23 May 2023
 | 23 May 2023
Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

A new accurate retrieval of bromine monoxide inside minor volcanic plumes from Sentinel-5 Precursor/TROPOMI

Simon Warnach, Holger Sihler, Christian Borger, Nicole Bobrowski, Steffen Beirle, Ulrich Platt, and Thomas Wagner

Abstract. 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.

Simon Warnach et al.

Status: open (until 30 Jun 2023)

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Simon Warnach et al.

Simon Warnach et al.


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Short summary
The paper discusses how to best detect gases inside volcanic plumes, namely BrO. Volcanic gases correspond to the magmatic signal and can be used to characterize the volcano. Satellite observations of high accuracy can provide a global inventory of this important quantity. The increase in accuracy provided in this paper can pave the way towards a most-complete inventory of volcanic gas composition.