Modelling the atmospheric ³⁴S-sulfur budget in a column model under volcanically quiescent conditions

Abstract. We investigated the sulfur isotope budget of atmospheric carbonyl sulfide (COS) and the role of COS as a precursor for stratospheric sulfate aerosols (SSA). Currently, the sulfur isotopic budgets for both SSA and tropospheric COS are unresolved. Moreover, there is some debate on the significance of COS on SSA formation. With the use of an atmospheric column model, we model the isotopic composition of COS to resolve some of the uncertainties in its budget. We attempt to constrain the isotopic budget (³²S and ³⁴S) of COS in the troposphere and the stratosphere. We are able to constrain the model results to match the observed COS isotopic signature at the surface, which has recently been measured to lie between δ³⁴S = 10–14 permil (‰). When we propagate this composition to SSA, we match the isotopic signal of SSA that was measured in volcanically quiescent times at 18 km as δ³⁴S = 2.6 ‰. Our results show that COS becomes isotopically enriched during destruction in the stratosphere, and this enriched isotopic signal of COS propagates through SO₂ to sulfate, creating strong positive isotopic gradients of both SO₂ and sulfate in the lower stratosphere. Sensitivity tests indicate that the enriched sulfur in the stratosphere is mostly sensitive to COS photolysis, and to a lesser extent to biosphere uptake and COS emission signature. A better quantification of these processes could further support the role of COS in sustaining the SSA layer. Hence, there is a need for isotopic measurements for both stratospheric COS and SSA to better constrain these contributions.