Volcanoes are known to be a major source of climatically important species, trace gases, and aerosol into the atmosphere, both through explosive eruptions and sustained quiescent degassing e.g.. The largest gaseous components of most eruptions are water, carbon dioxide and sulphur compounds, followed by hydrogen halides . These volcanogenic halides are known to present environmental hazards upon deposition to soils e.g.. Emissions of bromide, while weaker and less well studied than those of fluoride and chloride , are of interest to atmospheric chemistry due to the conversion of HBr to very reactive ozone-depleting bromine species .

The bromine explosion is an autocatalytic chemical reaction cycle which is known to convert bromine from HBr to very reactive forms . It is considered to be the reason for detections of elevated BrO associated with depleted O3 e.g.. BrO(g)+HO2(g)⟶HOBr(g)+O2(g)HOBr(g)⟶HOBr(aq)HBr(g)⟶HBr(aq)HOBr(aq)+Br-+H+⟶Br2(aq)+H2O(l)Br2(aq)⟶Br2(g)Br2(g)+hν⟶2Br(g)2(Br(g)+O3(g))⟶2(BrO(g)+O2(g)) There is no equivalent chlorine explosion as HCl is less reactive than HBr see.

In the troposphere, reactive halogen species catalyse ozone destructive cycles e.g. (X and Y are halogen atoms): XO(g)+YO(g)⟶X(g)+Y(g)+O2(g)XO(g)+YO(g)⟶XY(g)+O2(g)XY(g)+hν⟶X(g)+Y(g)X(g)+O3(g)⟶XO(g)+O2(g)Y(g)+O3(g)⟶YO(g)+O2(g). Reactive halogen chemistry and related O3 depletion has been studied in detail in the polar and marine boundary layers and over salt lakes seefor an overview.

BrO is detectable through spectroscopy, and detection of elevated levels are used as an indicator for the occurrence of reactive bromine chemistry. BrO was first detected in a volcanic plume by , and has, to date, been detected in the plumes of about 20 different volcanoes. A good overview of ground-based measurements of BrO in volcanic plumes can be found in and references therein. A systematic study of satellite data by greatly extended the data set of known BrO events detected via satellite measurements.

In both satellite and ground-based studies, in-plume ratios of BrO to SO2 have been found to be of the order of 10-5–several 10-4 once the plume has been transported a few km from the source e.g.. Measurements and modelling studies of in-plume BrO / SO2 ratios have shown BrO to be either present at low levels or undetectable within 1 km of volcanic vents whilst the BrO / SO2 ratio a few km downwind of the same vent is measured as up to several 10-4 e.g.. This suggests the formation of BrO is occurring within the time taken for this downwind transport. SO2 is widely used as a tracer for volcanic emissions due to its high concentration in volcanic plumes, low reactivity, and easily detectable spectroscopic signal .

Volcanoes are known to emit other halogens (F, Cl, I). The HF molecule is expected to be too stable for any in-plume reactive F chemistry to occur , whilst the magnitudes of volcanic HI emissions are probably too low to have a significant effect on plume chemistry . Measurements of oxidised Cl in volcanic plumes are varied, having been observed at Sakurajima , Etna , Puyehue-Cordón Caulle , and Soufière Hills . However a DOAS investigation by specifically looking for oxidised Cl did not detect any in the plume of Masaya Volcano. It is unknown whether this reflects natural variability or measurement issues. See for a discussion of reactive chlorine formation in volcanic plumes.

The conceptual model of halogen-O3 chemistry within volcanic plumes is complicated by the very high temperatures of the plume immediately after emission and mixing with the atmosphere. Such high temperature mixtures are believed to reach or approach thermodynamic equilibrium , resulting in the production of non-negligible quantities of oxidised halogen species which may begin the autocatalytic cycles described above . Because of the hazards involved, this “effective source region” within the crater cannot be investigated directly. Instead, inventories of volcanic emission species are based mostly on samples of fumarole emissions or crater-rim measurements . Processes occurring in the effective source region are inferred from these measurements, theory, and measurements of the plume further downwind.

