Behnke, W; Elend, M; Krüger, U; Zetzsch, C: The influence of NaBr/NaCl ratio on the Br-- catalysed production of halogenated radicals, Journal of Atmospheric Chemistry, 34, 87-99 (1999)
Abstract:
The activation of Br- and Cl- to at. Br and Cl in sea-spray aerosol was investigated in smog-chamber expts. In the presence of O3, hydrocarbons and NaCl aerosol alone no activation was obsd. By adding Br- to the aerosol, the chain reaction: Br + O3 .dblarw. BrO, BrO + HO2 .dblarw. HOBr, HOBr .dblarw. HOBr(aq), HOBr(aq) + H+ + Br-.dblarw. Br2 (6), HOBr(aq) + H+ + Cl-.dblarw. 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 sulfate, 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 expt. 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 flow expts., the redn. of Br2 by oxalic acid was obsd. to occur through a two-step mechanism: HC2O4- + Br2.dblarw. Br- + BrC2O4H (k22, k-22), BrC2O4H.dblarw. Br- + H+ + 2 CO2 (23) with the following rate consts. and ratios of rate consts., k  2: k22k23/k-22 = (2.9  0.3)  10-4 s-1, k-22/k23 = 7000 300013000 M-1, k22 = 2  14 M-1 s-1, and k23 > 0.1 s-1, k-22 > 600 M-1 s-1. Oxalic acid may be responsible for the inhibition of the chain reaction obsd. at the end of the expts.
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