Pre-industrial atmospheric Carbonyl Sulphide studied by 1-D isotopic photochemical model
Carbonyl sulﬁde (OCS) is the most stable and therefore abundant reduced sulfur compound in the atmosphere. OCS tropospheric chemical stability makes the stratospheric oxidation its main sink and the source of a sulfate aerosol layer between 17 and 30 km. This stratospheric sulfur aerosol (SSA) also known as the Junge layer aﬀects the planet’s Albedo While volcanic eruptions are an important source of stratospheric sulfate, they are sporadic and the source of sulfur in volcanically quiescent times has been a matter of debate.
Application of sulfur stable isotopes has provided evidence for OCS as the main source of the SSA. Ice core studies show that of pre-industrial tropospheric levels of OCS were 372 ppt while current levels are 500 ppt. In this study, we created a new 1-D photochemical model that includes stable isotopes has an additional tool for model calibration and consider the historical variation of anthropogenic emissions of OCS and its tropospheric precursor CS2. In order to calculate the OCS atmospheric vertical profile, we developed a one-dimensional photochemical model that takes into account chemistry, transport, deposition, stable isotopes and high-resolution absorption spectrum. The application of stable isotopes not only confirm as OCS as the main source of SSA during volcanic quiescent times but also confirms the isotopic mixing ratio of tropospheric OCS measured by Hattori et al, (2015) . Our findings show that about 33% of today’s SSA is anthropogenic. Furthermore, the anthropogenic contribution to SSA means that an estimate of -0.16 Wm2 of radiative forcing should be accounted in pre-industrial pollution free models. The model here presented is the first full attempt to develop a mass balance consistent with isotopic studies. This implementation allows for the study of isotopic imprints during specific chemical reactions and isotopic dilution during branching reactions. The isotopic Sulphur cycle was then studied after the model properly reproduced field measurements of known atmospheric Sulphur species. For an oxidizing type of atmosphere, the chemically stable end members of the reduced-oxidized states are OCS and H2SO4. The isotopic imprint of SSA(SO42-) is 2.6‰ which is reproduced if the isotopic signature of OCS injected in the troposphere is 6.5‰. This result is quite close to the value of 10.5 (±0.4) ‰ reported by Hattori et al., (2018) .