While BrO (and, with greater difficulty, OClO) can be measured remotely using passive spectroscopic techniques, direct measurements of O3 depletion within volcanic plumes have to be made in situ due to the large atmospheric O3 column, and as a result the existing body of literature on the depletion of ozone within volcanic plumes is small, see Table . Active DOAS measurement of O3 at a volcano is possible, however it presents significant difficulties in distinguishing the plume from background air. To the authors' knowledge the only published active DOAS O3 measurement at a volcano is . In this investigation, no O3 depletion could be determined – the authors of this publication attribute this to the difficulties of isolating a plume O3 signal from that of the background air.

This study expands this small collection of O3 observations. In addition, the O3 depletion data collected are associated with simultaneous measurements of an inert plume tracer (SO2). To the authors' knowledge, no prior data set exists of simultaneous measurements of O3 depletion and a tracer species within 1 km of volcanic vents. Wind speeds have also been measured, such that processing time can be determined for every datapoint. As in-plume halogen and O3 chemistry are closely linked, from these data we can investigate both qualitatively and quantitatively the fast chemical processes occurring within the early plume.

An overview of the near-crater results from this campaign can be seen in Table . Calculated plume ages were found to be of the order 1–4 min.

The plume was observed to be grounded at all downwind sites, enabling measurement. As both O3 and SO2 are known to exhibit deposition to the ground, there is the potential for the mixing ratios of these species at the elevation of the instruments to be perturbed by this physical process. However the ground at the peak of Etna is rocky and devoid of vegetation and could be classed in the scheme of as “barren land”. The bulk surface resistance of this category sufficiently high that, even if the aerodynamic and sublayer resistances are low, the deposition velocity will be too low to cause a significant perturbation during the transport from crater rim to the measurement sites.

Ozone depletion of up to 45 % was observed at all seven downwind sites. SO2 decreases with transport time (Fig. ), as would be expected due to dilution. The low SO2 mixing ratios at site d2-30 suggest this site was at the plume edge, this is consistent with visual observations of the plume. A plot of ΔO3 against transport time (Fig. ) shows no comparable trend as chemical destruction of O3 and dilution with background air occur simultaneously.

In Fig. , Δ[O3] / [SO2] vs. transport time is plotted. The linear trend is indicative of chemical destruction of O3 occurring within the first few minutes of plume transport (see Appendix).

The data on Fig.  appear to fit a linear trend. This indicates, that as a ratio to the inert plume tracer, the rate of chemical O3 destruction is constant within the first few minutes of plume evolution. In the expected chemical cycles Eq. (), the rate of O3 destruction is dependant on the concentration of halogens in highly reactive (non-hydrogen halide) forms. Therefore a constant rate of reaction indicates that the ratio of these species to SO2 is approximately constant during the first few minutes of plume evolution.

Depletion of ozone occurred on 30 July even when the relative humidity was <10 %. This is below the crystallisation point of any major aerosol component that would be expected to be present in the plume. It would therefore be expected that at such low humidities aqueous phase reactions (such as reaction ) would occur at a much slower rate, preventing the bromine explosion from converting HBr to very reactive forms of bromine. On inspection of the humidity measurements (Table ), no significant increase in humidity due to volcanic H2O was observed at the downwind sites (the humidity variation between up-24 and d1-24 could be volcanic in origin, however it is also in the range of normal meteorological variability). The possible impacts of a short-lived elevated humidity within the crater due to volcanogenic H2O are considered with the “dry” model runs in Section of this paper.

Data points from all measurement sites appear to approximately fit the same line-of-best fit on Fig. . This is despite the measurements spanning 2 separate days and these 2 days having very different circumstances. On 27 July the measurements were made in the plume of the North-East crater, and on 30 July the measurements were made in the plume of the central craters. The different craters are known to exhibit differing emission compositions . In addition, the relatively humidity on these 2 days was markedly different. It is unknown whether the cause of these two data sets' approximate alignment in Fig.  is due to mechanistic reasons relating to the “effective source region” (see below) or coincidence.

