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Air pollution is a significant environmental problem in Indonesia due in large part to increasing anthropogenic emissions associated with rapid population and industrial growth. Large-scale wildfires set by people for land-conversion are also contributors of pollutants during the dry season. Additional sources of air pollution are the region's volcanoes, the largest cluster of historically active vents in the world. Problems from air pollution include health hazards, reduced visibility, and acid deposition. Aerosol particles can also affect the climate by changing the way radiation is transmitted through the atmosphere. The relative effects of anthropogenic and volcanic emissions on acid deposition and direct radiative forcing in Indonesia in the year 1999 were studied using the regional atmospheric chemistry and climate model REMOTE. The modeled acid deposition of volcanic sulfur (S) does not exceed a critical load value of 0.4 g(S)/m2-yr in this region; while anthropogenic S is deposited above this threshold in Java, northeastern Sumatra, and peninsular Malaysia. Volcanic S is calculated to be approximately 11 % of the total S deposited in the modeled region, indicating that anthropogenic emissions strongly dominate acid S deposition in Indonesia. The modeled distributions of sulfate (H2SO4), organic carbon (OC), and black carbon (BC) have been used to calculate their direct shortwave radiative forcing using an off-line radiation transfer model. The direct radiative effect of aerosol particles is that they scatter and absorb solar radiation. The total anthropogenic aerosol has been calculated assuming an external aerosol mixture of sulfate, black, and organic carbon; while the volcanic aerosol is treated exclusively as sulfate. The total anthropogenic aerosol is found to be dominated by OC released by biomass burning especially during the dry season of June-October when land-clearing fires occur. The annual mean direct radiative forcing of the anthropogenic and volcanic sulfate aerosols are shown to be of the same order of magnitude, with the anthropogenic sulfate exerting its influence over a much larger area. Averaged over the model domain, the direct radiative forcing of the anthropogenic sulfate is about 5 times greater than that of the volcanic source.
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