BrCl chemistry

Crutzen and co-workers (Sander and Crutzen, 1996 Vogt et al., 1996) have developed a model for chemistry in the marine boundary layer at midlatitudes, in which autocatalytic cycles involving sea salt particles generate photochemically active gases such as BrCl, Br2, and Cl2. It is likely that such chemistry also occurs in the Arctic as well. In these cycles, reactions (125) and (126) in the condensed phase,... 

The situation regarding the importance of chlorine in polar springtime surface ozone chemistry remains rather confusing. Contrary to most information, newer measurements of BrCl volume mixing ratios by up to 35pmolmoP, comparable to those of Br2 (=27 pmol moP ),... 

Michalowski et al. (2000) calculated that the source of bromine from Br2 photolysis is about an order of magnitude larger than bromine release from BrCl after sunrise. Following BrO formation from the reaction of Br with O3 and HOBr formation by HO2 -b BrO HOBr -b O2 (namely, (4)), further reactions of HOBr with Cl and Br would give rise to enhanced levels of Br2 and BrCl. They also found that heterogeneous halogen chemistry within the snowpack was necessary for the O3 depletion events to occur and that the mass transfer of HOBr to the snowpack is a rate-limiting step. 

A crucial parameter is the sea-salt aerosol content, which is largely determined by the wind speed. If we assume a 5 times higher aerosol content (15 x 10 m (liquid), per m (air), that is, 47 pg NaCl per m (air)), which is at the upper range of values typically found in the MBL, the reactive halogen concentrations increase dramatically. We estimate maximum concentrations (pmol moF) after two days of [HCl] = 70, [HOCl] = 33, [HOBr] = 35, [HBr] = 3, [BrCl] = 38, [Br2] = 10 and [CI2] = 1.5. Catalytic ozone loss through bromine and chlorine during the second day increases to 1.4 or 0.6 nmol moF, respectively, corresponding to 58 % or 25 % of the ozone lost by photolysis and HOx chemistry. 

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