# The Future Space Launch Market and Stratospheric Ozone

Traffic within (and through) the stratosphere was perhaps the first recognized anthropogenic force for stratospheric perturbation. In the early 1970’s, a proposed fleet of 400 Supersonic Stratospheric Transports (SSTs) — or equivalently, High-Speed Civil Transports (HSCTs) — was severely scrutinized to address concerns which included the impact of SST HO$_\text{x}$ and NO$_\text{x}$ emissions on stratospheric ozone. Initial 1-D photochemical model estimates of steady state ozone loss arising from such a fleet frequently exceeded 50% or more and contributed to a moratorium on US development of SSTs.

Enhancements in stratospheric water and nitrogen oxides produce enhancements in the cycling of ozone via the general ozone-loss processing motif depicted in figure 1. In brief, precursor species Y may be activated to form active species X. Once formed, X reacts quickly with ozone to form XO. XO then reacts quickly with atomic oxygen to reform species X. The cycle produces a net loss of stratospheric ozone. Chain reaction termination steps are idiosyncratic to the physicochemical environment. In some cases, as depicted in figure 1,  X and XO will recombine to reform the long-lived precursor species.

Figure 1: general motif for ozone loss processing in the middle stratosphere. Klobas Thesis 2018

These initial model results featured exaggerated estimates of ozone loss for several reasons. The kinetic rates of the HO$_\text{x}$– and NO$_\text{x}$– mediated reactions were highly uncertain – and most importantly, a previously unknown source of atmospheric chlorine significantly reduced the efficiency by which atmospheric NO$_\text{x}$ might process ozone. Elevated concentrations of chlorine from chlorofluorocarbons served to lock away nitrogen dioxide (produced from SST emission of NO) and produce chlorine nitrate, per the following reactions.

$\text{NO} + \text{O}_3 \rightarrow \text{NO}_2 + \text{O}_2$

$\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2$

$\text{ClO} + \text{NO}_2 + \text{M} \rightarrow \text{ClONO}_2 + \text{M}^*$

Chlorine nitrate is a long-lived reservoir of both odd chlorine and odd nitrogen, and the discovery of this reaction essentially resolved the major threat to ozone previously predicted from high-altitude SSTs.

Space launch vehicles also perturb stratospheric inventories of trace gases. During the early 1970’s, and contemporaneous with the SST debates in congress, were initial studies on the implications of increased use of solid rocket motors (SRMs).  SRMs frequently employ ammonium perchlorate ($\text{NH}_4\text{ClO}_4$) oxidizers with alumina fuel, producing large local enhancements in chlorine and stratospheric surface area following a launch. The effects are quantifiable and a series of in situ rocket plume encounters in the late 1990’s revealed large, transient ozone holes spanning several hundred kilometers after a launch.

Contemporary commercial space launch vehicles tend to shy away from SRMs for many reasons including: low specific impulse, inability to prematurely terminate/restart burns, and environmental concerns. Liquid rocket motors (LRMs) are more gentle to the ozone layer than SRMs; however, they still produce trace gas emissions as they transit the stratosphere. HO$_\text{x}$ and NO$_\text{x}$ are the primary reactive families produced in a LRM plume, depending on the fuel source utilized. LO$_2$/RP-1, a kerosene fuel/liquid oxygen oxidizer, will emit water, nitrogen oxides, contributing to ozone loss processing. LO$_2$/LH$_2$ is cleaner burning,  but still produces large enhancements in HO$_\text{x}$.

This leads to an interesting scenario: as anthropogenic halogens decline throughout the century as a result of the highly-successful Montreal protocol (and subsequent amendments), the efficiency of the odd-nitrogen and odd-hydrogen catalytic cycles will increase. Also projected to increase is the rate of launches and the mass of material placed into orbit as the private space launch market grows exponentially.  Today’s launch of the Falcon Heavy by SpaceX is an impressive milestone in monitoring this trend.

The news surrounding this event reminded me of a paper from a while back — an econometric analysis of the space launch market and the impact its  expansion might have on the stratospheric ozone layer.

Ross et al. (2009) provide constraints on the mass which may be placed into orbit depending on future choices in fuel mix.  They provide the following figure (figure 2), which I reproduce for fair use academic purposes. In the figure, losses of ozone are presented as a function of payload rate, and fractional use of SRMs vs LRMs.

