Ion Chromatography, with concentrations ranging from 1.4 to 105.6 mu eq/l (mean = 17.6 mu eq/l) for formate and from 1.7 to 62.8 mu eq/l (mean = 16.7 mu eq/l) for acetate, respectively. Elevated concentrati ons of both formate and acetate were observed during the months of Septembe r and October when the biomass burning impacted air masses originating from Indonesia had arrived at Singapore and prevailed there. The formate to ace tate ratio was greater than 1 in biomass burning plumes suggesting that for mic acid was likely produced in the atmosphere in addition to its direct em ission from fires. Both formate and acetate together accounted for as much as 68 % of unneutralized total acidity in rain during the burning period. O ur results suggest that biomass burning is an important source of HCOOH and CH3COOH to the troposphere over Southeast Asia.