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

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) × 10-13 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 (2.6 × 10-13 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) × 10-11 cm3 s-1 and k-4 = (1.02 ± 0.07) × 104 s-1 were obtained, and were used to calculate the equilibrium constant K4 = (6.24 ± 0.56) × 10-15 cm3. The uncertainty is 2σ plus estimated systematic error. At high [Br] ((1−3) × 1012 cm-3), Br atoms are lost from equilibrium via the fast reaction Br + Br−DMS → Br2 + DMS (5), and a value of k5 = ( ) × 10-10 cm3 s-1 was obtained. The uncertainty is 2σ 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 300−450 nm showed a strong absorption centered at 365 nm due to Br−DMS. A value of = ( ) × 10-17 cm2 was obtained by fitting to the time-resolved absorption signal due to Br−DMS. The uncertainty is 2σ plus systematic error (as above).