Modeling the reactive halogen plume from Ambrym and its impact on the troposphere with the CCATT-BRAMS mesoscale model

Abstract

Abstract. Ambrym Volcano (Vanuatu, southwest Pacific) is one of the largest sources of continuous volcanic emissions worldwide. As well as releasing SO2 that is oxidized to sulfate, volcanic plumes in the troposphere are shown to undergo reactive halogen chemistry whose atmospheric impacts have been little explored to date. Here, we investigate with the regional-scale model CCATT-BRAMS (Coupled Chemistry Aerosol-Tracer Transport model, Brazilian developments on the Regional Atmospheric Modeling System, version 4.3) the chemical processing in the Ambrym plume and the impact of this volcano on the atmospheric chemistry on both local and regional scales. We focus on an episode of extreme passive degassing that occurred in early 2005 and for which airborne DOAS (differential optical absorption spectroscopy) measurements of SO2 and BrO columns in the near-downwind plume between 15 and 40 km from the vents have been reported. The model was developed to include reactive halogen chemistry and a volcanic emission source specific to this extreme degassing event. In order to test our understanding of the volcanic plume chemistry, we performed very high-resolution (500 m × 500 m) simulations using the model nesting grid capability and compared each DOAS measurement to its temporally and spatially interpolated model counterpart “point-by-point”. Simulated SO2 columns show very good quantitative agreement with the DOAS observations, suggesting that the plume direction as well as its dilution in the near-downwind plume are well captured. The model also reproduces the salient features of volcanic chemistry as reported in previous work, such as HOx and ozone depletion in the core of the plume. When a high-temperature chemistry initialization is included, the model is able to capture the observed BrO ∕ SO2 trend with distance from the vent. The main discrepancy between observations and model is the bias between the magnitudes of observed and simulated BrO columns that ranges from 60 % (relative to the observations) for the transect at 15 km to 14 % for the one at 40 km from the vents. We identify total in-plume depletion of ozone as a limiting factor for the partitioning of reactive bromine into BrO in the near-source (concentrated) plume under these conditions of extreme emissions and low background ozone concentrations (15 ppbv). Impacts of Ambrym in the southwest Pacific region were also analyzed. As the plume disperses regionally, reactive halogen chemistry continues on sulfate aerosols produced by SO2 oxidation and promotes BrCl formation. Ozone depletion is weaker than on the local scale but still between 10 and 40 % in an extensive region a few thousands of kilometers from Ambrym. The model also predicts the transport of bromine to the upper troposphere and stratosphere associated with convection events. In the upper troposphere, HBr is re-formed from Br and HO2. Comparison of SO2 regional-scale model fields with OMI (Ozone Monitoring Instrument) satellite SO2 fields confirms that the Ambrym SO2 emissions estimate based on the DOAS observations used here is realistic. The model confirms the potential of volcanic emissions to influence the oxidizing power of the atmosphere: methane lifetime (calculated with respect to OH and Cl) is increased overall in the model due to the volcanic emissions. When considering reactive halogen chemistry, which depletes HOx and ozone, the lengthening of methane lifetime with respect to OH is increased by a factor of 2.6 compared to a simulation including only volcanic SO2 emissions. Cl radicals produced in the plume counteract 41 % of the methane lifetime lengthening due to OH depletion. Including the reactive halogen chemistry in our simulation also increases the lifetime of SO2 in the plume with respect to oxidation by OH by 36 % compared to a simulation including only volcanic SO2 emissions. This study confirms the strong influence of Ambrym emissions during the extreme degassing event of early 2005 on the composition of the atmosphere on both local and regional scales. It also stresses the importance of considering reactive halogen chemistry when assessing the impact of volcanic emissions on climate.

Photochemical box-modelling of volcanic SO<sub>2</sub> oxidation: isotopic constraints

“…:8) A potentially significant limitation of the model simulations is the omission of volcanic halogens. Indeed, volcanic halogens 20 are known to undergo multi-phase chemistry, resulting in ozone depletion and possibly impacting the oxidation of volcanic SO 2 (Bobrowski et al, 2003;Millard et al, 2006;Roberts et al, 2009;von Glasow, 2010;Roberts et al, 2014;Jourdain et al, 2016). Halogen species such as HOBr may also directly oxidise SO 2 in the aqueous phase (Chen et al, 2017), but this oxidation pathway has not been quantified yet for volcanic plumes.…”

Implementation of electrochemical, optical and denuder-based sensors and sampling techniques on UAV for volcanic gas measurements: examples from Masaya, Turrialba and Stromboli volcanoes

“…However, an increase in the reactive bromine species BrO with distance from the crater rim has previously been observed by DOAS measurements at various volcanoes and is well described in the literature Bobrowski et al, 2007). Although the bromine speciation in volcanic plumes has been the subject of several ground-based (e.g., Gliß et al, 2015) airplane (e.g., General et al, 2015), satellite (e.g., Theys et al, 2009;Hörmann et al, 2013) and model studies (e.g., Bobrowski et al, 2007;Roberts et al, 2009von Glasow, 2010;Jourdain et al, 2016) in recent years, in situ measurements are scarce. The data presented here for the first minute after emission highlight the potential for UAV-based measurements to improve the sample acquisition and thus obtain a better understanding of plume aging.…”

“…Nyamuragira . In the last decade, several model studies (e.g., Bobrowski et al, 2007;Roberts et al, 2009;von Glasow, 2010;Jourdain et al, 2016) have engaged with the variation in halogen variability in volcanic plumes with respect to various atmospheric and magmatic parameters. In the case of bromine, it was modeled that the initial emitted hydrogen bromide is depleted shortly after emission under consumption of tropospheric ozone and is transformed to reactive species such as BrO, HOBr, Br 2 , BrCl and BrONO 2 .…”

Multicopter measurements of volcanic gas emissions at Masaya (Nicaragua), Turrialba (Costa Rica) and Stromboli (Italy) volcanoes: Applications for volcano monitoring and insights into halogen speciation

“…Theys et al, 2009;Hörmann et al, 2013) and model studies (e.g. Roberts et al, 2009;von Glasow, 2010;Roberts et al, 2014;Jourdain et al, 2016) in recent years, in-situ measurements are scarce. The here presented data for the first minute after emission highlights the potential of UAV-based measurements to improve sample acquisition and thus a better understanding of plume aging.…”

“…In the last decade, several model studies (e.g. , Roberts et al, 2009von Glasow, 2010;Roberts et al, 2014;Jourdain et al, 2016) have engaged on the variation of halogen variability in volcanic plumes with respect to various atmospheric and magmatic parameters. In the case of bromine, it was modelled that the initial emitted hydrogen bromide is depleted shortly after emission under consumption of tropospheric ozone and is transformed to reactive species such as BrO, HOBr, Br2, BrCl and BrONO2.…”