We interpret HCHO column variations observed by the Ozone Monitoring Instrument (OMI), aboard the NASA Aura satellite, over India during 2014 using the GEOS-Chem atmospheric chemistry and transport model. We use a nested version of the model with a horizontal resolution of approximately 25 km. HCHO columns are related to local emissions of volatile organic compounds (VOCs) with a spatial smearing that increases with the VOC lifetime. Over India, HCHO has biogenic, pyrogenic, and anthropogenic VOC sources. Using a 0-D photochemistry model, we find that isoprene has the largest molar yield of HCHO which is typically realized within a few hours. We also find that forested regions that neighbour major urban conurbations are exposed to high levels of nitrogen oxides. This results in depleted hydroxyl radical concentrations and a delay in the production of HCHO from isoprene oxidation. We find that propene is the only anthropogenic VOC emitted in major Indian cities that produces HCHO at a comparable (but slower) rate to isoprene. The GEOS-Chem model reproduces the broad-scale annual mean HCHO column distribution observed by OMI (r = 0.6), which is dominated by a distinctive meridional gradient in the northern half of the country, and by localized regions of high columns that coincide with forests. Major discrepancies are noted over the Indo-Gangetic Plain (IGP) and Delhi. We find that the model has more skill at reproducing observations during winter (JF) and pre-monsoon (MAM) months with Pearson correlations r > 0.5 but with a positive model bias of ≃ 1×1015 molec cm−2. During the monsoon season (JJAS) we reproduce only a diffuse version of the observed meridional gradient (r = 0.4). We find that on a continental scale most of the HCHO column seasonal cycle is explained by monthly variations in surface temperature (r = 0.9), suggesting a role for biogenic VOCs, in agreement with the 0-D and GEOS-Chem model calculations. We also find that the seasonal cycle during 2014 is not significantly different from the 2008 to 2015 mean seasonal variation. There are two main loci for biomass burning (the states of Punjab and Haryana, and northeastern India), which we find makes a significant contribution (up to 1×1015 molec cm−2) to observed HCHO columns only during March and April over northeastern India. The slow production of HCHO from propene oxidation results in a smeared hotspot over Delhi that we resolve only on an annual mean timescale by using a temporal oversampling method. Using a linear regression model to relate GEOS-Chem isoprene emissions to HCHO columns we infer seasonal isoprene emissions over two key forest regions from the OMI HCHO column data. We find that the a posteriori emissions are typically lower than the a priori emissions, with a much stronger reduction of emissions during the monsoon season. We find that this reduction in emissions during monsoon months coincides with a large drop in satellite observations of leaf phenology that recovers in post monsoon months. This may signal a forest-scale response to monsoon conditions.

Volcanoes are an important source of inorganic halogen species into the atmosphere. Chemical processing of these species generates oxidised, highly reactive, halogen species which catalyse considerable O₃ destruction within volcanic plumes. A campaign of ground-based in situ O₃, SO₂ and meteorology measurements was undertaken at the summit of Mount Etna volcano in July/August 2012. At the same time, spectroscopic measurements were made of BrO and SO₂ columns in the plume downwind. Depletions of ozone were seen at all in-plume measurement locations, with average O₃ depletions ranging from 11–35 nmol mol⁻¹ (15–45%). Atmospheric processing times of the plume were estimated to be between 1 and 4 min. A 1-D numerical model of early plume evolution was also used. It was found that in the early plume O₃ was destroyed at an approximately constant rate relative to an inert plume tracer. This is ascribed to reactive halogen chemistry, and the data suggests the majority of the reactive halogen that destroys O₃ in the early plume is generated within the crater, including a substantial proportion generated in a high-temperature "effective source region" immediately after emission. The model could approximately reproduce the main measured features of the ozone chemistry. Model results show a strong dependence of the near-vent bromine chemistry on the presence or absence of volcanic NO_x emissions and suggest that near-vent ozone measurements can be used as a qualitative indicator of NO_x emission.

