The Kinetics and Products of the Hydroxyl Initiated Oxidation of Carbon Disulfide in Molecular Oxygen

Carbon disulfide (CS_2) is a trace constituent of the earth's atmosphere and is believed to be an important precursor of stratospheric aerosol. Laboratory experiments indicate that CS _2 is rapidly oxidized by OH in air. The following mechanism, OH + CS_2 + M = CS_2OH + M (i) CS_2 OH + O_2 to products (ii) has been proposed to explain the strong (O_2) and (M) dependencies of the CS_2 loss rate observed in the laboratory experiments. Our work was undertaken to resolve the oxidation mechanism and measure the kinetics and products of the elementary steps. The mechanism was first studied by monitoring (OH) vs time in CS_2 and O _2 using a pulsed laser photolysis-pulsed laser induced fluorescence (PLPLIF) apparatus. All observations were consistent with reactions (i) and (ii). The equilibrium constant for reaction (i) was measured as a function of temperature, and a Van't Hoff analysis yielded {Delta}H _sp{298}{circ} = -10.9 +/- 1.0 kcal mol ^{-1} and {Delta }S_sp{298}{circ } = -24.0 +/- 4.4 cal K^{-1}mol^ {-1}. The rate coefficient for reaction (ii) was also measured using the PLPLIF apparatus, giving k_{rm ii} = (2.6 +/- 1.0) times 10^{-14}cm^3 molecule^{-1}s ^{-1} (249-299 K, 50 Torr He) which was independent of H/D isotopic substitution (k_{rm H}/k _{rm D} = 1.05 +/- 0.18). The products of reaction (ii) and subsequent steps involving O_2 were also investigated. HO _2 production was identified using a laser magnetic resonance (LMR) spectrometer coupled to a discharge-flow reactor. The HO_2 yield was measured with the PLPLIF apparatus by modeling the OH temporal profiles with NO added to convert the HO _2 product to OH. The yield of HO _2 per CS_2OH consumed by O_2 is 95 +/- 15%. SO_2 was also identified as a major product using a discharge-flow reactor coupled to a chemical ionization mass spectrometer (CIMS). Analysis of the (OH) and (SO_2) as a function of (CS_2), (O_2), and time indicated that the SO_2 yield per CS_2OH oxidized by O _2 is 90 +/- 20%. Similar experiments with ^{18}OH demonstrated that the hydroxyl oxygen atom is found in the SO_2 product. These results are discussed with respect to the understanding of the oxidation mechanism and the atmospheric chemistry of CS_2.