A linear line of best fit was calculated using a Williamson–York iterative bivariate analysis , its gradient is (-1.02±0.07)×10-4 s-1. The r2 value of this fit is 0.76. This linear trend can be interpreted as there being a rate of destruction of O3 within the plume of approximately 10-4 molecules per molecule of SO2 (the inert plume tracer) per second in the first few minutes of plume evolution. SO2 is only used as a measure of plume intensity, SO2 does not play a role in O3 destruction.

Extrapolating the line of best fit on Fig.  results in an intercept of the x axis of approximately -60 s. Assuming chemical O3 destruction is not instantaneous in the plume, it would be expected that the line of best fit would pass through the origin. We determine that this offset is most likely the result of inaccuracy in the calculation of processing time. Firstly, the calculation assumes that air travels in a straight line between vent and sampling point; however, the path length is likely to be greater than this due to turbulence, the topology of the mountaintop, and variation in wind direction. Second, the distances between vents and sampling points were calculated only from their lateral displacement and did not consider the time taken for vertical transport from the vent to the crater rim. From our visual observations of the plume, we estimate this vertical transport time is likely to have been of the order of a minute.

Table  lists the composition of volcanic volatiles. These figures are calculated from June 2012 values of SO2, HF, HCl, and HBr measured using various alkaline traps (Bocca Nuova crater), as well as the MULTIGAS measurements of CO2 and SO2 made on the main campaign (using all available data). As complete characterisation of the volcanic volatiles could not be made from the June 2012 measurements alone, the ratio of the gases to CO2 were used in these calculations with absolute values of H2O and CO2 calculated from the values reported in . Additionally, H2 / SO2 was taken from , and H2S / SO2 was calculated from the values reported in . These values were then normalised to ensure that the sum equals unity.

The remote sensing results are shown in Table , along with the correlation between the BrO and SO2 columns and the estimated plume age (based upon distance from the crater and wind speed, see Sect. ). The BrO / SO2 column ratios are within the range typically measured both in the Etna plume and volcanic plumes in general , though they are at the lower end of these ranges.

We used a vertically resolved 1-D numerical model MISTRA to simulate the chemical evolution of the volcanic plume in the atmosphere and to test the model's performance against the data presented above. The model is the same as used in with minor modifications. Based on previous work we used the thermodynamic equilibrium model HSC to calculate the composition of the mixture of volcanic volatiles and ambient air in the “effective source region” which is located in the crater where temperatures are high enough so that the assumption of immediate thermodynamic equilibrium is still largely valid but also enough ambient air has been entrained to oxidise the main volcanic reduced gases (mainly H2S and H2) which leads to a dramatic change in halogen speciation. Recent studies e.g. have shown that the assumption of thermodynamic equilibrium is incorrect for a number of compounds, especially for H2S and NOx which makes the use of an equilibrium model such as HSC problematic but it is still regarded to provide a better approximation of the composition of volcanic volatiles than not assuming any equilibration. See also related comments in . Compared to we used a higher ratio of volcanic volatiles : ambient air (95 : 5), which is very rich in volcanic volatiles but already oxidised.

The model is run in Lagrangian mode, following the evolution of a “puff” of plume downwind of the volcano and explicitly modelling the interaction of the plume with background air see. The modelled wind speed is 10 m s-1, and the plume dispersal is assumed to be Gaussian and follows using the Pasquill–Gifford scheme to calculate the parameters of the Gaussian plume (see e.g. ).

The S and Br elemental ratios of the volcanic volatiles used in the HSC calculation are taken from the average values measured at the crater rim (Table , column “low Br / S”) on 26 and 27 June 2012. We also did a second set of model runs initialised with a much higher Br / S ratio (Table , column “high Br / S”) based on unpublished data from Alessandro Aiuppa. Table S1 in the Supplement gives the composition of the volcanic volatiles and the results of the HSC calculations used as input for the 1-D model.

The relative humidity varied significantly during the campaign (see Table ), which is important as the bromine explosion is efficient in the presence of aqueous aerosol.

Following a crystallisation humidity of 40 % for sulphate-containing particles is assumed in the model, but other studies suggest sulphate-containing particles to be liquid for lower relative humidities e.g..

