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Bromine monoxide (BrO) is a key radical in the atmosphere, influencing the chemical state of the atmosphere, most notably the abundance of ozone (O3). O3 depletion caused by the release of bromine has been observed and modeled in polar regions, salt pans, and in particular inside volcanic plumes. Furthermore, the molar ratio of BrO and SO2 – which can be detected simultaneously via spectroscopic measurements using the differential optical absorption spectroscopy (DOAS) method – is a proxy for the magmatic composition of a volcano and potentially an eruption forecast parameter. The detection of BrO in volcanic plumes from satellite spectroscopic observations is limited by the precision and sensitivity of the retrieval, which so far only allowed for the detection of BrO during major eruptions. The unprecedented spatial resolution of up to 3.5km×5.5km and the high signal-to-noise ratio of the TROPOspheric Monitoring Instrument (TROPOMI) on board Sentinel-5 Precursor (S-5P) enable observing and monitoring volcanic bromine release globally even for minor eruptions or even quiescent degassing. In this study, we investigate how far the BrO retrieval can be improved using TROPOMI data and how well BrO can be detected, even in small eruptions and during quiescent volcanic degassing. There are two steps for which improvements in accuracy are investigated and applied: the improvement and quantitative determination of (1) the detection limit of the DOAS BrO column retrieval and (2) the correction of the non-volcanic background BrO signal. First, the DOAS retrieval settings are varied, and their influence on accuracy and precision is investigated with respect to the detection limit and potential systematic influences. Based on these results, we propose a dedicated DOAS evaluation scheme optimized for the detection of BrO in volcanic plumes. For the DOAS retrieval, we propose the use of a large fit window from 323–360 nm , yielding a statistical uncertainty lower by a factor of 1.8 compared to previous BrO DOAS algorithms while not enhancing systematic influences. Second, the effect of the background BrO is reduced by a latitude-dependent empirical correction scheme correlated to cloud information as well as information on the O3 column. Via these improvements, the combined statistical and systematic uncertainties in the resulting BrO vertical column density is on the order of 7×1012moleculescm-2. We present a new and accurate retrieval algorithm of BrO columns from TROPOMI observations which allows for the detection of even slightly enhanced BrO amounts inside minor eruptive plumes of bromine-rich volcanoes. While designed specifically for TROPOMI observations, the retrieval algorithm is in general also applicable to other hyperspectral satellite observations. However, some parts might require adaptation. [ABSTRACT FROM AUTHOR]

Bromine monoxide (BrO) is a key radical in the atmosphere, influencing the chemical state of the atmosphere, most notably the abundance of ozone. The main effect of BrO onto tropospheric ozone concentrations occurs in bromine release events in polar regions, salt pans and volcanic plumes. Ozone depletion caused by halogen release has been observed and modeled for such conditions, in particular inside volcanic plumes. Furthermore, the molar bromine to sulphur ratio in volcanic plumes is a proxy for the magmatic composition of a volcano and potentially an eruption forecast parameter. The integrated column of BrO in the atmosphere, which in turn serves as an estimate for the bromine content, can be detected simultaneously with SO2 via spectroscopic measurements using the Differential Optical Absorption Spectroscopy (DOAS). Thus, a direct derivation of the BrO/SO2 ratio can be performed from a single measurement. Satellite spectroscopic observations offer the potential to observe and monitor volcanic bromine release globally. The detection of BrO in volcanic plumes is limited by the precision and sensitivity of the retrieval, which so far only allowed for the detection of BrO during major eruptions, leading to a potential sampling bias when looking at the BrO/SO2 ratio. The The TROPospheric Monitoring Instrument (TROPOMI) onboard Sentinel-5 Precursor (S-5P) however, with its unprecedented spatial resolution of up to 3.5 × 5.5 km2 and high signal-to-noise ratio, enables the detection of BrO in minor eruptions or even quiescent degassing. In this study, we investigate, how far the BrO retrieval can be improved using TROPOMI data and how well BrO can be detected, even in small eruptions and during quiescent volcanic degassing. There are two steps, for which improvements in accuracy are investigated and applied: the improvement and quantitative determination of (1) the detection limit of the DOAS BrO column retrieval and (2) the correction of non-volcanic background BrO signal. First, the DOAS retrieval settings are varied and their influence on accuracy and precision is investigated with respect to the detection limit and potential systematic influences. Based on these results, we propose a dedicated DOAS evaluation scheme optimized for the detection of BrO in volcanic plumes. For the DOAS retrieval, we propose the use of a large fit window from 323–360 nm, yielding a factor of 1.8 lower statistical uncertainty compared to previous BrO DOAS algorithms, while not enhancing systematic influences. Second, the effect of the background BrO is reduced by a latitude dependent empirical correction scheme correlated to cloud information as well as information on the ozone column. Via these improvements, the combined statistical and systematic uncertainties of the resulting BrO vertical column density is in the order of 7 × 1012 molecules cm−2, which allows for the detection of even slightly enhanced BrO amounts inside minor eruptive plumes of bromine-rich volcanoes. [ABSTRACT FROM AUTHOR]

