Photochemistry of tropospheric CS2, a new chemical pathway

<p><strong>Photochemistry of tropospheric CS<sub>2</sub>, a new chemical pathway</strong></p><p><strong> </strong></p><p>Yuanzhe Li<sup>1</sup>, Kazuki Kamezaki<sup>1</sup> and Sebastian Danielache<sup>1</sup></p><p><sup>1</sup> Faculty of Science and Technology, Sophia University</p><p> </p><p><strong>Abstract</strong></p><p>Carbon disulfide (CS<sub>2</sub>) 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 (SO<sub>2</sub>). Therefore, CS<sub>2</sub> 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 CS<sub>2</sub> is the reaction with the OH radical which shares of 75-88% CS<sub>2</sub> global removal (Khan et al., 2017). This reaction pathway generates an adduct SCSOH, followed by oxidation with O<sub>2</sub> to form OCS and SO<sub>2</sub>. UV induced processes are usually considered irrelevant in the troposphere. Tropospheric CS<sub>2</sub> photo-oxidation mechanism was first suggested by Wine et al. (1981). The CS<sub>2</sub> UV-absorption spectrum has a strong absorption band (280-360 nm), which generates a photo-excited (CS<sub>2</sub>(<sup>3</sup>A<sub>2</sub>) often presented as CS<sub>2</sub><sup>* </sup>state) fragment, which gets further oxidized by O<sub>2</sub> to produce OCS and SO<sub>2</sub>. The solar flux spectrum in the troposphere satisfies conditions for a CS<sub>2</sub> photo-excitation, enabling a potential CS<sub>2</sub> photo-oxidation pathway in the troposphere.</p><p>In this study, CS<sub>2</sub> 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 (CS<sub>2</sub>, OCS and SO<sub>2</sub>) reproduce field measurements. Under strong light conditions, the CS<sub>2</sub> photo-excitation reaction is followed by two CS<sub>2</sub><sup>*</sup> excited state quenching reactions. The reaction rate <em>r</em> for the net CS<sub>2</sub> photo-induced oxidation and CS<sub>2 </sub>+ OH reactions at 1 km are 71 and 26 molecule cm<sup>-3</sup> s<sup>-1</sup> respectively. These results indicate that, under favorable light conditions photochemistry is a relevant tropospheric sink of CS<sub>2</sub>.</p><p> </p><p>References</p><p>Khan, A., Razis, B., Gillespie, S., Percival, C., Shallcross, D., Global analysis of carbon disulfide (CS<sub>2</sub>) using the 3-D chemistry transport model STOCHEM, <em>Aims Environ. Sci.</em> <strong>2017</strong>, <em>4</em>, 484–501.</p><p> </p><p>Wine, P. H., Chameides, W. L., Ravishankara, A. R., Potential role of CS<sub>2</sub> photooxidation in tropospheric sulfur chemistry, <em>Geophys. Res. Lett.</em> <strong>1981</strong>, <em>8</em>, 543-546.</p><p> </p><div> <div> </div> </div>