Spatial Distribution of Halogen Oxides in the Plume of Mount Pagan Volcano, Mariana Islands

Abstract

Halogens are emitted from volcanoes primarily as hydrogen halides (HCl, HF, HBr, and HI). Upon mixing with the atmosphere, chlorine and bromine species are partially converted to the halogen oxides OClO and BrO. Here we report on the spatial distribution of BrO and OClO in the gas plume emitted from Mount Pagan volcano, Northern Mariana Islands. We found enhanced BrO/SO 2 ratios near the plume edges and a lack of OClO in the plume's core. Our results highlight the importance of in-mixing of atmospheric oxidants for halogen oxide formation. They indicate that OClO can only be formed after most bromide dissolved in plume aerosols has been released to the gas phase. We conclude that Mount Pagan's gas emissions originated from a shallow magma body and were transported to the surface along dry degassing pathways and that the volcano's halogen emissions likely had significant impact on the oxidation capacity of the downwind atmosphere.Plain Language Summary The halogens chlorine, fluorine, bromine, and iodine are commonly degassed from volcanoes and can cause ozone destruction in downwind areas. This occurs through a series of complex chemical reactions taking place within the first few minutes after emission that convert part of the emitted bromine and chlorine into bromine monoxide (BrO) and chlorine dioxide (OClO). These halogen oxides can be measured using remote sensing instruments commonly used to monitor volcanic degassing around the world. In this study, we examine the spatial distribution of BrO and OClO in the plume of Mount Pagan volcano. Our measurements allow insights into the chemical processes responsible for BrO and OClO formation. We find that in-mixing of background air is of fundamental importance for the reaction scheme and that BrO is formed preferentially over OClO. This information will allow volcanologists to better link bromine emissions to associated volcanic activity and help in assessing the impact of volcanic halogen degassing on atmospheric chemistry.

Advances in Bromine Speciation in Volcanic Plumes

“…The measurements are very consistent with the horizontal spatial distribution of BrO-OClO predicted by Roberts et al (2018). Kern and Lyons (2018) propose that measurements of OClO alongside BrO can help determine the degree of depletion of Br (aq) (and HBr) and therefore guide interpretation of BrO observations in terms of total bromine emission.…”

“…Modeled OClO/SO 2 formation near downwind is enhanced at greater relative humidity because this enhances the aerosol surface that supports the heterogeneous reactions, leading to a more rapid bromine cycling and faster depletion of HBr and enabling BrCl formation (Roberts et al, 2018). Kern and Lyons (2018) report observations of OClO (with BrO) at the plume edge, whilst only BrO is observed at the plume center. The measurements are very consistent with the horizontal spatial distribution of BrO-OClO predicted by Roberts et al (2018).…”

“…Vertical profiles were calculated from two-dimensional BrO and SO 2 distributions measured by imaging DOAS instruments Louban et al, 2009;General et al, 2015). Gliß et al (2015) and recently Kern and Lyons (2018) investigated the vertical BrO/SO 2 distribution by using a 1 dimensional scanner unit. found highest BrO/SO 2 values at the edges of the plume (3 × 10 −4 ) compared to lower values in the center (2 × 10 −4 ) and reproduced this trend with their model data.…”

Reaction Rates Control High-Temperature Chemistry of Volcanic Gases in Air

“…Furthermore, high-temperature production of sulfateprecursor SO 3 generates near-source (so-called "primary") sulfate aerosols that promote heterogeneous reactions that drive the bromine explosion chemistry. Observations of BrO (and occasionally OClO) in tropospheric volcanic plumes globally (e.g., Bobrowski et al, 2007;Boichu et al, 2011;Hörmann et al, 2013;Donovan et al, 2014;Kern and Lyons, 2018) indicate pervasive emissions of volcanic halogens to the troposphere accompanied by both high-and low-temperature chemistry in volcanic plumes.…”

“…In descending order of typical abundance (which varies with volcano setting, magma redox conditions, and eruptive style) volcanic gas emissions consist of: H 2 O, CO 2 , SO 2 , HCl, HF, H 2 S, OCS, CO, and HBr, as well as other trace species e.g., metals. Observations of volcanic plumes identify several additional species (e.g., NO, NO 2 , HNO 3 , BrO, OClO, SO 4 2− , HO 2 NO 2 , and H 2 O 2 ), (Allen et al, 2000;Mather et al, 2004b;Bobrowski et al, 2007;Oppenheimer et al, 2010;Carn et al, 2011;Martin et al, 2012;Kern and Lyons, 2018). These species are formed by oxidizing chemical reactions as the magmatic gases mix with air, first at high temperatures near to the source and then at low temperatures as the cooled plume disperses further into the background atmosphere.…”