The bromine monoxide radical (BrO) is well known for its role in stratospheric ozone destruction, in particular in combination with ClO. In contrast to chlorine, bromine is in its active forms in the sunlit stratosphere and BrO is one of the dominant bromine species during daytime. While overall the contribution of bromine to stratospheric ozone loss is smaller than that of chlorine as result of its lower abundance, emission of source gases is not yet declining as is the case for chlorine. Thus, the relative role of bromine could still be increasing.
Large concentrations of BrO are also observed in the polar boundary layer in spring time during so called ozone depletion events when ozone levels close to the surface are strongly reduced. This bromine originates from sea salt and is released to the gas phase through heterogeneous processes on ice or snow the details of which are not yet fully understood. One hypothesis is that concentrated brine on frost flowers acts as the source either directly or once the frost flowers disintegrate into wind blown aerosol. Once bromine is released to the gas phase, BrO is formed and can be amplified through an autocatalytic release from aerosols leading to "bromine explosions" with rapid ozone loss. An example of typical BrO fields during polar spring are given in the figure above. Please refer to the data section for a discussion of issues related to BrO retrieval from satellite.
From ground-based DOAS measurements, comparison of satellite and ground-based measurements and also from balloon-borne measurements there is indication for the presence of a BrO background of about 1ppt in the free troposphere. The source of this BrO could be either short lived organic compounds, transport of air affected by polar bromine explosions or volcanic eruptions. In the absence of direct measurements, this tropospheric background is still under debate.
Multi-axis DOAS measurements near active volcanoes provide evidence for very large BrO concentrations in some volcanic plumes. The signal has been observed at several different volcanoes, albeit at different levels relative to SO2 concentrations. No BrO could be observed in GOME and SCIAMACHY measurements during large volcanic eruptions, indicating that under these conditions BrO emissions are much smaller than during degassing.
When using satellite retrieved BrO columns, one as to keep in mind that measurements in the UV/visible spectral range do not provide vertical resolution for BrO. Thus, it can not easily be decided which part of the signal originates from the boundary layer, the free troposphere or the stratosphere. Unfortunately, the sensitivity of the measurements to BrO varies with altitude as a function of surface reflectivity (this is illustrated in our interactive airmass factor calculator). Proper analysis of the measurements therefore requires an assumption on the vertical distribution of the BrO (e.g. from model calculations) and knowledge of surface albedo and cloud cover, cloud top altitude, and cloud optical depth. While we a re currently working on such a detailed retrieval using nadir measurements in combination with SCIAMACHY limb profiles, the data shown on this page and in the data browser are calculated assuming a stratospheric profile. They quite accurate for regions where the stratosphere and upper troposphere dominates, underestimate the total column over dark surfaces if significant BrO amounts are present in the troposphere and overestimate the total column over bright surfaces if BrO is present in the lower atmosphere.
An additional complication arises for SCIAMACHY data from an instrumental feature in the spectral region traditionally used for the BrO retrieval (345 - 359 nm). To avoid the large noise resulting from this feature, the BrO retrieval for SCIAMACHY is performed in the 336 - 347 nm region where a low bias in BrO is observed in the presence of large HCHO signals, e.g. over rain forests or during large biomass burning.
A short description of the algorithm used for the BrO data product can be found in the BrO Algorithm Description.
If you are interested in more information or SCIAMACHY BrO data, please contact Andreas Richter.