Figure 2, reproduced from Ross et al. (2009)

It should be noted that the projected losses are parameterized from model studies of 20th century atmospheres. Effects from the changing climate, such as stratospheric cooling, changes in the strength of the Brewer Dobson Circulation, and declining halogen burdens may significantly change the calculus. Unfortunately these studies have yet to be performed.

The SpaceX Falcon Heavy launch window opens in several hours and, if successful, will be a transformational moment in commercial spaceflight, opening up deep space to private venture. Tune in to the launch livestream here.

# Impacts from Comets and Asteroids

The evolution of the Earth, and life on Earth, has been punctuated by periodic impacts with extraterrestrial bodies — some leading to mass extinction events. Most famous among these is the 65 Mya Cretaceous-Tertiary (K-T) extinction, during which three quarters of all surface species were eliminated following the impact of a 15 km diameter asteroid travelling at 20 km/s at the Chicxulub site (the cenote ring — sinkholes in the underlying standstone —  is traced on figure 1).

Figure 1: approximate location of cenote ring (and extrapolation of the semicircle to the marine environment) at the Chicxulub site in the Yucatan penninsula

Asteroid densities range between 3000 kg / m$^3$ (rocky) to 8000 kg / m$^3$ (metallic), depending on their composition. A back-of-the-envelope calculation provides an estimate of the impactor’s energy:

$\text{E}_\text{kinetic} = \frac{1}{2}\text{m}{v}^2$

where m ranges between $4\times 10^{16} - 1\times 10^{17}$ kg and v = 20,000 m / s and the kinetic energy ranges between $8\times 10^{24} - 2\times 10^{25}$ J [one should note that the impact velocity of an asteroid averages 18 km /s but may range between the Earth’s escape velocity (~10 km/s)  and the solar system’s escape velocity (~ 70 km/s]. Another back-of-the-envelope conversion (one ton of TNT = $4\times 10^9$ J) reveals that this impact released an energy equivalent to $2\times 10^9 - 5\times 10^9$ megatons of TNT (not a typo).

An interesting thing happens when explosions produce large fireballs. If the fireball is greater than the scale height of the atmosphere (~ 8 km), the fireball will backfire, producing a vacuum straw which draws material from within the fireball directly to the stratosphere and mesosphere. This was certainly the case with the K-T impactor.

The chemical and climate effects of this event were evaluated by a team from the NCAR and the University of Colorado in 2017. Bardeen and coworkers found catastrophic decadal reductions in surface and sea-surface temperatures following the massive injection of aciniform carbon, black carbon, and charcoal into the mesosphere and stratosphere by the event. They determined that this layer of carbon reduced surface insolation by one-billionfold for several months, and this impact winter didn’t recover to even one percent of the present downwelling flux until two years later. Meanwhile, the carbon layer in the atmosphere produced stratospheric heating between 50 — 100 K, cooking away the ozone layer: enhanced rates of HOx-mediated ozone destruction and reduced rates of ozone production resulted in surface UV flux increases of up to 300% for a period of about six years following.

Such impacts of large, monolithic asteroids or comets are infrequent. The time between expected impacts can be expressed as a power law function of impactor diameter in which D is given in meters and alpha is the fractional composition of ocean (0.7) or land (0.3), depending on the type of impact for which one wishes to compute the recurrence interval. If the impactor is greater than the average depth of the ocean (3.6 km), alpha is unity.

$\tau_\text{impact} = \frac{3.71\text{x}10^{-2}\text{D}^{2.377}}{\alpha}$

The threat from an asteroid or comet impact is ranked between 0 — 10 on the Torino Scale, in which an increasing metric conveys both the increasing probability of impact and a higher potential destructiveness. Though only known Earth-crossing bodies are ranked on the Torino scale, from the recurrence interval relation, it is evident that the catastrophic Chicxulub event is not a major concern ($\tau = 3.13\times 10^8$ years); however, smaller events are expected to pose a threat especially to the ozone layer.

