Daily ambient PM2.5 was collected during 8 September to 5 October (Julian Days, JDs 252e279), 2008 in Singapore to investigate impacts of transboundary biomass burning smoke on distribution among C2eC5 dicarboxylic acids (DCAs) and corresponding dicarboxylate salts (DCS) in the tropical urban atmosphere. Quantification of DCAs and DCS were performed using solvent and water extraction followed by chromatography analyses via GCeMS and ion chromatography. The averaged PM2.5 and PM10 concentrations from September 19e30 (JDs 263e274) were concurrently elevated by w40% due to the transboundary smoke. During this same period, C2eC5 total dicarboxylates (TDCAS, summation of DCAs and DCS), on average, increased more than two times, with C2-TDCAS accounted for in average 80% of the C2eC5 TDCAS. This demonstrates that the transboundary smoke enriched C2eC5 TDCAS more than PM2.5 in the urban environment. In the presence of the transboundary smoke, the averaged concentration ratios of C2eC5 DCS to corresponding DCA were 13.4, 2.9, 1.0, and 1.4 with oxalate salts exhibiting the highest concentration (355.0 ng m
3). The transboundary smoke increased malic acid concentration more than 3.5 times, which is the largest relative increase among the quantified TDCAS. Considering that malic acid is mainly generated through ambient photooxidation, such significant increase in malic acid demonstrates more prominent photooxidation incurred by the smoke.