Partitioning of Halogens in Volcanic Gas Emissions

Volcanic eruption columns are complex and dynamic chemical environments. Along with sulfur dioxide and water, volcanic eruption columns may contain large quantities of the halogens chlorine and bromine, and to a lesser extent iodine. Plume temperatures range from very hot at the crater rim to ambient within the umbrella cloud. Entrainment of environmental air during convection is likely to produce enhancements in water, rapid parcel cooling and resultant kinetic freeze-out of chemical species.  Until recently, it had been assumed that volcanic halogen gases primarily existed in their hydrogen halide forms, i.e., hydrogen chloride, hydrogen bromide, and hydrogen iodide.

This is important because hydrogen halides are extraordinarily soluble in water and water-soluble species are effectively screened from the stratosphere during eruption column ascent. Indeed, conventional wisdom held that volcanic eruptions posed an insignificant source of stratospheric halogen gases until Textor et al. (2003) demonstrated that, under some conditions, significant quantities of hydrogen chloride might survive transport in the eruption column to the stratosphere.

Bobrowski et al. (2003) first detected an oxidized halogen — the bromine monoxide radical, BrO — in a volcanic plume using DOAS. Subsequently, Bobrowski et al. (2007) explained the presence of BrO by invoking a bromine explosion mechanism similar to the one described by Barrie and Platt (1997) in regard to the very different phenomenon of polar sea-salt processing.

The mechanism goes like this:

Some quantity of volcanic HBr is thermally dissociated to produce the Br radical.

\text{HBr} \xrightarrow[]{\Delta} \text{H} + \text{Br}

This radical then reacts with entrained ozone to form BrO.

\text{Br} + \text{O}_3 \rightarrow \text{BrO} + \text{O}_2

The BrO may react with the hydroperoxyl radical, which may also form due to thermal chemistry in the volcanic plume, producing hypobromous acid.

\text{BrO} + \text{HO}_2 \rightarrow \text{HOBr} + \text{O}_2

HOBr can then react with dissolved bromine (e.g., in an acidic droplet on a particle surface) to produce molecular bromine.

\text{HOBr}_\text{(g)} + \text{Br}^-_{\text{(aq)}} + \text{H}^+_{\text{(aq)}} \rightarrow \text{Br}_{2\text{(g)}} + \text{H}_2\text{O}_\text{(l)}

Molecular bromine is rapidly photolyzed to produce two more bromine radicals, which react with ozone.

\text{Br}_2 + \text{h}\nu \rightarrow 2\text{Br}

2\text{Br} + 2\text{O}_3 \rightarrow 2\text{BrO} + 2\text{O}_2

Thus, for every initial bromine atom, two bromine monoxide molecules are produced, provided there is enough ozone in the entrained air volume.

Finally, bromine atoms regenerate themselves to perpetuate the autocatalytic cycle:

\text{BrO} + \text{h}\nu \rightarrow \text{Br} + \text{O}

This O atom may produce more ozone.

\text{Net}: \text{Br} + \text{Br}^-_\text{(aq)} + \text{O}_3 + \text{H}^+_\text{(aq)} \xrightarrow[]{surface} 2\text{BrO} + \text{products}

If significant quantities of volcanic halogen species were to partition to the stratosphere, they would contribute to ozone loss processes, as demonstrated by Klobas et al. (2017). Figure 1, below, demonstrates how the co-injection of volcanic halogens may significantly perturb column ozone following a Pinatubo-sized volcanic eruption in a RCP 2.6 climate change scenario. Panels a — b relate the effect from sulfate aerosol alone in the year 2018 (a) and 2100 (b). Notably, as the stratospheric burden of chlorine declines due to the decay of anthropogenic chlorofluorocarbons (CFCs), sulfate-only volcanic eruptions will have less and less impact on total column ozone — but, if the volcanic eruption also injects halogens, as in panels c — f, ozone depletion is predicted even when background halogen levels are low.



Figure 1: Ozone response to stratospheric halogen injection in contemporary and future RCP 2.6 atmospheres. (a and b) Pinatubolike eruptions (SO2 injection only), (c and d) coinjection of 0.014 HCl:SO2 (EESC effectively increased by ~0.3 ppbv), and (e and f) coinjection of 0.14 HCl:SO2 (EESC effectively increased by ~3 ppbv). Note that the color scale is nonlinear in order to encompass the broad range of ozone changes. Global averages (90°S–90°N) of total column ozone perturbation are traced atop each panel as a function of time. Temporal average ozone anomalies are traced right. Global-temporal averages are enumerated in the top right. Black triangles indicate injection latitude and time. (Klobas thesis 2018)

The existence of oxidized halogens in a volcanic plume is a game-changer when it comes to stratospheric injection efficiency. While hydrogen halides possess Henry’s law constants in the 1e13 — 1e15 mol/atm neighborhood, halogen monoxides are closer to 0.5 — 0.8 mol/atm. Compare this to the Henry’s law constant for sulfur dioxide, which we know partitions effectively to the stratosphere, of about 1e5 mol/atm. IF — and it’s a big if — volcanic halogen species can be found in great quantity in their non-hydride forms, the stratospheric injection efficiency for these halogen gases may be much higher than previously considered.  Indeed, in the recent past, groups have reported finding UTLS BrO, OClO, and IO in aged volcanic plumes with SCIAMCHY and GOME-2.


So, an interesting paper showed up in a google scholar alert in my email inbox this morning: Bobrowski and colleagues measured the evolution of halogen gases using both in situ and remote sensing techniques AND quantified the oxidized fractions. What they found confirmed my suspicions: while chlorine exists mainly in the form of hydrogen chloride, up to 35% of total bromine and 18% of total iodine are bound in oxidized species. Mind you this wasn’t an explosively erupting volcano with stratospheric significance — they measured emissions from a lava lake — so this isn’t direct evidence of oxidized halogen species in a stratospheric eruption column (and eruption columns are dark and ozone-poor, so photochemistry will be suppressed everywhere except the boundaries), but it’s another step in that direction.

The stratospheric injection efficiency of volcanic halogens is a developing story and more research is needed to really constrain the magnitude of the threat to stratospheric ozone. The fact that oxidized halogen species are significantly present in volcanic emissions is another indication that prior estimates of the very low halogen injection efficiency may have been too conservative.