MECHANISTIC STUDIES OF THE OH-INITIATED OXIDATION OF CS2 IN THE PRESENCE OF O-2

We have investigated production of carbon- and sulfur-containing end p roducts of the OH-initiated oxidation of CS2 in the presence of Oz; an important atmospheric chemical reaction which is known to proceed via the following three elementary steps: OH + CS2 + M <-> CS2OH + M; CS2 OH + O-2 --> products. Two different experimental approaches were empl oyed. In one set of experiments (CP-FTIR studies) continuous photolysi s of CH3ONO/NO/CS2/Air mixtures at 298 K and 700-Torr total pressure w as combined with product detection by Fourier transform infrared;spect roscopy; these studies measured mores of products formed per mole of C S2 consumed. In a second set of experiments (LFP-TDLAS studies) 248-nm laser flash photolysis of H2O2/CS2/N2O/He/O-2 mixtures at 298 K and 2 5-100-Torr total pressure was combined with product detection by time- resolved tunable diode laser absorption spectroscopy; in this case, th e quantity measured was moles of product formed per mole of OK consume d. In both studies OCS and CO are observed as carbon-containing produc ts with yields of 0.83 +/- 0.08 and 0.16 +/- 0.03, respectively; uncer tainties represent estimates of absolute accuracy at the 95% confidenc e level. The LFP-TDLAS experiments demonstrate that the above yields r epresent: ''prompt'' product formation; i.e., OCS and CO are formed ei ther as primary products of the CS2OH + O-2 reaction or as products of a fast (k > 10(-15) cm(3) molecule(-1) s(-1)) secondary reaction of a primary product with O-2 The CP-FTIR experiments show that, under atm ospheric conditions, SO2 is produced with a yield of 1.15 +/- 0.10; in this case, the LFP-TDLAS results strongly suggest that only about thr ee-fourths of the SO2 is formed as a prompt product, with the remainde r generated via slow reaction of SO (generated as a prompt product of the CS2OH + O-2 reaction) with O-2. The implications of our results fo r understanding the detailed mechanism of the very complex CS2OH + O-2 reaction are discussed, as are their implications for understanding t he atmospheric cycles of CS2 and OCS.