17:15 〜 18:30
[AAS05-P10] Study of tropospheric CS2 photooxidation chemistry
キーワード：CS2, Photochemistry, Modeling, PATMO, Sulfur chemistry
Abstract
Carbon disulfide (CS2) is an atmospheric trace gas whose main sources in the atmosphere are from oceans and soils (Khalil et al., 1984). Anthropogenic emissions have increased in recent years, producing a strong regional distribution. CS2 is a reactive atmospheric sulfur gas, and as such it has a relatively short lifetime, ranging from a few days to half a month (Khan et al., 2017). Its oxidation end products in the atmosphere are carbonyl sulfide (COS) and sulfur dioxide (SO2). Therefore, CS2 indirectly contributes to the production of sulfate aerosols, which influence atmospheric radiative properties and stratospheric ozone depletion.
The OH-initiated oxidation of CS2 in the presence of O2 is considered to be the main CS2 sink pathway. By forming an intermediate SCSOH at first, followed by a series of oxidation reactions with O2 to produce COS and SO2, this process removes 75~88% of atmospheric CS2 (Khan et al., 2017). The current literature suggests that the conversion ratio of CS2 to COS is 0.83 (Stickel et al., 1993), but this result may be underestimated because only reaction with OH radicals considered and CS2 photochemistry is neglected.
The mechanism of CS2 photooxidation was first proposed by Wine et al. (1981). According to the CS2 UV-Vis absorption spectrum, there is a strong absorption band at 280~360 nm, which would first photo excite CS2 from ground state to CS2(3A2) state (usually presented as CS2* state). Then the majority of CS2* particles will be de-excited into CS2 ground state molecules after collision with air molecules (N2, O2 and H2O). Because of the rapid photochemical reaction rate of CS2, a small portion of CS2* is oxidized with O2 to produce COS and SO2 during the pseudo-steady state process of continuous CS2 photo excitation reaction and CS2* quenching reaction. In addition, the solar flux spectrum in the troposphere is strong enough to trigger the above CS2 photochemistry.
In this study, an updated CS2 photochemical network is studied by a 1-D atmospheric photochemical model (PATMO). Reaction path analysis is carried out using an open-source reaction network viewer (ReNView). From the simulated result, major reduced sulfur species (CS2, COS and SO2) reproduce field measurements. When the zenith angle of sunlight is 0°, our result shows that the conversion ratio of CS2 to COS is 0.87. The reaction rate r for the net CS2 photo-induced oxidation and CS2 + OH reactions at 1 km are about 18 and 144 molecule cm-3 s-1 respectively. These results indicate that, under favorable light conditions photochemistry is a relevant tropospheric sink of CS2.
References
Khalil, M. and Rasmussen, R. (1984). Global sources, lifetimes and mass balances of carbonyl 761 sulfide (OCS) and carbon disulfide (CS2) in the earth’s atmosphere. Atmospheric Environment 762 (1967), 18(9):1805–1813.
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.
Stickel, R. E. et al. (1993), Journal of Physical Chemistry, 97(51), pp. 13653–13661.
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.
Carbon disulfide (CS2) is an atmospheric trace gas whose main sources in the atmosphere are from oceans and soils (Khalil et al., 1984). Anthropogenic emissions have increased in recent years, producing a strong regional distribution. CS2 is a reactive atmospheric sulfur gas, and as such it has a relatively short lifetime, ranging from a few days to half a month (Khan et al., 2017). Its oxidation end products in the atmosphere are carbonyl sulfide (COS) and sulfur dioxide (SO2). Therefore, CS2 indirectly contributes to the production of sulfate aerosols, which influence atmospheric radiative properties and stratospheric ozone depletion.
The OH-initiated oxidation of CS2 in the presence of O2 is considered to be the main CS2 sink pathway. By forming an intermediate SCSOH at first, followed by a series of oxidation reactions with O2 to produce COS and SO2, this process removes 75~88% of atmospheric CS2 (Khan et al., 2017). The current literature suggests that the conversion ratio of CS2 to COS is 0.83 (Stickel et al., 1993), but this result may be underestimated because only reaction with OH radicals considered and CS2 photochemistry is neglected.
The mechanism of CS2 photooxidation was first proposed by Wine et al. (1981). According to the CS2 UV-Vis absorption spectrum, there is a strong absorption band at 280~360 nm, which would first photo excite CS2 from ground state to CS2(3A2) state (usually presented as CS2* state). Then the majority of CS2* particles will be de-excited into CS2 ground state molecules after collision with air molecules (N2, O2 and H2O). Because of the rapid photochemical reaction rate of CS2, a small portion of CS2* is oxidized with O2 to produce COS and SO2 during the pseudo-steady state process of continuous CS2 photo excitation reaction and CS2* quenching reaction. In addition, the solar flux spectrum in the troposphere is strong enough to trigger the above CS2 photochemistry.
In this study, an updated CS2 photochemical network is studied by a 1-D atmospheric photochemical model (PATMO). Reaction path analysis is carried out using an open-source reaction network viewer (ReNView). From the simulated result, major reduced sulfur species (CS2, COS and SO2) reproduce field measurements. When the zenith angle of sunlight is 0°, our result shows that the conversion ratio of CS2 to COS is 0.87. The reaction rate r for the net CS2 photo-induced oxidation and CS2 + OH reactions at 1 km are about 18 and 144 molecule cm-3 s-1 respectively. These results indicate that, under favorable light conditions photochemistry is a relevant tropospheric sink of CS2.
References
Khalil, M. and Rasmussen, R. (1984). Global sources, lifetimes and mass balances of carbonyl 761 sulfide (OCS) and carbon disulfide (CS2) in the earth’s atmosphere. Atmospheric Environment 762 (1967), 18(9):1805–1813.
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.
Stickel, R. E. et al. (1993), Journal of Physical Chemistry, 97(51), pp. 13653–13661.
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.