Volcanic sulfur emissions: Estimates of source strength and its contribution to the global sulfate distribution

Anthropogenic emission of SO₂ and conversion into SO₄²⁻ is argued to be the most important factor damping and modulating the global greenhouse effect. Recent estimates of the relative strength of the three important sources of volatile sulfur (SO₂ from fossil fuel combustion ∼78 Tg S/yr, from biomass burning ∼2 Tg S/yr, and from natural sources ∼25 Tg S/yr) suggest an over-whelming effect of the anthropogenic emissions for climate forcing. However, the radiauvely relevant product SO₄²⁻ may have different patterns due to the distribution of the sources (some very dense areas near the surface for anthropogenic SO₂, formation of SO₂ from dimethylsulfide in the marine boundary layer, and emission of volcanic SO₂ mostly in the free atmosphere in rural areas). In this paper we study the relative contribution of volcanic SO₂ emissions to the atmospheric sulfur budget applying an atmospheric general circulation model including a full sulfur cycle and prescribed source distributions. An off-line analysis tool is applied to determine the radiative forcing of sulfate aerosols. The results show that natural S sources are at least as important as the anthropogenic ones, even though their source strength is much smaller. The reasons are different lifetimes due to different production and emission processes. Therefore, we should improve our knowledge about the volcanic volatile sources and their time-space variability.