Bromine Atom Production and Chain Propagation During Springtime Arctic Ozone Depletion Events in Barrow, Alaska
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Atmos. Chem. Phys., 17, 3401–3421, 2017 www.atmos-chem-phys.net/17/3401/2017/ doi:10.5194/acp-17-3401-2017 © Author(s) 2017. CC Attribution 3.0 License. Bromine atom production and chain propagation during springtime Arctic ozone depletion events in Barrow, Alaska Chelsea R. Thompson1,a,b, Paul B. Shepson1,2, Jin Liao3,c,d, L. Greg Huey3, Chris Cantrell4,e, Frank Flocke4, and John Orlando4 1Department of Chemistry, Purdue University, West Lafayette, IN, USA 2Department of Earth and Atmospheric Sciences and Purdue Climate Change Research Center, Purdue University, West Lafayette, IN, USA 3School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA 4National Center for Atmospheric Research, Boulder, CO, USA anow at: Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA bnow at: Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA cnow at: Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA dnow at: Universities Space Research Association, Columbia, MD, USA enow at: Department of Atmospheric and Ocean Sciences, University of Colorado Boulder, Boulder, CO, USA Correspondence to: Chelsea R. Thompson ([email protected]) Received: 22 December 2015 – Discussion started: 18 January 2016 Revised: 1 November 2016 – Accepted: 9 February 2017 – Published: 9 March 2017 Abstract. Ozone depletion events (ODEs) in the Arctic are termined to be quite small, at < 1.5, and highly dependent primarily controlled by a bromine radical-catalyzed destruc- on ambient O3 concentrations. Furthermore, we find that Br tion mechanism that depends on the efficient production and atom production from photolysis of Br2 and BrCl, which is recycling of Br atoms. Numerous laboratory and modeling predominately emitted from snow and/or aerosol surfaces, studies have suggested the importance of heterogeneous re- can account for between 30 and 90 % of total Br atom pro- cycling of Br through HOBr reaction with bromide on saline duction. This analysis suggests that condensed-phase pro- surfaces. On the other hand, the gas-phase regeneration of duction of bromine is at least as important as, and at times bromine atoms through BrO–BrO radical reactions has been greater than, gas-phase recycling for the occurrence of Arc- assumed to be an efficient, if not dominant, pathway for tic ODEs. Therefore, the rate of the BrO self-reaction is not Br reformation and thus ozone destruction. Indeed, it has a sufficient estimate for the rate of O3 depletion. been estimated that the rate of ozone depletion is approxi- mately equal to twice the rate of the BrO self-reaction. Here, we use a zero-dimensional, photochemical model, largely 1 Introduction constrained to observations of stable atmospheric species from the 2009 Ocean–Atmosphere–Sea Ice–Snowpack (OA- The springtime depletion of boundary layer ozone in the SIS) campaign in Barrow, Alaska, to investigate gas-phase Arctic has been the subject of intense research for several bromine radical propagation and recycling mechanisms of decades. Early observations revealed a strong correlation be- bromine atoms for a 7-day period during late March. This tween ozone depletion events (ODEs) and enhancements in work is a continuation of that presented in Thompson et filterable bromine (Barrie et al., 1988). This discovery led re- al. (2015) and utilizes the same model construct. Here, we searchers to propose a mechanism for the bromine-catalyzed use the gas-phase radical chain length as a metric for ob- destruction of ozone. jectively quantifying the efficiency of gas-phase recycling of bromine atoms. The gas-phase bromine chain length is de- Published by Copernicus Publications on behalf of the European Geosciences Union. 3402 C. R. Thompson et al.: Bromine atom production and chain propagation in a manner that is consistent with the stoichiometry of Re- action (R7). Br2 C hv ! 2Br (R1) To efficiently sustain the ozone destruction cycle to Br C O3 ! BrO C O2 (R2) the point of near-complete loss of boundary layer ozone ([O ] < 2 ppb), bromine atoms must be continually recycled BrO C BrO ! Br2.or Br C Br/ C O2 (R3) 3 through some combination of the above mechanisms. The This reaction cycle requires an initial source of bromine gas-phase reaction cycle described by Reactions (R1)–(R3) atoms to the boundary layer. Laboratory and theoretical stud- has generally been considered to be the dominant pathway ies have suggested that Br2 could be produced through oxida- for Br reformation following the initial activation of Br2 from tion of bromide present in salt-enriched snow, ice, or aerosol the surface (the mechanism for which is still not fully under- surfaces by gas-phase ozone (Hirokawa et al., 1998; Oum et stood). Thus, it has been assumed that the rate of ozone de- al., 1998b; Gladich et al., 2015). struction can be estimated as Eq. (1) (see Eq. 15 in Hausmann and Platt, 1994, Eq. 3 in Le Bras and Platt, 1995, and Eq. 7 in C − C C !! C C O3 2Br.aq/ 2H.aq/ Br2 O2 H2O (R4) Zeng et al., 2006) or as Eq. (2), if chlorine chemistry is con- sidered through Reaction (R9) (Eq. IX in Platt and Janssen, Field observations by Pratt et al. (2013) using a controlled 1995). snow chamber experiment with natural tundra snow collected near Barrow, AK, lend further evidence to this mechanism, dTO U 3 D 2 · k3 ·TBrOU2 (1) and also suggest Br2 production from OH produced photo- dt chemically within the snowpack. This mechanism was fur- dTO3U ther explored in the modeling study of Toyota et al. (2014), D 2 k3 ·TBrOU2 C k9 ·TBrOU·TClOU (2) dt which suggested an important role for this activation path- way in producing bromine within the snowpack interstitial C ! C air. BrO ClO Br OClO (R9) Once present in the gas-phase, bromine atoms can be re- However, these approximations assume that the ozone de- generated through radical–radical reactions of BrO with XO struction rate is dominated by the BrO C XO reaction, which (where X D Br, Cl, or iodine: I), NO, OH, or CH OO to prop- 3 in turn necessitates efficient gas-phase recycling of Br; there- agate the chain reaction and continue the destruction cycle of fore, a relatively long bromine chain length would be re- ozone. If BrO photolyzes or reacts with NO, O is regener- 3 quired to account for observed rates of ozone destruction. ated, and there is a null cycle with respect to O . However, 3 It is, however, possible that Br atoms are generated mostly although O is not destroyed, these two pathways represent 3 by Br photolysis, followed by BrO termination, e.g., by Re- efficient routes for Br atom propagation. Thus, Reaction (R3) 2 action (R5), in which case a short gas-phase bromine rad- serves to make Reaction (R2) effective in destruction of O . 3 ical chain length would be implied. The chain length for At the same time, Br atoms could be recycled through hetero- any process depends on the rates of the propagation relative geneous reactions of HOBr with bromide in the condensed to the production and destruction reactions (Kuo, 1986). It phase to release Br to the gas-phase via the now well-known 2 is important to note that, in our definition, the chain length “bromine explosion” mechanism (Vogt et al., 1996; Tang and refers to radical propagation reactions occurring solely in the McConnell, 1996; Fan and Jacob, 1992). gas phase, and is a quantity completely independent of any BrO C HO2 ! HOBr C O2 (R5) condensed-phase chemistry. In the stratosphere, the Br–BrO catalytic cycle can have a chain length ranging from 102 to HOBr ! HOBr (R6) .g/ .aq/ 104 (Lary, 1996). In the troposphere, there is significantly C − C C ! C HOBr.aq/ Br.aq/ H.aq/ Br2.aq/ H2O (R7) less solar radiation and many more available sinks; thus, radi- cal chain lengths can be much shorter. For example, the chain Br2.aq/ ! Br2.g/ (R8) length of the tropospheric HOx cycle has been estimated to Evidence for reaction sequence (R5)–(R8) has been provided be ∼ 4–5 (Ehhalt, 1999; Monks, 2005), increasing to 10–20 through laboratory studies, which found that Br2 was pro- near the tropopause (Wennberg et al., 1998). The halogen duced when frozen bromide solutions were exposed to gas- radical chain lengths in the Arctic troposphere have so far phase HOBr (Huff and Abbatt, 2002; Adams et al., 2002). not been determined; thus, it is difficult to evaluate whether This mechanism is believed to proceed rapidly to produce Eqs. (1) and (2) are appropriate for estimating ozone deple- Br2 so long as sufficient bromide is present in an accessible tion rates. condensed phase. The efficiency of this heterogeneous recy- The importance of heterogeneous reactions for recycling cling mechanism has also been found to have a dependence reactive bromine has been demonstrated in the recent liter- on the acidity of the surface, as was shown using natural en- ature (see review by Abbatt et al., 2012). Modeling studies vironmental snow samples in Pratt et al. (2013) and investi- using typical Arctic springtime conditions to simulate ODEs gated in the modeling studies of Toyota et al. (2011, 2014), have concluded that ozone depletion cannot be sustained Atmos. Chem. Phys., 17, 3401–3421, 2017 www.atmos-chem-phys.net/17/3401/2017/ C. R. Thompson et al.: Bromine atom production and chain propagation 3403 without considering the heterogeneous recycling of reactive 2 Experimental bromine on snow or aerosol surfaces (e.g., Michalowski et al., 2000; Piot and Von Glasow, 2008; Liao et al., 2012b; 2.1 Measurements and site description Toyota et al., 2014).