Impacts of smaller bodies with the ocean will vaporize large amounts of ocean water and the trace species within it. Pierazzo et al. (2010) quantify the risk to ozone following such an event, finding that impactors greater than 500 m in diameter ($\tau$ = 97 ky, ~ 60,000 MT TNT) would increases the stratospheric burden of NOx, ClOx, BrOx, and HOx to such an extent that years-long decreases in total column ozone of up to 30% at midlatitudes would occur, resulting in twofold increases in erythemal radiation.

Similar results are found by Birks et al. (2007), who explore impactors between 150 — 1200 m in diameter. They find that asteroids of 450 m diameter ($\tau$ = 75 ky, ~ 40,000 MT TNT) are likely to cause significant years-long perturbations in total column ozone, and that smaller impactors are likely to only have continental or regional effects, though they do not model the effect of stratospheric soot injections, and these effects may be very significant, depending on the area over which biomass burning occurs.

This leads me to two very informative articles which were published a few days ago in the Journal of Geology. Wolbach and coworkers produced two comprehensive papers (paper one & paper two) which provide an explanation for the Younger Dryas cooling event (13 kya), which is characterized by a 1300 year disruption in climate, the collapse of the thermohaline circulation, the disappearance of the Clovis culture (note: not the Klobas culture), and the extinction of large animals across the world. The authors provide trace element and isotopic data from ~200 sedimentary and ice core sites around the world indicating that an abrupt and widespread input of carbon to the atmosphere coincided with an enhancement in platinum resultant from comet impact.

They posit that an Earth-crossing comet on the order of 100 km in size fragmented and produced a debris swarm several hundred Earth radii in length, estimating that the recurrence time between an encounter of the Earth and such a debris swarm is on the order of 50 ky. For each encounter, some $10^{13} - 10^{14}$ g of comet bits will rain down at 30 km/s (comets have an average velocity around 30 km/s at 1 AU) and produce a series of air bursts and land impacts. The authors find from ice core and sedimentary record that up to 10 million square kilometers of the Earth burned following the impact 13 kya (about 5% of the total land surface area of the planet at the time), producing a long-lived impact winter with extreme effect on the climate. Though the direct effects of the comet on the atmosphere dissipated within a decade, feedbacks were perturbed in such a way that the climate system was disrupted for a millennium afterward (causing iceberg calving, meltwater flooding, and subsequent thermohaline compensation).

The authors also briefly discuss the ozone impact of the comet encounter, noting that while soot falls out of the atmosphere after a few months the recovery of ozone lags behind for up to a decade — and that ozone depletion may play a primary role in the extinctions observed during this period.

So, I guess it’s time to add another entry to the stratospheric threat matrix. More on that later, when the work is complete.

# Climate Feedbacks from Stratospheric Ozone Loss Following Energetic Particle Precipitation

Incoming energetic particles (EPs), primarily protons and electrons, mediate some chemical and physical processes in the Earth’s atmosphere. Variations in the incoming flux of EPs can thus result in perturbations to the chemical partitioning of the stratosphere.

Though EPs arise from multiple origins, each source process produces a characteristic energy spectrum. Galactic cosmic rays (GCRs) are a highly energetic source of protons originating from outside the solar system, with characteristic energies between $10^{-12}$ J — $10^2$ J per nucleon. Solar cosmic rays (SCRs) originating from coronal mass ejections have energy spectra which typically peak around $10^{-7}$ J per nucleon. Fluxes of GCRs and SCRs are inversely correlated with 11-year periodicity. When the 11-year sunspot cycle is at a maximum, so too are SCR fluxes, while GCR fluxes are at a minimum due to pressure from solar winds.

Regardless of their origin, sufficiently energetic EPs ($> 10^{-11}$ J) will collide with atmospheric gases and produce charged secondary products, classified by their soft (electrons, positrons, and photons) and hard (muons, pions, etc.) components. These secondary products will then interact with atmospheric species in a variety of ways. One such scheme resulting in the production of NO$_\text{x}$ is presented below.

The reaction pathway is initiated following interaction with energetic secondary electrons, denoted $\text{e}^*$.

$\text{e}^* + \text{N}_2 \rightarrow \text{N}^+ + \text{N} + 2\text{e}^-$

$\text{e}^* + \text{N}_2 \rightarrow \text{N} + \text{N} + \text{e}^-$

$\text{e}^* + \text{N}_2 \rightarrow \text{N}_2^+ + 2\text{e}^-$

$\text{e}^* + \text{O}_2 \rightarrow \text{O}_2^+ + 2\text{e}^-$

$\text{e}^* + \text{O}_2 \rightarrow \text{O} + \text{O}^+ + 2\text{e}^-$
Subsequent recombination/exchange chemistry further enhances atomic nitrogen.