Volcanoes emit halogens into the atmosphere that undergo complex chemical cycling in plumes and cause destruction of ozone. We present a case study of the Mount Etna plume in the summer of 2012, when the volcano was passively degassing, using aircraft observations and numerical simulations with a new 3D model “WRF-Chem Volcano” (WCV), incorporating volcanic emissions and multi-phase halogen chemistry. Measurements of SO₂ – an indicator of plume intensity – and ozone were made in the plume a few tens of kilometres from Etna, revealing a strong negative correlation between ozone and SO₂ levels. From these observations, using SO₂ as a tracer species, we estimate a mean in-plume ozone loss rate of 1.3×10⁻⁵ molecules of O₃ per second per molecule of SO₂. This value is similar to observation-based estimates reported very close to Etna's vents, indicating continual ozone loss in the plume up to at least tens of kilometres downwind. The WCV model is run with nested grids to simulate the plume close to the volcano at 1 km resolution. The focus is on the early evolution of passively degassing plumes aged less than 1 h and up to tens of kilometres downwind. The model is able to reproduce the so-called “bromine explosion”: the daytime conversion of HBr into bromine radicals that continuously cycle in the plume. These forms include the radical BrO, a species whose ratio with SO₂ is commonly measured in volcanic plumes as an indicator of halogen ozone-destroying chemistry. The species BrO is produced in the ambient-temperature chemistry, with in-plume BrO $/$ SO₂ ratios on the order of 10⁻⁴ mol/mol, similar to those observed previously in Etna plumes. Wind speed and time of day are identified as non-linear controls on this ratio. Sensitivity simulations confirm the importance of near-vent radical products from high-temperature chemistry in initiating the ambient-temperature plume halogen cycling. Heterogeneous reactions that activate bromine also activate a small fraction of the emitted chlorine; the resulting production of chlorine radical Cl strongly enhances the methane oxidation and hence the formation of formaldehyde (HCHO) in the plume. Modelled rates of ozone depletion are found to be similar to those derived from aircraft observations. Ozone destruction in the model is controlled by the processes that recycle bromine, with about three-quarters of this recycling occurring via reactions between halogen oxide radicals. Through sensitivity simulations, a relationship between the magnitude of halogen emissions and ozone loss is established. Volcanic halogen cycling profoundly impacts the overall plume chemistry in the model, notably hydrogen oxide radicals (HO_x), nitrogen oxides (NO_x), sulfur, and mercury chemistry. In the model, it depletes HO_x within the plume, increasing the lifetime of SO₂ and hence slowing sulfate aerosol formation. Halogen chemistry also promotes the conversion of NO_x into nitric acid (HNO₃). This, along with the displacement of nitrate out of background aerosols in the plume, results in enhanced HNO₃ levels and an almost total depletion of NO_x in the plume. The halogen–mercury model scheme is simple but includes newly identified photo-reductions of mercury halides. With this set-up, the mercury oxidation is found to be slow and in near-balance with the photo-reduction of the plume. Overall, the model findings demonstrate that halogen chemistry has to be considered for a complete understanding of sulfur, HO_x, reactive nitrogen, and mercury chemistry and of the formation of sulfate particles in volcanic plumes.

Volcanoes are known to be important emitters of atmospheric gases and aerosols, which for certain volcanoes can include halogen gases and in particular HBr. HBr emitted in this way can undergo rapid atmospheric oxidation chemistry (known as the bromine explosion) within the volcanic emission plume, leading to the production of bromine oxide (BrO) and ozone depletion. In this work, we present the results of a modelling study of a volcanic eruption from Mt Etna that occurred around Christmas 2018 and lasted 6 d. The aims of this study are to demonstrate and evaluate the ability of the regional 3D chemistry transport model Modèle de Chimie Atmosphérique de Grande Echelle (MOCAGE) to simulate the volcanic halogen chemistry in this case study, to analyse the variability of the chemical processes during the plume transport, and to quantify its impact on the composition of the troposphere at a regional scale over the Mediterranean basin. The comparison of the tropospheric SO₂ and BrO columns from 25 to 30 December 2018 from the MOCAGE simulation with the columns derived from the TROPOspheric Monitoring Instrument (TROPOMI) satellite measurements shows a very good agreement for the transport of the plume and a good consistency for the concentrations if considering the uncertainties in the flux estimates and the TROPOMI columns. The analysis of the bromine species' partitioning and of the associated chemical reaction rates provides a detailed picture of the simulated bromine chemistry throughout the diurnal cycle and at different stages of the volcanic plume's evolution. The partitioning of the bromine species is modulated by the time evolution of the emissions during the 6 d of the eruption; by the meteorological conditions; and by the distance of the plume from the vent, which is equivalent to the time since the emission. As the plume travels further from the vent, the halogen source gas HBr becomes depleted, BrO production in the plume becomes less efficient, and ozone depletion (proceeding via the Br+O₃ reaction followed by the BrO self-reaction) decreases. The depletion of HBr relative to the other prevalent hydracid HCl leads to a shift in the relative concentrations of the Br⁻ and Cl⁻ ions, which in turn leads to reduced production of Br₂ relative to BrCl. The MOCAGE simulations show a regional impact of the volcanic eruption on the oxidants OH and O₃ with a reduced burden of both gases that is caused by the chemistry in the volcanic plume. This reduction in atmospheric oxidation capacity results in a reduced CH₄ burden. Finally, sensitivity tests on the composition of the emissions carried out in this work show that the production of BrO is higher when the volcanic emissions of sulfate aerosols are increased but occurs very slowly when no sulfate and Br radicals are assumed to be in the emissions. Both sensitivity tests highlight a significant impact on the oxidants in the troposphere at the regional scale of these assumptions. All the results of this modelling study, in particular the rapid formation of BrO, which leads to a significant loss of tropospheric ozone, are consistent with previous studies carried out on the modelling of volcanic halogens.

