Photolysis of SO2 Isotopologues in a Low O2 Atmosphere

Radiative transfer calculations were performed in the context of modeling the photochemistry of sulfur in the early Earth atmosphere. Rather than present an in-depth discussion of fhe many photochemical possibilities [29], my emphasis here is on the dissociation rates of SO2 isotopologues. However, because absorption by CO2, H2O and other possible compounds will alter the dissociation rates of SO2, particularly at short wavelengths ( 200 nm), the radiative transfer and photochemistry are necessarily linked. The photodissociation rate coefficient for the S02 isotopologue (x = 32, 33,34 or 36) from 190 to 220 nm is given by 

The photodissociation rate coefficients are included as source and sink terms in a system of time-dependent continuity equations for the atmosphere. Modem values for vertical (eddy) diffusion and solar photon flux are utilized. The system of 2nd-order ordinary differential equations is solved by integration, and yields chemical species abundances as a function of time and altitude. The isotope atmospheric chemistry includes only SO2 isotopologue photodissociation reactions and production of SO isotopologues. Additional isotopic reactions such as SO2 oxidation by OH, SO photolysis, SO disproportionation during self-reaction, and SO dimmer formation, have been neglected. My objective here is to focus only on SO2 photolysis as a S-MIF mechanism. 

FIGURE 5.5 The variation with altitude of SO2 isotopologue photolysis rates and rate coefficient delta values in the model atmosphere. Results are shown at a time corresponding to a 10% reduction in the initial SO2 column density, (a) Photolysis rates for each isotopologue, illustrating that the rate increases with the magnitude of the band shift, (b) Rate coefficient 

The model also predicts A S/A S---2, which lies between the Antarctic ice 

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