Regardless of the exact composition and crystallisation humidity of the volcanic aerosol present on the days of this study when simultaneous SO2 and O3 measurements were made, we wanted to test the implications of crystallised aerosol in the model. We therefore ran the model at the following three different settings regarding the deliquescence of the aerosol:

“moist”: relative humidity ∼ 57 % (above the sulphate crystallisation humidity assumed in the model); aerosol aqueous phase chemistry occurs throughout the plume;

Being remote from major sources of anthropogenic pollution and above the planetary boundary layer, the ambient NOx levels at Etna are expected to be negligible compared to the amount of nitrogen oxides (NOx) that thermodynamic models such as HSC predict would be formed in the effective source regions of volcanic vents. However and discussed the problems associated with this, mainly that the time required to reach equilibrium for NOx under the given temperatures in the effective source region is, by far, too long. The generation of NOx could significantly perturb bromine chemistry due to the formation of BrNO2 see e.g. Fig. 2 in. A detailed discussion of volcanic plume Br–NOx chemistry, including a modelling study that includes additional BrNO2 reactions, can be found in . Use of their scheme would likely result in less bromine being present as BrNO2 and might lead to somewhat different O3 development. For each of the three aerosol settings described above, two model runs were done – one with volcanic NOx in the quantities predicted in the thermodynamic model and one with no volcanic NOx. These six runs were performed for both of the plume initialisations shown in Table .

In this study of chemical evolution in the plumes of Mount Etna during a passively degassing phase we have confirmed O3 depletion to occur less than a few minutes after emission. This extends the limited data set regarding volcanic O3 depletion and is the first data set to date based on ground-based data to include simultaneous measurements of O3 and a plume tracer. The depletion process is ongoing over this time period, and the data suggests it occurred at an approximately constant rate. For the Etna craters measured in late July 2012, this rate was found to be a depletion of approximately 10-4 molecules of O3 per molecule of SO2 (the inert plume tracer) per second.

Substantial depletion of O3 was observed within minutes after emission, and occurred at very low humidities at which the heterogeneous processes of the bromine explosion would be significantly slowed. On the basis of this, an approximately constant rate of ozone depletion (as a ratio of the inert tracer), and analysis of our modelling study we conclude that most of the very reactive halogens that caused the observed O3 depletion within the 1–4 min old plume were not a product of chemical cycling in the plume once it had emerged from the crater but rather generated in effective source regions near the vents in high-temperature processes that ceased shortly after emission, and/or very fast low-temperature chemical processes within the crater. This is in agreement with the thermodynamic modelling studies of , , and which predict oxidised halogens being produced by high-temperature effective source regions in volcanoes. The in situ measurements in this study were taken at distances less than 500 m from the vents. Measurements of BrO / SO2 at such distances tend to be low compared to those measured further downwind . That substantial O3 depletion can occur in such regions suggests these low levels of reactive bromine (and possibly chlorine) species that are generated within the crater are sufficient to cause the O3 depletion observed in this study.

We have also shown that further downwind, contemporaneous spectroscopic observation of the plume shows BrO / SO2 columns are within the typical range that has been observed within volcanic plumes. Our near-crater measurements are therefore likely to be typical for Mount Etna in its passively degassing phase.

Results from our modelling study suggest that a significant factor influencing the bromine-ozone chemistry in the very early plume is the presence or absence of volcanic NOx and, if it is present, its source strength relative to that of Br. The modelling study shows that volcanic NOx would be expected to significantly retard early O3 depletion and therefore plume O3 and Br data could be compared to model output to yield information on NOx within a plume. The measured data from this campaign are insufficient to definitively ascertain whether or not volcanic NOx generation is occurring but it appears unlikely to be the case as near-crater O3 depletion was observed whilst volcanic NOx emissions suppressed such O3 depletion in the modelling study. The modelling study also shows how variations in humidity, and the consequent variations in heterogeneous chemistry, may impact the plume chemistry in detectable ways. A comparison of the results from our three different humidity/heterogeneous chemistry cases gives an indication as to the extents that O3 depletion can be ascribed to reactive halogens generated in high-temperature processes within the crater, low-temperature processes within the crater, and low temperature processes further downwind.