$\text{N}_2^+ + \text{O} \rightarrow \text{NO}^+ + \text{N}$

$\text{N}_2 + \text{N}^+ \rightarrow \text{N}_2^+ + \text{N}$
$\text{N}_2^+ + \text{e}^- \rightarrow \text{N} + \text{N}$
$\text{NO}^+ + \text{e}^- \rightarrow \text{N} + \text{O}$

$\text{N}^+ + \text{O} \rightarrow \text{N} + \text{O}^+$

Finally, reaction of atomic nitrogen with molecular oxygen produces nitric oxide.

$\text{N} + \text{O}_2 \rightarrow \text{NO} + \text{O}$

Similarly, EPP events may produce HO$_\text{x}$. For more details, Mironova et al. (2015) provide a very thorough review on the interaction of EPPs with the atmosphere.

Once formed, HO$_\text{x}$ and NO$_\text{x}$ will engage mesospheric ozone loss processes. A schematic for the HO$_\text{x}$-mediated destruction of ozone in the middle atmosphere is presented in figure 1 — and NO$_\text{x}$-mediated processes will be quite similar. Ozone loss processes produced in the immediate environment of EPP interactions are frequently called direct effects. Direct effects from NO$_\text{x}$ and HO$_\text{x}$ produced by EPPs are a significant source of variability in mesospheric ozone (upwards of 25% of mesospheric ozone).

Figure 1: A graph theory representation of the HOx-mediated catalytic destruction of ozone in the middle stratosphere. Klobas thesis 2018.

Mesospheric HO$_\text{x}$ from EPPs is not long-lived and does not have effects beyond the immediate environment in which it is formed; however, NO$_\text{x}$ has a long lifetime and will transport to the middle stratosphere, where it will further engage ozone loss processing. Ozone loss following transport from the region in which the NO$_\text{x}$ was formed is referred to as an indirect effect of EPPs. Ozone losses from indirect effects are estimated to be as large as 15% in the middle and upper stratosphere.

The action of these events is largely confined to the polar regions because (1) EPs are entrained to geomagnetic fields which direct particle fluxes to the magnetic poles, and (2) mesospheric and stratospheric subsidence of NO$_\text{x}$ occurs at the poles.

One of the most fascinating papers I’ve ever read on the potentially disastrous effects of EPPs concerns a seemingly unlikely situation: a magnetic field reversal of the Earth concurrent with the transit of the Solar System through a region of high interstellar density.  The story goes like this:

The density of the interstellar medium is heterogeneous. If the solar system were to transit a region with higher density, enhanced rates of EP precipitation would be expected as the heliosphere contracts. The coincidence of both situations seems highly improbable at first glance; however, a review of the expected frequencies and durations of these events provides the evidence to the contrary. The solar system has transited regions of enhanced density ($\geq 100$ H atoms/cm$^3$ vs 0.3 H atoms/cm$^3$ currently) $\approx$ 135 times in the previous 4.5 Gy and the average duration of a transit was $\approx$ 1 My. The Earth’s magnetic field reverses stochastically, however on average once every 300 ky, the event itself lasting several thousand years. A statistical treatment thus indicates that there may have been as many as 7 instances in the prior 250 My of a magnetic field reversal coinciding with the transit of the solar system through a region of enhanced density — and that EP fluxes would remain elevated for up to a period of thousands of years.

Such 1000-year periods of enhanced GCR precipitation are projected to lead to widespread, long-term depletions of the stratospheric ozone layer. Pavlov et al. (2005) use a 2-D dynamical/chemical model to evaluate the new steady-state ozone solution following a 300-fold enhancement of GCR, finding that total column ozone decreases by 40\% globally, and by up to 80% at high latitudes after a 5 — 10 year adjustment period.

