The influence of NaBr/NaCl ratio on the Br(-)-catalysed production of halogenated radicals

The activation of Br- and Cl- to atomic Br and Cl in sea-spray aerosol was investigated in smog-chamber experiments. In the presence of O-3, hydrocarbons and NaCl aerosol alone no activation was observed. By adding Bi to the aerosol, the chain reaction: Br + O-3 double right arrow BrO3 BrO + HO2 double right arrow HOBr, HOBr double right arrow HOBr(aq), HOBr(aq) + H-= + Br- double right arrow Br-2 (6), HOBr(aq) + H-= + Cl- double right arrow BrCl (7) was verified. The step from reaction (6) to (7) is accompanied by a decrease of the Br-/Cl- ratio from 1/600 to less than 1/2000. In the absence of sulphate, the chain is initiated by the reaction of OH(aq) with Br-. The pH value decreases to less than 2 during the first minutes of the experiment and later on to almost 1 (in the absence of NOx or SO2). This is caused by the formation of oxalic acid from alkanes and toluene. In stopped now experiments, the reduction of Br-2 by oxalic acid was observed to occur through a two-step mechanism: HC(2)O4(-) + Br-2 double left right arrow Br- + BrC2O4H (k(22), k(-22)), BrC2O4H double right arrow Br- + H+ + 2 CO2 (23) with the following rate constants and ratios of rate constants, k +/- 2 sigma: k(22)k(23)/k(-22) = (2.9 +/- 0.3) 10(-4) s(-1), k(-22)/k(23) = 7000 +/- (13000)(3000) M-1, k(22) = 2 +/- (4)(1) M-1 s(-1), and k(23) > 0.1 2(-1), k(-22) > 600 M-1 s(-1). Oxalic acid may be responsible for the inhibition of the chain reaction observed at the end of the experiments.