Photochemistry of tropospheric CS2, a new chemical pathway

Abstract

<p><strong>Photochemistry of tropospheric CS₂, a new chemical pathway</strong></p><p><strong>&#160;</strong></p><p>Yuanzhe Li¹, Kazuki Kamezaki¹ and Sebastian Danielache¹</p><p>¹ Faculty of Science and Technology, Sophia University</p><p>&#160;</p><p><strong>Abstract</strong></p><p>Carbon disulfide (CS₂) is an atmospheric trace gas and is mainly produced by anthropogenic emissions. Its oxidation end-products in the atmosphere are carbonyl sulfide (OCS) and sulfur dioxide (SO₂). Therefore, CS₂ indirectly contributes to the production of sulfate aerosol, which influences atmospheric radiative properties and stratospheric ozone depletion.</p><p>Current understanding suggests that the main sink of CS₂ is the reaction with the OH radical which shares of 75-88% CS₂ global removal (Khan et al., 2017). This reaction pathway generates an adduct SCSOH, followed by oxidation with O₂ to form OCS and SO₂. UV induced processes are usually considered irrelevant in the troposphere. Tropospheric CS₂ photo-oxidation mechanism was first suggested by Wine et al. (1981). The CS₂ UV-absorption spectrum has a strong absorption band (280-360 nm), which generates a photo-excited (CS₂(³A₂) often presented as CS₂^*state) fragment, which gets further oxidized by O₂ to produce OCS and SO₂. The solar flux spectrum in the troposphere satisfies conditions for a CS₂ photo-excitation, enabling a potential CS₂ photo-oxidation pathway in the troposphere.</p><p>In this study, CS₂ photochemistry is revised and studied by a 1-D atmospheric model (PATMO) capable of handling photochemistry with a high-resolution spectrum. Simulated main reduced sulfur species (CS₂, OCS and SO₂) reproduce field measurements. Under strong light conditions, the CS₂ photo-excitation reaction is followed by two CS₂^* excited state quenching reactions. The reaction rate <em>r</em> for the net CS₂ photo-induced oxidation and CS₂+ OH reactions at 1 km are 71 and 26 molecule cm^-3 s^-1 respectively. These results indicate that, under favorable light conditions photochemistry is a relevant tropospheric sink of CS₂.</p><p>&#160;</p><p>References</p><p>Khan, A., Razis, B., Gillespie, S., Percival, C., Shallcross, D., Global analysis of carbon disulfide (CS₂) using the 3-D chemistry transport model STOCHEM, <em>Aims Environ. Sci.</em> <strong>2017</strong>, <em>4</em>, 484&#8211;501.</p><p>&#160;</p><p>Wine, P. H., Chameides, W. L., Ravishankara, A. R., Potential role of CS₂ photooxidation in tropospheric sulfur chemistry, <em>Geophys. Res. Lett.</em> <strong>1981</strong>, <em>8</em>, 543-546.</p><p>&#160;</p><div>\n<div>&#160;</div>\n</div>