Photochemistry of tropospheric CS2, a new chemical pathway

Photochemistry of tropospheric CS₂, a new chemical pathway

 

Yuanzhe Li¹, Kazuki Kamezaki¹ and Sebastian Danielache¹

¹ Faculty of Science and Technology, Sophia University

 

Abstract

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.

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.

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 r 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.

 

References

Khan, A., Razis, B., Gillespie, S., Percival, C., Shallcross, D., Global analysis of carbon disulfide (CS₂) using the 3-D chemistry transport model STOCHEM, Aims Environ. Sci. 2017, 4, 484–501.

 

Wine, P. H., Chameides, W. L., Ravishankara, A. R., Potential role of CS₂ photooxidation in tropospheric sulfur chemistry, Geophys. Res. Lett. 1981, 8, 543-546.