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Direct Detection of Atmospheric Atomic Bromine Leading to Mercury and Ozone Depletion

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Direct Detection of Atmospheric Atomic Bromine Leading to Mercury and Ozone Depletion Direct detection of atmospheric atomic bromine leading to mercury and ozone depletion Siyuan Wanga,1, Stephen M. McNamaraa, Christopher W. Mooreb,2, Daniel Obristb,3, Alexandra Steffenc, Paul B. Shepsond,e,f,4, Ralf M. Staeblerc, Angela R. W. Rasod, and Kerri A. Pratta,g,5 aDepartment of Chemistry, University of Michigan, Ann Arbor, MI 48109; bDivision of Atmospheric Science, Desert Research Institute, Reno, NV 89523; cAir Quality Processes Research Section, Environment and Climate Change Canada, Toronto, ON M3H5T4, Canada; dDepartment of Chemistry, Purdue University, West Lafayette, IN 47907; eDepartment of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907; fPurdue Climate Change Research Center, Purdue University, West Lafayette, IN 47907; and gDepartment of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109 Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved May 29, 2019 (received for review January 12, 2019) Bromine atoms play a central role in atmospheric reactive halogen O3 producing BrO ([R2]), which can undergo photolysis to chemistry, depleting ozone and elemental mercury, thereby reform Br ([R3]). Br is also regenerated by BrO reaction with enhancing deposition of toxic mercury, particularly in the Arctic NO ([R4]), or a halogen monoxide (XO = BrO, ClO, or IO; near-surface troposphere. However, direct bromine atom mea- [R5]) (19). surements have been missing to date, due to the lack of analytical capability with sufficient sensitivity for ambient measurements. Br2 + hv → Br + Br, [R1] Here we present direct atmospheric bromine atom measurements, conducted in the springtime Arctic. Measured bromine atom levels − Br + O → BrO + O , [R2] reached 14 parts per trillion (ppt, pmol mol 1;4.2× 108 atoms 3 2 − per cm 3) and were up to 3–10 times higher than estimates using + + → + [R3] previous indirect measurements not considering the critical role of BrO hv O2 Br O3, molecular bromine. Observed ozone and elemental mercury deple- tion rates are quantitatively explained by the measured bromine BrO + NO → Br + NO2, [R4] atoms, providing field validation of highly uncertain mercury chem- EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES istry. Following complete ozone depletion, elevated bromine con- BrO + XO → Br + X + O2. [R5] centrations are sustained by photochemical snowpack emissions of molecular bromine and nitrogen oxides, resulting in continued at- Br atom concentrations have previously been estimated based on mospheric mercury depletion. This study provides a breakthrough in hydrocarbon measurements (known as the “hydrocarbon clock” quantitatively constraining bromine chemistry in the polar atmo- sphere, where this chemistry connects the rapidly changing surface Significance to pollutant fate. atmosphere | bromine | ozone | mercury We report atmospheric bromine atom measurements, which quantitatively explain the removal of the ozone and mercury from the near-surface Arctic troposphere. Bromine atoms are apid ozone (O3) depletion in the springtime Arctic tropo- proposed to dominate mercury oxidation and its deposition Rsphere was discovered several decades ago (1), and has been into the ecosystem on the global scale. Therefore, the analyt- attributed to reactive bromine chemistry (2, 3). However, most ical capability of in situ bromine atom measurements and the global models do not reproduce trends in Arctic ozone due to results herein have broad significance beyond the Arctic, par- missing and/or incomplete descriptions of bromine chemistry (4). ticularly in areas of abundant bromine chemistry, including the Reactive bromine chemistry is also prevalent in the springtime Antarctic near-surface troposphere, volcanic plumes, above Antarctic near-surface troposphere (5), tropical and subtropical saline lakes, and especially the tropical upper troposphere, free troposphere (6, 7), volcanic plumes (8), and above saline where bromine atoms are predicted to exist in high relative lakes (9). In addition to regulating the fate of atmospheric pol- abundance. lutants, tropospheric O3 contribution to radiative forcing is particularly large in the polar regions, due to the low absolute Author contributions: S.W. and K.A.P. designed research; P.B.S. led the field campaign; humidity and high surface albedo over the snowpack and sea-ice S.W., S.M.M., C.W.M., D.O., A.S., R.M.S., and K.A.P. performed research; S.W., C.W.M., surfaces (10). In the polar lower atmosphere, ozone depletion A.R.W.R., and K.A.P. analyzed data; and S.W. and K.A.P. wrote the paper. events (ODEs) often occur simultaneously with the loss of at- The authors declare no conflict of interest. 