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Abstract:
Halogens in the atmosphere chemically destroy ozone. In the troposphere,
bromine has higher ozone destruction efficiency than chlorine and is the halogen
species with the widest geographical spread of natural sources. We investigate the relative strength of various sources of reactive tropospheric bromine and the
influence of bromine on tropospheric chemistry using a 6-year simulation with the
global chemistry transport model MOZART4. We consider the following sources: short-lived bromocarbons (CHBr3, CH2BrCl, CHBr2Cl, CHBrCl2, andCH2Br2) and CH3Br, bromine from airborne sea salt particles, and frost flowers and sea salt on or in the snowpack in polar regions. The total bromine emissions in our simulations
add up to 31.7 Gmol(Br)/yr: 63 % from polar sources, 24.6 % from short-lived
bromocarbons and 12.4 % from airborne sea salt particles. We conclude from our
analysis that our global bromine emission is likely to be on the lower end of the range,
because of too low emissions from airborne sea salt. Bromine chemistry has an effect
on the oxidation capacity of the troposphere, not only due to its direct influence on ozone concentrations, but also by reactions with other key chemical species like HOx and NOx. Globally, the impact of bromine chemistry on tropospheric O3 is comparable to the impact of gas-phase sulfur chemistry, since the inclusion of bromine chemistry in MOZART4 leads to a decrease of the O3 burden in the troposphere by 6 Tg, while we get an increase by 5 Tg if gas-phase sulfur chemistry is switched off in the standard model. With decreased ozone burden, the simulated oxidizing capacity of the atmosphere decreases thus affecting species associated with the oxidation capacity of the atmosphere (CH3OOH, H2O2).
