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Tropospheric Halogen Oxides BrO and IO

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The CO measurements over clouded ocean scenes have been compared with co-located modelled CO columns over the same clouds and agree well. Using clouded ocean scenes quadruples the number of useful CO measurements compared to land-only measurements. The five-year dataset shows significant inter-annual variability over land and over clouded ocean areas, like Asian outflow of pollution over the northern Pacific, biomass-burning outflow over the Indian Ocean originating from Indonesia, and biomass burning in Brazil. In general, there is good agreement between observed and modelled seasonal cycles and inter-annual variability.
Assimilation of SCIAMACHY CO data was also used to improve CO emission estimates in the Middle East (Tangborn et al. 2009). One remarkable result is the finding that CO emissions over Dubai have more than doubled in 2004 compared to those in the available emissions inventory based on data from 1998.

Tropospheric Halogen Oxides – BrO and IO

BrO and IO are reactive halogen radicals which have gained growing interest in recent years (Simpson et al. 2007). Both compounds impact on tropospheric chemistry. They react with ozone and change the oxidation pathways of several atmospheric species. Bromine compounds have been identified as initiator of strong boundary layer ozone depletion events in polar spring (Barrie et al. 1988). The IO molecule constitutes the starting point for iodine nucleation and the formation of fine aerosol particles, which affect the atmospheric radiation balance and which may potentially grow to cloud condensation nuclei (O’Dowd et al. 2002). Widespread plumes of enhanced BrO are regularly observed in the Arctic, as well as in the Antarctic, shortly after polar sunrise where they persist for several months. The spatial distributions and locations of these plumes move rapidly on a daily basis (Begoin et al. 2010), with the BrO probably not only being situated in the boundary layer but also at higher altitudes. A strong link exists between the BrO patterns and sea ice cover. Sources of BrO are most likely of inorganic nature. Current discussions consider young sea ice, frost flowers, aerosols and brine (Kaleschke et al. 2004; Piot and von Glasow, 2008).

Recent analyses of SCIAMACHY nadir observations using spectral data around 420 nm have enabled the detection of tropospheric IO columns (Saiz-Lopez et al. 2007, Schönhardt et al. 2008). IO amounts are small and close to the instrument’s detection limit, but through efficient reaction cycles, even these small amounts still have a considerable impact on the polar tropospheric chemistry. A variety of details in the temporal and spatial distribution of both IO and BrO over the Antarctic polar region is revealed in fig. 3-17. Monthly means from October through austral summer until March are shown, in each case data is averaged over four subsequent years. As for BrO, largest amounts of IO appear in Antarctic spring time. Besides this general similarity, spatial distributions are quite different. BrO is observed predominantly above sea ice regions during spring, and furthermore along coast lines and on shelf ice regions in summer. Abundances vanish towards autumn. IO, however, shows larger variability throughout the time series. Regions with enhanced IO include the sea ice, ice shelves, coast lines, but also the continent (in October). Enhanced IO above the sea ice in the characteristic ring-like pattern only occurs much later in spring (November) in contrast to BrO, where enhancements can already be observed in August well before the time period illustrated in fig. 3-13. When southern autumn approaches, IO concentrations begin to increase again. While BrO behaves similarly on both hemispheres, no widespread enhanced IO abundances are observed in the Arctic spring time.

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