Kinetics of the Gas-Phase Reaction of BrNO2 with NO

Abstract

BrNO2 was prepared in situ in a static reactor (v = 420 L) by photolyzing Br2/NO2/N2 mixtures in the wavelength range 500−700 nm at temperatures between 263 and 294 K. After the lights were switched off, the excess NO was added, and IR and UV spectra were monitored simultaneously as a function of time. From the pseudo-first-order decay of the IR absorption of BrNO2 in the presence of a large excess of NO, the second-order rate constant for reaction 4, BrNO2 + NO ⇒ BrNO + NO2, was determined to be k4 = 2.3 × 10-12 exp[(−17.8 ± 2.1) kJ mol-1/RT] cm3molecule-1s-1 (2σ). The measured yields of BrNO were close to 100% (98 ± 5%). These results suggest that reaction 4 is unimportant as a loss process of BrNO2 under most tropospheric conditions. Additional experiments on the thermal stability of BrNO2 led to an upper limit of 4.0 × 10-4 s-1 for its thermal gas-phase decomposition rate constant at 298 K in 1 atm of synthetic air. Finally, the mechanism of the Br + NO2 reaction and the thermochemistry of BrNO2 and BrONO are discussed in light of the results of the present experiments and of previous work from the literature.