Tropospheric bromine release and ozone depletion events (ODEs) as they commonly occur in the Arctic spring are studied using the regional software WRF-Chem. For this purpose, the MOZART-MOSAIC chemical reaction mechanism is extended by bromine and chlorine reactions as well as an emission mechanism for reactive bromine via heterogeneous reactions on ice and snow surfaces. The simulation domain covers an area of 5,040 km x 4,960 km, centered north of Utqiagvik (formerly Barrow), Alaska, and the time interval from February through May, 2009. Several simulations for different strengths of the bromine emission are conducted and evaluated by comparison with in-situ and ozone-sonde measurements of ozone mixing ratios as well as by comparison with tropospheric BrO vertical column densities (VCDs) from the Global Ozone Monitoring Experiment-2 (GOME-2) satellite instrument. The base bromine emission scheme includes the direct emission of bromine due to bromide oxidation by ozone through the reactive surface ratio β of the ice/snow surface relative to a flat surface. 10 Results of simulations with β = 1.0 agree well with the observations, however, a value of 1.5 performs somewhat better. The bromine emission due to bromide oxidation by ozone is found to be important to provide an initial seed for the bromine explosion. Consideration of halogen chemistry substantially improves the prediction of the ozone mixing ratio with respect to the observations. Meteorological nudging is found to be essential for a good prediction of ODEs over the three months period. [ABSTRACT FROM AUTHOR]

Clouds impact the radiative transfer of the Earth's atmosphere and strongly influence satellite measurements in the UV visible and IR spectral ranges. For satellite measurements of trace gases absorbing in the UV/vis spectral range, particularly clouds ultimately determine the vertical sensitivity profile, mainly by reducing the sensitivity for trace gas columns below the cloud. The Mainz Iterative Cloud Retrieval Utilities (MICRU) algorithm is specifically designed to reduce the error budget of trace gas retrievals, such as those for nitrogen dioxide (NO2), which strongly depends on the accuracy of small cloud fractions (CF) in particular. The accuracy of MICRU is governed by an empirical parametrisation of the viewing geometry dependent background surface reflectivity taking instrumental and physical effects into account. Instrumental effects are mainly degradation and polarisation effects, physical effects are due to the anisotropy of the surface reflectivity, e.g. shadowing of plants and sun glitter. MICRU is applied to main science channel (MSC) and polarisation measuring device (PMD) data collected between April 2007 and June 2013 by the GOME-2A instrument onboard the MetOp-A satellite. CF are retrieved at different spectral bands between 374 and 758 nm. The MICRU results for MSC and PMD at different wavelengths are inter-compared to study CF precision and accuracy, which depend on wavelength and spatial correlation. Furthermore, MICRU results are compared to FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A band) and OCRA (Optical Cloud Recognition Algorithm) operational cloud products. We show that MICRU retrieves small CF with an accuracy of 0.04 or better for the entire 1920 km wide swath with a potential bias between -0.01 and -0.03. CF retrieved at shorter wavelengths are less affected by adverse surface heterogeneities. The comparison to the operational CF algorithms shows that MICRU significantly reduces the dependence on viewing angle, time, and sun glitter. Systematic effects along coasts are particularly small for MICRU due to its dedicated treatment of land and ocean surfaces. The MICRU algorithm is designed for spectroscopic instruments ranging from the GOME to TROPOMI/Sentinel-5P, but is also applicable to UV/vis imagers like, for example, AVHRR, MODIS, VIIRS, and Sentinel-2. [ABSTRACT FROM AUTHOR]