Indonesian smoke aerosols from peat fires and the contribution from volcanic sulfur emissions

The island arc volcanoes in and around Indonesia have been permanently degassing for thousands of years, thereby contributing significantly to the total emissions of sulfur species in that region. The hot and wet tropical weather conditions with high solar irradiation and regular daily precipitation during the wet season lead to efficient removal of oxidised sulfate by wet deposition. This is accumulated in the Indonesian peat areas, which serve as natural sponges, soaking up rain during the wet season and slowly releasing moisture into the atmosphere during the dry season. When peat forests are drained for land clearing purposes, the peat quickly dries out and becomes extremely flammable. When ignited, the composition of the burning peat mainly determines the fire aerosol chemical composition and microphysical properties. In this paper we investigate the contribution of volcanic sulfur emissions to wet deposition of sulfur in Indonesian peat swamp areas based on numerical simulations carried out with a global atmospheric circulation model including the tropospheric sulfur cycle. Our study suggests that the observed hygroscopicity and elevated sulfur content of the Indonesian peat fire aerosols is due to accumulated volcanic sulfur.