Kinetics and Products of the Reactions BrO + DMS and Br + DMS at 298 K

T. Ingham, D. Bauer, R. Sander, P. J. Crutzen & J. N. Crowley

J. Phys. Chem. A, 103, 7199-7209 (1999)
Abstract. The kinetics of the reaction BrO + DMS -> products (1), were examined by use of Pulsed-Laser Photolytic generation and time-resolved detection of the BrO radical by absorption spectroscopy at total pressures of 60, 100, and 200 Torr N2 (1 Torr = 133.322 Pa). A value of k1 = (4.40±0.66)x10-13 cm3 s-1 was obtained independent of pressure at 295 K. This value is significantly higher than that determined previously in low pressure (< 4 Torr He), discharge flow measurements (2.6x10-13 cm3 s-1). By observing the formation of DMSO directly, we obtain a value of 1.17± 0.34 for its yield in reaction (1); the major uncertainty is the ± 30% in the DMSO cross section. The impact of these results on the chemistry of the remote marine boundary layer was assessed using a chemical box model. At daytime concentrations of 1-2 pmol/mol, the BrO radical was found to be an important sink for DMS and the dominant source of DMSO.

In a second set of experiments, Pulsed-Laser Photolysis coupled with Resonance Fluorescence detection of Br-atoms was employed to study the equilibrium kinetics of Br + DMS + M -> Br-DMS + M (4, -4) at 100 Torr N2 and 295 K. Values of k4 = (6.36±0.43)x10-11 cm3 s-1 and k-4 = (1.02±0.07)x104 s-1 were obtained, and were used to calculate the equilibrium constant K4 = (6.24±0.56)x10-15 cm3. The uncertainty is 2 sigma plus estimated systematic error. At high [Br] (1-3x1012 cm-3), Br-atoms are lost from equilibrium via the fast reaction Br + Br-DMS <-> Br2 + DMS (5), and a value of k5 = 4.2x10-10 cm3 s-1 was obtained. The uncertainty is 2 sigma plus the major systematic error incurred by estimating [Br]o from laser fluence measurements. Pulsed-Laser Photolysis combined with time-resolved UV-absorption at selected wavelengths and diode array measurements in the wavelength range 300 to 450 nm showed strong absorption centred at 365 nm due to Br-DMS. A value of sigmamax(365nm) = 2.74x10-17 cm2 was obtained via fits to the time-resolved absorption signal due to Br-DMS. The uncertainty is 2 sigma plus systematic error (as above).

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Rolf Sander (20 Oct 1999)