Tropospheric Halogen Photochemistry in the Rapidly Changing Arctic

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Tropospheric Halogen Photochemistry in the Rapidly Changing Arctic Kerri A. Pratt 1,*,@ The Arctic is undergoing rapid change, with increasing temperatures, rapid sea ice loss, and resulting development. The sea ice, snowpack, and atmosphere are connected through multiphase chemistry, including unique halogen photochemistry that changes atmospheric composition and pollutant fate. Field-based mass spectrometry is leading new insights into this halogen chemistry. The Arctic is warming at a faster rate than anywhere else on Earth, with rapidly shrinking sea ice transforming the region [1]. The Arctic surface, characterized by sea ice, snow, seawater, and tundra, serves as a source of atmospheric trace gases and particles, as well as a surface for multiphase reactions. The Arctic recently transitioned from being dominated by sea ice persisting for multiple years (multiyear sea ice) to first-year sea ice (i.e., b1-year old) [1]. In the springtime, this thinner first-year sea ice is more likely to fracture, resulting in leads (areas of open water surrounded by sea ice). Unprecedented summertime sea ice loss is increasing the extent of open ocean, which emits trace gases and sea spray aerosol. This open water also increases accessibility to shipping and resource extraction, with combustion emissions that pose a serious concern for local Arctic air quality and acceleration of climate change. With these profound changes, there is an urgent need to study the chemistry connecting the Arctic sea ice, snowpack, and atmosphere.

Yet, due to extreme logistical and measurement challenges, Arctic atmospheric chemical composition data are scarce, resulting in an inadequate understanding of Arctic atmospheric processes and significant uncertainties in model predictions.

[4]. However, under polluted conditions, the production of NOx reservoir species, including HNO3, HO2NO2, and peroxyacetyl nitrate (C2H3NO5), slows bromine recycling [5]. This poorly understood, yet sensitive, dependence of bromine chemistry on NOx levels is important to consider given increasing local Arctic combustion Following springtime polar sunrise, the emissions. Arctic lower troposphere experiences ozone depletion events, during which Highly reactive iodine and chlorine atoms ozone (O3) episodically and rapidly de- also dramatically alter Arctic atmospheric clines to near-zero levels [2]. Typically, re- composition through iodine atom reaction active trace gases are removed through with ozone and contribution to new partireaction with the hydroxyl radical (OH), cle formation and growth and chlorine formed from O3 photolysis and subse- atom reaction with long-lived hydrocarquent reaction of excited atomic oxygen bons [3]. Early evidence of active chlorine [O(1D)] with water vapor. However, in the chemistry in the springtime Arctic tropopolar regions, low temperatures suppress sphere was observed from the depletion tropospheric water vapor content and of short alkanes, which were correlated OH formation. Barrie and colleagues [2] with chlorine atom reaction rate constants, hypothesized that the ozone depletion is and corresponding increases in oxidized caused by bromine chemistry because of hydrocarbons [3]. For many hydrocarthe inverse relationship between filterable bons, chlorine atom reaction rate conbromide (HBr + particulate Br–) and O3. stants are several orders of magnitude Through satellite, aircraft, and ground- greater than those for OH, resulting in based remote sensing (differential optical rapid destruction of typically long-lived absorption spectroscopy; DOAS) observa- hydrocarbons and potential contribution tions of bromine monoxide (BrO), wide- to secondary aerosol production [3]. spread bromine chemistry during spring in Early measurements of total photolyzable the polar regions is well-documented [3]. chlorine (e.g., Cl 2 and HOCl) and broHowever, there are significant knowledge mine (e.g., Br 2 and HOBr) provided gaps in our understanding of how halogen conclusive evidence of Arctic springtime chemistry is being altered by the changing halogen chemistry and confirmed that Arctic surface. organohalogen compounds play a minor role [3]. Autocatalytic destruction of tropospheric ozone involves the conversion of condensed-phase bromide to gasphase bromine, while increasingly depleting atmospheric ozone, through the ‘bromine explosion’ (see Equations I–VII in Box 1) [3]. Bromine atoms are also implicated in the oxidation of long-lived elemental mercury [3]. Notably, even under background NOx (NO + NO2) conditions, the NO2 pathway for Br2 production (see Equations V–VII in Box 1) is competitive

