Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor (S-5P) platform will measure ultraviolet earthshine radiances at high spectral and improved spatial resolution (pixel size of 7 km × 3.5 km at nadir) compared to its predecessors OMI and GOME-2. This paper presents the sulfur dioxide (SO 2 ) vertical column retrieval algorithm implemented in the S-5P operational processor UPAS (Universal Processor for UV/VIS Atmospheric Spectrometers) and comprehensively describes its various retrieval steps. The spectral fitting is performed using the differential optical absorption spectroscopy (DOAS) method including multiple fitting windows to cope with the large range of atmospheric SO 2 columns encountered. It is followed by a slant column background correction scheme to reduce possible biases or across-track-dependent artifacts in the data. The SO 2 vertical columns are obtained by applying air mass factors (AMFs) calculated for a set of representative a priori profiles and accounting for various parameters influencing the retrieval sensitivity to SO 2 . Finally, the algorithm includes an error analysis module which is fully described here. We also discuss verification results (as part of the algorithm development) and future validation needs of the TROPOMI SO 2 algorithm.

During recent years, volcanic emissions turned out to be a natural source of bromine compounds in the atmosphere. While the initial formation process of bromine monoxide (BrO) has been successfully studied in local ground-based measurements at quiescent degassing volcanoes worldwide, literature on the chemical evolution of BrO on large spatial and temporal scales is sparse. The first space-based observation of a volcanic BrO plume following the Kasatochi eruption in 2008 demonstrated the capability of satellite instruments to monitor such events on a global scale.

In this study, we systematically examined GOME-2 observations from January 2007 until June 2011 for significantly enhanced BrO slant column densities (SCDs) in the vicinity of volcanic plumes. In total, 772 plumes from at least 37 volcanoes have been found by using sulphur dioxide (SO₂) as a tracer for a volcanic plume. All captured SO₂ plumes were subsequently analysed for a simultaneous enhancement of BrO and the data were checked for a possible spatial correlation between the two species. Additionally, the mean BrO/SO₂ ratios for all volcanic plumes have been calculated by the application of a bivariate linear fit.

A total number of 64 volcanic plumes from at least 11 different volcanoes showed clear evidence for BrO of volcanic origin, revealing large differences in the BrO/SO₂ ratios (ranging from some 10⁻⁵ to several 10⁻⁴) and the spatial distribution of both species. A close correlation between SO₂ and BrO occurred only for some of the observed eruptions or just in certain parts of the examined plumes. For other cases, only a rough spatial relationship was found. We discuss possible explanations for the occurrence of the different spatial SO₂ and BrO distributions in aged volcanic plumes

Abstract.We present an analysis of SO 2 column densities derived from GOME-2 satellite measurements for the Kılauea volcano (Hawai'i) for [2007][2008][2009][2010][2011][2012]. During a period of enhanced degassing activity in March-November 2008, monthly mean SO 2 emission rates and effective SO 2 lifetimes are determined simultaneously from the observed downwind plume evolution and meteorological wind fields, without further model input. Kılauea is particularly suited for quantitative investigations from satellite observations owing to the absence of interfering sources, the clearly defined downwind plumes caused by steady trade winds, and generally low cloud fractions. For March-November 2008, the effective SO 2 lifetime is 1-2 days, and Kılauea SO 2 emission rates are 9-21 kt day −1 , which is about 3 times higher than initially reported from ground-based monitoring systems.