0 mospheric mercury (Hg ) (11, 12), a global pollutant primarily This article is a PNAS Direct Submission. emitted from anthropogenic (including fossil-fuel combustion) Published under the PNAS license. and natural (including geogenic) sources (13). Globally, Br Data deposition: Br atom data have been deposited in the NSF Arctic Data Center, avail- atoms are proposed to be the primary atmospheric oxidant able at https://arcticdata.io/catalog/view/doi:10.18739/A2D79598P. 0 II reacting with Hg to produce oxidized mercury (Hg ,or 1Present address: Atmospheric Chemistry Observations & Modeling, National Center for HgXY, for example, HgBr2 or HgBrNO2)(14–16), yet direct Atmospheric Research, Boulder, CO 80307. observational evidence of this reaction is unavailable due to the 2Present address: Gas Technology Institute, Des Plaines, IL 60018. lack of Br measurements. Long-term records show that mer- 3Present address: Department of Environmental, Earth and Atmospheric Sciences, Univer- cury is readily transported to the Arctic (12), and the Arctic sity of Massachusetts, Lowell, MA 01854. II snowpack serves as a net sink for atmospheric Hg during 4Present address: School of Marine and Atmospheric Sciences, Stony Brook University, springtime (17), with deposition introducing toxic mercury into Stony Brook, NY 11794. the ecosystem (13). 5To whom correspondence may be addressed. Email: [email protected]. Atomic Br plays a central role in atmospheric bromine This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. chemistry. Following photochemical snowpack production (18), 1073/pnas.1900613116/-/DCSupplemental. Br2 is photolyzed to produce Br ([R1]). Br rapidly reacts with www.pnas.org/cgi/doi/10.1073/pnas.1900613116 PNAS Latest Articles | 1of6 Downloaded by guest on September 27, 2021 method) (20), and recent molecular halogen (Br2, BrCl) measure- Measured Br mole ratios peaked during daytime (maximum: − ments (21). Alternatively, Br can be inferred from BrO, commonly 14 ppt, 4.2 × 108 atoms per cm 3), with near-zero levels at night, measured from satellite, airborne, and ground-based instruments and the diurnal variation in Br closely tracked BrO (Fig. 1). At (19). However, Br regeneration from BrO likely plays a minor role low O3 levels and/or low temperatures (28), Br is expected to be when O3 is depleted (22), suggesting that Br estimates from BrO more abundant than BrO (2). Measured Br/BrO ratios (Fig. 1) observations alone may be biased. ranged from 7.5 (during an ODE on 20 March) to 0.1 (during background O3). For example, on 20 March 2012 (Fig. 2), O3 was Results depleted to less than 1 part per billion (ppb) and observed Br Here, we present an in-depth analysis of reactive bromine reached 9 ppt in the morning, while BrO remained at less than 5 chemistry and its impacts on ozone and mercury from 14 to 28 ppt, and Hg0 quickly dropped to below detectable concentra- March 2012 during the Bromine, Ozone, and Mercury Experi- tions. Br2, the primary precursor to Br, reached 35 ppt in the ment (BROMEX) conducted near Utqiagvik_ (Barrow), Alaska early morning, with decreasing levels observed as the sun rose, SI Appendix ( , Fig. S1). This examination employs an unprece- due to increasing Br2 photolysis competing with Br2 photo- dented suite of halogen and mercury measurements, including chemical snowpack production (18, 26). In the afternoon, O3 direct measurements of Br, along with Br2, BrO, HOBr, Cl2, increased to 5 ppb, with BrO reaching 12 ppt, causing the 0 II – ClO, O3, and mercury (Hg and Hg ) (23 26). Br was measured measured Br/BrO ratio to decrease. using chemical ionization mass spectrometry (CIMS), with monitoring of bromine isotopes for unambiguous identification Importance of Br2. BrO observations can be used to estimate Br (SI Appendix, Fig. S2). Laboratory experiments were performed atom concentrations by multiplying by a modeled Br/BrO ratio for calibration and to rule out potential interferences (Methods (29); however, this ratio has not been tested due to the lack and SI Appendix). Sampled air masses traveled across the of Br measurements. As shown in Fig. 3A, the observed Br/ Beaufort Sea and snow-covered tundra, providing Arctic back- BrO ratios decrease with increasing ozone, and are generally ground conditions (23). During this time, the spatial scale of explained by an observationally constrained model (SI Appen- active bromine chemistry was typically larger than 30 km, based dix, Methods). When O3 was 3–7 ppb, Br production from Br2 on BrO and O3 observations at multiple nearby locations (25, photolysis ([R1]) was equivalent to BrO production from Br 27). This is also consistent with springtime satellite BrO column reaction with O3 ([R2]). When O3 was below 2–3 ppb, mea- observations which often show widespread surface BrO plumes
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