It’s a completely insane story with extraordinarily dire consequences. The fact that it’s not only possible, but expected to occur on a basis as frequent as the collision of a 5 km asteroid (one third of a Chicxulub impactor), with 1000-year implications is alarming; while Bruce Willis might be able to save us from an incoming asteroid, there’s nothing we can do to restart the geomagnetic field or redirect the velocity of the Solar System through less dense space.

To put this all in context, the Chicxulub event is estimated to have perturbed the ozone layer, the climate, and pretty much everything on Earth for about a decade or so, but I’ll write a post about that another time..

Two papers recently appeared on my google scholar alerts on the topic of EPPs.

The first, by Andersson et al. (2018), provides an accounting of total chemical forcing from EPP processes on ozone, while exploring the effect of medium-energy electrons (MEE, 300 — 1000 keV), whose effect has only recently been incorporated into chemical models [WACCM, this paper, SOCOL in Arsenovic et al. (2016)]. Andersson and workers find that the inclusion of MEE results in a strong enhancement of indirect ozone processing rates as low as 30 km — accounting for up to 8% of the ozone depending on the pressure level.

The second, by Meraner and Schmidt (2018), explores the climate impact of the radiative forcing effects from EPP-induced ozone loss. The authors find that the winter stratosphere increases in temperature as a result of ozone loss and gradient-driven transport is subsequently reduced.This effect could slightly reduce the strength of the polar vortex, though they state that the modeled response is small and that the effect of EPPs on climate may be minimal. This finding was surprising to me as previous works (though with less-sophisticated models) predicted stratospheric cooling  with resultant strengthening of the polar vortex and a much larger surface warming effect.

# Perturbation of Ozone by Enhancements in Meteoric Smoke Following Major Meteor Showers

Between 5 – 300 tons of meteoric material is deposited in the Earth’s atmosphere each
day. The uncertainty in that estimate is large due to limitations in the different instrumental and analytical techniques used to obtain meteoric mass fluxes — and the fact that no single technique can provide an integrated estimate over the entire size distribution. Recent estimates of mass fluxes seem to be biased toward the lower quarter of this quantity (e.g., 40 — 50 tons per day) — and this estimate contains both contributions from meteorite and cosmic dust infall.  In the case of cometary dust, the average velocity of an incoming meteorite is less than 15 km/s (though greater than 11 km /s — Earth’s escape velocity), and some 20% of the infalling mass is converted to meteoric smoke (nanometer-scale metallic particles of meteoric origin) via ablative processes (whose efficiency increases as a function of velocity). This accounts for roughly 10 tons meteoric smoke per day.

These particles subsequently sediment toward the poles during their 4-year lifetime, serving as mesospheric and stratospheric cloud nuclei and possibly participating directly in ozone chemistry.

Meteor showers are periodic events that occur when the Earth’s orbit sweeps past the path of a cometary debris trail and can present orders-of-magnitude enhancements in the daily meteoric flux rate over zonally localized regions. This morning I woke up to see a paper on correlations between large meteor showers and changes in total ozone: Gorbanev et al. (2017) explore TOMS measurements over several decades, using autocorrelation techniques. The decreases they find are significant — on the order of 5 DU. Figure 1, below, demonstrates the autocorrelation peaks of total ozone with the radar-returned meteor infall rate.

Figure 1: The autocorrelation functions from the total
ozone measured during annual meteor showers as a function of Time lag (days). (From Gorbanev et al. [2017])

The authors then demonstrate the disruption of the seasonal Autumnal enhancement in northern hemispheric ozone by the occurrence of the Leonid showers (figure 2, below). Following peak meteor activity, total ozone declines by about 5 DU over a period of 14 days (November 18 — December 2). Following the disturbance, total ozone resumes its seasonal trend.

Figure 2: 1999 northern hemispheric total ozone.  The Leonid meteor shower disrupts the seasonal increase in ozone by about 5 DU over a period of two weeks. The seasonal trend recovers and resumes afterward. (From Gorbanev et al. [2017])

The authors conclude that this signal can be used to identify interactions between meteoric material and the atmosphere. Their story is open access and available at this link.

# 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.

# Interactions between Climate Change and Volcanism

An interesting review detailing the inter-linkage between climate change and volcanic activity hit my google scholar alert inbox.  Cooper et al. (2018) discuss not only how large explosive volcanic eruptions might impact surface radiative forcing via sulfate aerosols scattering incoming sunlight, but also how symptoms of the changing climate might influence the frequency of volcanic eruptions.

