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A laser flash photolysis/long path absorption technique has been employed to study at 298 K the kinetics of aqueous phase reactions of the SO4− radical with a number of species commonly found in cloud water. Much lower radical concentrations were employed than in all previous direct studies of SO4− reaction kinetics. In the zero ionic strength limit, rate coefficients for SO4− reactions with the anions HSO3−, Cl−, NO2−, HCOO−, and CH3COO− are found to be 7.5 × 108, 2.6 × 108, 9.8 × 108, 1.1 × 108, and 3.7 × 106 M−1 s−1, respectively. Rate coefficients for SO4− reactions with the neutral species H2O2, CH3OH, HCOOH, and CH3COOH are found to be 1.2 × 107, 8.8 × 106, 4.6 × 105, and 1.4 × 104 M−1 s−1, respectively. For many of the reactions studied, agreement with previously reported rate data is poor. Of particular importance for cloud chemistry is the fact that the SO4− + HSO3− reaction proceeds more slowly than previously believed, while the SO4− + Cl− reaction is somewhat faster than previously thought. Incorporation of these results into cloud chemistry models should reduce the calculated efficiency of free radical chain reactions as an oxidation mechanism for S IV.

The recombination of methyl radicals is the major loss process for methyl in the atmospheres of Saturn and Neptune. The serious disagreement between observed and calculated levels of CH has led to suggestions that the atmospheric models greatly underestimated the loss of CH due to poor knowledge of the rate of the reaction CH + CH + M → CH + M at the low temperatures and pressures of these atmospheric systems. In an attempt to resolve this problem, the absolute rate constant for the self-reaction of CH has been measured using the discharge-flow kinetic technique coupled to mass spectrometric detection at T = 202 and 298 K and P = 0.6-2.0 Torr nominal pressure (He). CH was produced by the reaction of F with CH, with [CH] in large excess over [F], and detected by low energy (11 eV) electron impact ionization at m/ z = 15. The results were obtained by graphical analysis of plots of the reciprocal of the CH signal vs reaction time. At T = 298 K, k (0.6 Torr) = (2.15 ± 0.42) × 10 cm molecule s and k (1 Torr) = (2.44 ± 0.52) × 10 cm molecule s. At T = 202 K, the rate constant increased from k (0.6 Torr) = (5.04 ± 1.15) × 10 cm molecule s to k (1.0 Torr) = (5.25 ± 1.43) × 10 cm molecule s to k (2.0 Torr) = (6.52 ± 1.54) × 10 cm molecule s, indicating that the reaction is in the falloff region. Klippenstein and Harding had previously calculated rate constant falloff curves for this self-reaction in Ar buffer gas. Transforming these results for a He buffer gas suggest little change in the energy removal per collision, -〈Δ E〉, with decreasing temperature and also indicate that -〈Δ E〉 for He buffer gas is approximately half of that for Argon. Since the experimental results seem to at least partially affirm the validity of the Klippenstein and Harding calculations, we suggest that, in atmospheric models of the outer planets, use of the theoretical results for k is preferable to extrapolation of laboratory data to pressures and temperatures well beyond the range of the experiments.