Kinetics and products of the reactions BrO+DMS and Br+DMS at 298 K

The kinetics of the reaction BrO + DMS --> products(1), were examined by us e of pulsed-laser photolytic generation and time-resolved detection of the BrO radical by absorption spectroscopy at total pressures of 60, 100, and 2 00 Torr N-2 (1 Torr = 133.322 Pa). A value of k(1) = (4.40 +/- 0.66) x 10(- 13) cm(3) s(-1) was obtained independent of pressure at 295 K. This value i s significantly higher than that determined previously in low-pressure (<4 Ton He), discharge flow measurements (2.6 x 10(-13) cm(3) s(-1)). By observ ing 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 cr oss section. The impact of these results on the chemistry of the remote mar ine boundary layer was assessed using a chemical box model. At daytime conc entrations of 1-2 pmol/mol, the BrO radical was found to be an important si nk for DMS and the dominant source of DMSO. In a second set of experiments, pulsed-laser photolysis coupled with resonance fluorescence detection of B r atoms was employed to study the equilibrium kinetics of Br + DMS + M <-> Br-DMS + M (4, -4) at 100 Torr N-2 and 295 K. Values of k(4) = (6.36 +/- 0. 43) x 10(-11) cm(3) s(-1) and k(-4) = (1.02 +/- 0.07) x 10(4) s(-1) were ob tained, and were used to calculate the equilibrium constant K-4 = (6.24 +/- 0.56) x 10(-15) cm(3). The uncertainty is 2 sigma plus estimated systemati c error. At high [Br] ((1-3) x 10(12) cm(-3)), Br atoms are lost from equil ibrium via the fast reaction Br + Br-DMS --> Br-2 + DMS (5), and a value of k(5) = (4.2(-1.2)(+2.3)) x 10(-10) cm(3) s(-1) was obtained. The uncertain ty is 2 sigma plus the major systematic error incurred by estimating [Br](o ) from laser fluence measurements. Pulsed-laser photolysis combined with ti me-resolved UV absorption at selected wavelengths and diode array measureme nts in the wavelength range 300-450 nm showed a strong absorption centered at 365 nm due to Br-DMS. A value of sigma(max)(365nm) = (2.74(-1.1)(+1.6)) x 10(-17) cm(2) was obtained by fitting to the time-resolved absorption sig nal due to Br-DMS. The uncertainty is 2 sigma plus systematic error (as abo ve).