Volcanoes are an important source of inorganic halogen species into the atmosphere. Chemical processing of these species generates oxidised, highly reactive, halogen species which catalyse considerable O3 destruction within volcanic plumes. A campaign of ground-based in situ O3, SO2 and meteorology measurements was undertaken at the summit of Mount Etna volcano in July–August 2012. At the same time, spectroscopic measurements were made of BrO and SO2 columns in the plume downwind. Depletions of O3 were seen at all in-plume measurement locations, with average O3 depletions ranging from 11–35 nmol mol−1 (15–45%). Atmospheric processing times of the plume were estimated to be between 1 and 4 min. A 1-D numerical model of early plume evolution was also used. It was found that in the early plume O3 was destroyed at an approximately constant rate relative to an inert plume tracer. This is ascribed to reactive halogen chemistry, and the data suggests the majority of the reactive halogen that destroys O3 in the early plume is generated within the crater, including a substantial proportion generated in a high-temperature "effective source region" immediately after emission. The model could approximately reproduce the main measured features of the O3 chemistry. Model results show a strong dependence of the near-vent bromine chemistry on the presence or absence of volcanic NOx emissions and suggest that near-vent O3 measurements can be used as a qualitative indicator of NOx emission.

HBr emissions from volcanoes lead rapidly to the formation of BrO within volcanic plumes and have an impact on tropospheric chemistry, at least at the local and regional scales. The motivation of this paper is to prepare a framework for further 3D modelling of volcanic halogen emissions in order to determine their fate within the volcanic plume and then in the atmosphere at the regional and global scales. The main aim is to evaluate the ability of the model to produce a realistic partitioning of bromine species within a grid box size typical of MOCAGE (Model Of atmospheric Chemistry At larGE scale) 3D (0.5^∘ × 0.5^∘). This work is based on a 1D single-column configuration of the global chemistry-transport model MOCAGE that has low enough computational cost to allow us to perform a large set of sensitivity simulations. This paper uses the emissions from the Mount Etna eruption on 10 May 2008. Several reactions are added to MOCAGE to represent the volcanic plume halogen chemistry. A simple plume parameterisation is also implemented and tested. The use of this parameterisation tends to only slightly limit the efficiency of BrO net production. Both simulations with and without the parameterisation give results for the partitioning of the bromine species, of ozone depletion and of the $\mathrm{BrO}/{\mathrm{SO}}_{\mathrm{2}}$ ratio that are consistent with previous studies. A series of test experiments were performed to evaluate the sensitivity of the results to the composition of the emissions (primary sulfate aerosols, Br radical and NO) and to the effective radius assumed for the volcanic sulfate aerosols. Simulations show that the plume chemistry is sensitive to all these parameters. We also find that the maximum altitude of the eruption changes the BrO production, which is linked to the vertical variability of the concentrations of oxidants in the background air. These sensitivity tests display changes in the bromine chemistry cycles that are generally at least as important as the plume parameterisation. Overall, the version of the MOCAGE chemistry developed for this study is suitable to produce the expected halogen chemistry in volcanic plumes during daytime and night-time.