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.
The study of production and depletion of chemical species is vital for the understanding of composition and evolution of planetary atmospheres. We present an implementation of photoinduced isotopic effects into the PATMO code (Ávila et al., 2021) code designed for the study of stable isotopes and photo-induced isotopic effects. With respect to the original code PATMO, where the photochemistry was not included, this report extends capability of the model to set photochemical processes for stable isotopes and thus enhancing its applicability. The PATMO code is flexible and allows the edition of new chemical reactions without need for hard code them. We also test how changes in spectral resolution affects the calculation of isotopic effects during the photodissociation of oxygen. We found that for a highly structured spectrum such as the Schumann-Runge band a spectral resolution larger than 0.005 nm is necessary for accurately modeling these isotopic effects. We also show that SO2 and SO photodissociation couples in a complex shielding fashion and significantly affects the photo-induced isotopic effects. The model was also benchmarked against today's Earth atmosphere, where the solar UV flux and the ozone profile of the US Standard atmosphere of 1976 was reproduced with the simple Chapman mechanism and improved with the implementation of NOx and HOx radicals.
<p>It is debated how much stratospheric sulfate aerosol (SSA) in volcanically quiescent times is replenished by carbonyl sulfide (COS) oxidation products. The atmospheric COS budget is also currently uncertain, with missing sources and sinks. Isotopic analysis can be used to allocate the missing sources of COS and also to further constrain the relevance of COS to SSA. The measured tropospheric isotopic signature of COS (&#948;<sup>34</sup>S) ranges from 10-14 &#8240; (Kamezaki et al., 2019; Angert et al.,2019; Hattori et al., 2020; Davidson et al., 2020), whereas SSA &#948;<sup>34</sup>S is constrained by only one single measurement at 18 km of 2.6 &#8240; (Castleman, 1974). We use an atmospheric column model to constrain the COS isotopic budget and understand the contribution of COS to sulfate. We find that the COS tropospheric signal is determined by the signatures of its precursors (carbon disulfide, CS<sub>2</sub>, and dimethyl sulfide, DMS) and fractionation during plant uptake and oxidation. Photolysis of COS is important in the stratosphere; the isotopic signal of COS propagates through sulfur dioxide (SO<sub>2</sub>) to sulfate in the stratosphere. The model can reproduce &#948;<sup>34</sup>S between 1-5 &#8240; in the lower stratosphere, which encapsulates the observations from Castleman (1974).</p><p><strong>References</strong></p><ul><li>Angert, A., Said-Ahmad, W., Davidson, C., & Amrani, A. (2019). Sulfur isotopes ratio of atmospheric carbonyl sulfide constrains its sources. Scientific reports, 9(1), 1-8.</li> <li>Castleman Jr, A. W., Munkelwitz, H. R., & Manowitz, B. (1974). Isotopic studies of the sulfur component of the stratospheric aerosol layer.&#160;Tellus,&#160;26(1-2), 222-234.</li> <li>Davidson, C., Amrani, A., & Angert, A. (2020). Tropospheric carbonyl sulfide mass-balance based on direct measurements of sulfur isotopes.</li> <li>Hattori, S., Kamezaki, K., & Yoshida, N. (2020). Constraining the atmospheric OCS budget from sulfur isotopes.&#160;Proceedings of the National Academy of Sciences,&#160;117(34), 20447-20452.</li> <li>Kamezaki, K., Hattori, S., Bahlmann, E., & Yoshida, N. (2019). Large-volume air sample system for measuring 34S&#8725; 32S isotope ratio of carbonyl sulfide.&#160;Atmospheric Measurement Techniques,&#160;12(2), 1141-1154</li> </ul>
In Section 1 we describe the extended model set-up, including diffusion (Section 1.1), wet deposition calculation (Section 1.2), the performance of the PATMO model for modelling isotopic sulfur (Figure S3) and the vertical profile of fractionation of the COS oxidation reactions (Figure S4). We then discuss the model performance (Section 2), the derivation of the δ budget equation (Section 3), the tropospheric budgets of SO 2 and sulfate (Table 2), and the budget profile of total S, bulk and isotopic (Section 4.1).
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