The story goes like this: as the cryosphere is destabilized, landlocked ice sheets will melt. In places with very thick ice sheets, this will produce a very abrupt (on a geological timescale) shift in lithospheric pressure loading. As one might infer from figure 1, below, such a shift might significantly enhance the fraction of melt partitioning in the vicinity. More melt, of course, translates to more frequent volcanism, though there isn’t an instantaneous response — it takes centuries for the new magma to reach the surface.

Figure 1: Example of decompression melting. An abrupt change in surface loading, (e.g., isothermal decompression) provokes a phase transition.  (adapted from Arham Bahar / Universiti Malaysia Kelantan)

As I was reading this review I was reminded of a work relating another lesser-known link between climate change and volcanism. Aubry et al. (2016) explore how climate change might influence eruption column height.

Initially, a volcanic plume may be considered in loose analogy to a molecular beam — the expansion of a high pressure gas into a vacuum through an aperture. The expanding volcanic gas carries with it some momentum, thrusting itself upward. At some point, friction with (and entrainment of) the surrounding atmosphere counters this momentum. Depending on the strength of the eruption, the gas thrust region may extend from the crater rim to several km above the crater rim. Above the gas thrust region, convective thrust drives vertical transport within the eruption column. Air parcels within the eruption column are considerably buoyant and will convect to their point of neutral buoyancy. Finally, at this point, as in a cloud top, the parcels will diffuse horizontally, forming the umbrella cloud.

The maximum height of an eruption column is limited by the amount of heat transferred to the atmosphere per unit time (power, Watts), as it is convection which ultimately drives this process. Ultimate column height can be expressed according to the following equation, in which N is the the Brunt–Väisälä frequency, M is the mass ejection rate, and horizontal transport coefficients k1 + k2 = 1.

$\text{H}\propto\text{N}^{-\text{k}_1}\text{M}_0^{\text{k}_2}$

The Brunt–Väisälä frequency is dependent on the lapse rate ($\Gamma = -\frac{\text{dT}}{\text{dz}}$), the specific heat capacity $\text{c}_\text{p}$, the gravitational constant g, and the zonal temperature T.
$\text{N}^2 = \frac{\text{g}}{\text{T}}\left(\frac{\text{g}}{\text{c}_\text{p}}-\Gamma \right)$
In order for a volcanic eruption to influence global climate (and stratospheric chemistry), its eruption column must penetrate the tropopause (defined as the point of inflection in the lapse rate) — and climate change is expected to produce changes in both the tropopause height and the lapse rate. In the future, in most regions, the Brunt–Väisälä frequency will decrease. In turn, the plume height for any given eruption mass ejection rate will be shorter. Aubry et al. (2016) conclude that stratospheric input of volcanic gases will be less frequent in a future climate change scenario.

Figure 2: Lapse rate ($-\frac{dT}{dz}$) as a function of year within the RCP 6.0 emissions storyline. Note that lapse rate increases as the climate changes. (Klobas thesis 2018)

Finally, since we’re on the topic of unexpected climate-change/volcanism interactions, I’d like to bring attention to a work by Laakso et al. (2016) in which they simulate what might happen if a large volcanic eruption were to occur during albedo modification geoengineering by stratospheric aerosol injection –aka solar radiation management (SRM).

Figure 3 below provides the total aerosol burden of sulfate in Tg. For controls, they simulate a volcano-only scenario (black solid), a SRM-only scenario (black dashes), the sum of the SRM-only and volcano-only scenarios (magenta dashes). There are two experimental perturbations: SRM is immediately stopped when the volcano erupts (blue solid) and SRM continues as normal despite the volcanic eruption (red solid).

Surprisingly, the time required to recover to the pre-eruption sulfate burden during SRM is halved relative to the volcano-only case, even if the SRM program continues as normal after the eruption. Why? Because coagulation processes will produce larger aerosols more quickly, reducing the lifetime of sulfate aerosol via gravitational sedimentation. They go on to show that the cessation of SRM following a volcanic eruption could cause very rapid surface warming as a result of reduced aerosol lifetimes. The authors obviously explore more than this — and it’s open access — so I encourage you to read it yourself for more info.