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

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 HO_x 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 HO_x 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.