Analytical advances in real-time, fielddeployable mass spectrometry employing chemical ionization have enabled recent insights into our understanding of Arctic tropospheric halogen chemistry. At Alert, Nunavut, Canada, Foster and colleagues [6] first measured Br2 and BrCl, with elevated levels observed following polar sunrise. Br2 was observed at up to 25 ppt [6], suggesting that it is the primary source of bromine atoms. Later, during Trends in Chemistry, September 2019, Vol. 1, No. 6

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Box 1. The ‘Bromine Explosion’ Cycle The ‘bromine explosion’ cycle is initiated by molecular bromine (Br2) photolysis to produce bromine atoms (Br) that react with O3 to produce BrO (Equations I and II). While BrO self-reaction regenerates Br atoms (Equation III), BrO also reacts with HO2 and NO2, to produce HOBr and BrONO2, respectively, which can react on acidic bromide (Br–)-containing surfaces to reproduce Br2 (Equations IV–VII). Br2 þ hv → 2Br

½I

Br þ O3 → BrO þ O2

½II

BrO þ BrO → 2Br þ O2

½III

BrO þ HO2 → HOBr þ O2

½IV

BrO þ NO2 → BrONO2

½V

BrONO2 þ H2 O → HOBrðaqÞ þ HNO3ðaqÞ

½VI

HOBr þ Br– ðaqÞ þ Hþ ðaqÞ → Br2 þ H2 O

½VII

spring near Utqiaġvik (Barrow), Alaska, Liao and colleagues [7] measured Cl2 at highly elevated levels (up to 440 ppt) above the coastal snowpack during daytime under background ozone conditions. Custard and colleagues [8] then measured chlorine monoxide (ClO), resulting in the first simultaneous high-time resolution Cl2 and ClO observations, showing that Cl2 photolysis is the primary source of chlorine atoms and providing an improved constraint on atmospheric chlorine atom levels [1]. During February near Utqiaġvik, Raso and colleagues [9] measured I2 at 0.3–1.0 ppt, correlated with sunlight (Figure 1A). Sipilä and colleagues [10] detected iodic acid (HIO3), coincident with new particle formation and growth, at Station Nord in northern Greenland during spring (February–May; Figure 1B). Together, these results suggest that snowpack I2 is likely the primary iodine atom source in the springtime Arctic leading to iodic acid and new particle formation. Overall, these new measurements highlight how developments in analytical methodology are advancing understanding Arctic halogen chemistry. However, these observations are limited, and many critical species (e.g. X, HOX, 546

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XONO2, XNO2, HgX2, and HgXNO2, where X is Br, Cl, or I) have yet to be observed, limiting model evaluations and highlighting the need for additional observations and further method development. Field-deployable mass spectrometry has been critical in isolating the essential role of the snowpack in Arctic halogen chemistry. Foster and colleagues [6] first observed elevated Br2 levels within the top 10 cm of the Arctic snowpack, consistent with airborne DOAS observations of BrO elevated near the snowpack surface [11]. Through outdoor chamber experiments testing snow and ice samples, Pratt and colleagues [11] observed efficient Br2 production from sunlit, acidic, surface snow, collected over both tundra and first-year sea ice (multiyear ice was not available). Br2 was not observed, above the limit of detection, for highly saline samples [sea ice, brine icicles (frost flower proxies), basal snow], likely due to buffering of the ice/snow surface [11]. Br2 production occurred by bromide oxidation by a condensed-phase oxidant produced photochemically (e.g., OH production by nitrite photolysis at the

