Distribution of dicarboxylic acids and carbon isotopic ratios in aerosols from 1997 Indonesian forest fires

Fine aerosol particles collected in Southeast Asia during 1997 Indonesian forest fires were studied for the concentrations of total carbon (TC), water-soluble organic carbon (WSOC) and low molecular weight dicarboxylic acids (C2-C12) as well as carbon isotopic ratios of TC (δCTC). TC and WSOC showed a large increase during the heavy forest fire event. At the same period, dicarboxylic acids, dominated by oxalic (C2) followed by succinic (C4) and malonic (C3) acids, also showed a concentration increase. Furthermore, the δCTC showed a decrease from ca. –25.5 to –27.5‰ during an intensified forest fire event, suggesting an addition of organic aerosols derived from C3 plants whose δC are lower. These results indicate that the aerosol particles in Southeast Asia were significantly affected by the combustion processes of vegetations during the 1997 Indonesian forest fires that were extensively induced by El Ninõ event. Introduction During a dry season of Southeast Asia, forest fires frequently occur and cause changes in the concentration and composition of aerosols in the tropical atmosphere. Such changes in the atmosphere should alter the optical and microphysical properties of tropical clouds and have a strong impact on radiation balance [Andreae and Crutzen, 1997]. Novakov and Corrigan [1996] reported that water-soluble organic components in smoke particles from smoldering biomass combustion are responsible for their intrinsic cloud condensation nuclei (CCN) activity. However, there are no detailed reports on the molecular composition of water-soluble organic fraction in aerosols associated with biomass burning. Characterization of water-soluble organic aerosols at molecular levels is necessary to better understand the changes in the atmospheric composition caused by forest fires. Water-soluble organic aerosols that account for 20-67% of total carbon in aerosols [Saxena and Hildemann, 1996], have capability to act as CCN [Novakov and Penner, 1993; Cruz and Pandis, 1997; Matsumoto et al., 1997]. Low molecular weight dicarboxylic acids are one of the most abundant water-soluble organic compound classes identified in aerosols [Sempéré and Kawamura, 1994], and may strongly contribute to CCN due to the hydrophilicity of their carboxyl groups. Dicarboxylic acids have been reported abundantly in aerosols from the urban [Kawamura and Kaplan, 1987; Kawamura and Ikushima, 1993;