snow grain surface) and was enhanced by ozone addition due to a bromine explosion (see Equations I–VII in Box 1) occurring within the snowpack interstitial air [11]. Toyota and colleagues [12] implemented snowpack Br2 production into a regional model, showing agreement with BrO spatial distributions. Arctic snowpack bromide measurements show transport of gas-phase bromine inland, beyond sea spray aerosol transport [3]. Indeed, elevated BrO near the snowpack surface was recently observed over far inland coastal tundra [13]. It is likely that continued changes to the sea ice and snow surface will impact the spatial distribution of Arctic halogen chemistry, with modeling efforts needed to explore impacts of these inland regions comprising a large fraction of the future snow-covered Arctic. During field experiments exposing the Arctic snowpack to sunlight and artificial light, photochemical snowpack production of Cl2, BrCl, and I2 was also observed [9,14]. The relative production of Br2 and BrCl in the Arctic snowpack was enhanced, with Cl2 production limited, by the addition of O3 and at air temperatures below which NaCl·2H2O precipitates, limiting chloride availability for reaction [14]. From vertical gradient measurements above the coastal snowpack, emission rates of Br2 and Cl2 were derived by Custard and colleagues [14] and shown to explain observed near-surface reactive halogen levels [4]. Based on these data, simulations show that multiphase reactions of ClONO2 are key to snowpack Cl2 production, with Cl2 uptake to atmospheric particles suggested to be an important sink of Cl2 and source of BrCl [4]. Together these results show that the surface snowpack is the primary source of molecular halogens to the near-surface Arctic troposphere. To improve simulations, measurements of the dry deposition of molecular halogen precursors are needed to

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and colleagues [15] observed transport of a lofted BrO plume, showing the first observational evidence of trace reactive bromine undergoing multiphase reactions aloft on atmospheric particles, a previously long-standing theory. This mechanism for atmospheric transport of BrO provides an explanation for satellite observations and measured free tropospheric bromine chemistry [3], and a means to extend the spatial distribution of halogen chemistry across the Arctic region [15]. The source(s) of bromidecontaining aerosol for bromine recycling aloft has remained uncertain, due to limited parallel chemical measurements of trace gases and particles. Blowing snow and open leads have been proposed as sources of particulate bromide [3]; however, these hypotheses have yet to be tested empirically through coincident chemical measurements of snow, aerosols, and trace gases in the Arctic. Reactive bromine has primarily been observed in the Arctic during calm wind periods [3,6,11,13], leaving questions about the vertical propagation of surface-based halogen chemistry and source(s) of aerosols contributing to recycling.

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Figure 1. Springtime Arctic Snowpack Photochemical I2 Production, HIO3, and New Particle Formation and Growth. (A) Radiation and atmospheric I2 mole ratios, near Utqiaġvik, Alaska, on 1 and 2 February 2014, were observed at 1 m above the snowpack (ambient) and at 10 cm below the snowpack surface (interstitial air), with temporal variations associated with sunlight. Error bars are propagated measurement uncertainties. Adapted, with permission, from [9]. (B) New particle formation and growth event (shown as aerosol number concentrations as a function of particle diameter and time) were observed simultaneously with elevated HIO3 concentrations at Station Nord, Greenland, on 31 March 2015. Adapted, with permission, from [10].

constrain recycling in models, and the enrichment of bromide in the surface snowpack in the dark winter [14] needs to be investigated. Another significant challenge is the current inability to measure the chemical composition and pH of the snow grain surface in the field, or recreate these complex chemical and

physical properties of snow grains in the laboratory. While the snowpack has been identified as a significant source of molecular halogens to the Arctic troposphere, our understanding of the vertical propagation of this chemistry is lacking. Peterson

Many knowledge gaps remain in our understanding of the interactions between atmospheric trace gases, aerosols, clouds, and the surface snowpack in the Arctic. To date, most efforts have focused on trace halogen gas measurements, with few simultaneous observations with aerosols and clouds. The Arctic surface is quickly transforming with sea ice loss, increasing marine trace gas and aerosol emissions, and opening the region to development, with associated combustion emissions. Increasing open water also modifies atmospheric turbulence, clouds, and precipitation, which are coupled to Arctic halogen chemistry. Therefore, there is great urgency to observe and study Arctic atmospheric composition to predict future climate impacts, while also Trends in Chemistry, September 2019, Vol. 1, No. 6

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probing halogen chemistry that is relevant to the remote marine atmosphere across the globe. Acknowledgments K.A.P. is grateful for support from an Alfred P. Sloan Foundation Research Fellowship in Chemistry.

1 Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, USA

*Correspondence: [email protected] (K.A. Pratt). @ Twitter: @ArcticKerri https://doi.org/10.1016/j.trechm.2019.06.001 © 2019 Elsevier Inc. All rights reserved.

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