The relative importance of chlorine and bromine radicals in the oxidation of atmospheric mercury at Barrow, Alaska

Stephens, C. R.; Shepson, P. B.; Steffen, Alexandra; Bottenheim, J. W.; Liao, J.; Huey, L. G.; Apel, E. C.; Weinheimer, A. J.; Hall, Samuel R.; Cantrell, C. A.; Sive, B. C.; Knapp, D. J.; Montzka, D. D.; Hornbrook, R. S.

doi:10.1029/2011jd016649

Cited by 73 publications

(84 citation statements)

References 135 publications

(218 reference statements)

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“…The model predicts Cl atom concentrations of 2 × 10 5 to 6 × 10 5 molecules cm −3 , which is also higher than previous estimates of 1 × 10 3 to 1 × 10 5 determined from hydrocarbon measurements (Jobson et al, 1994;Ariya et al, 1998;Rudolph et al, 1999;Boudries and Bottenheim, 2000;Keil and Shepson, 2006). As discussed in Stephens et al (2012), the hydrocarbon-based methods average over the transport path, which can be aloft and consist of several days, and thus should be lower than that observed near the surface, if the surface is the Cl 2 and Br 2 source. Fig.…”

Section: Comparison Of Modeled and Observed Mole Ratios For Select Spmentioning

confidence: 99%

Interactions of bromine, chlorine, and iodine photochemistry during ozone depletions in Barrow, Alaska

et al. 2015

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Abstract. The springtime depletion of tropospheric ozone in the Arctic is known to be caused by active halogen photochemistry resulting from halogen atom precursors emitted from snow, ice, or aerosol surfaces. The role of bromine in driving ozone depletion events (ODEs) has been generally accepted, but much less is known about the role of chlorine radicals in ozone depletion chemistry. While the potential impact of iodine in the High Arctic is more uncertain, there have been indications of active iodine chemistry through observed enhancements in filterable iodide, probable detection of tropospheric IO, and recently, observation of snowpack photochemical production of I 2 . Despite decades of research, significant uncertainty remains regarding the chemical mechanisms associated with the bromine-catalyzed depletion of ozone, as well as the complex interactions that occur in the polar boundary layer due to halogen chemistry. To investigate this, we developed a zero-dimensional photochemical model, constrained with measurements from the 2009 OA-SIS field campaign in Barrow, Alaska. We simulated a 7-day period during late March that included a full ozone depletion event lasting 3 days and subsequent ozone recovery to study the interactions of halogen radicals under these different conditions. In addition, the effects of iodine added to our Base Model were investigated. While bromine atoms were primarily responsible for ODEs, chlorine and iodine were found to enhance the depletion rates and iodine was found to be more efficient per atom at depleting ozone than Br. The interaction between chlorine and bromine is complex, as the presence of chlorine can increase the recycling and production of Br atoms, while also increasing reactive bromine sinks under certain conditions. Chlorine chemistry was also found to have significant impacts on both HO 2 and RO 2 , with organic compounds serving as the primary reaction partner for Cl atoms. The results of this work highlight the need for future studies on the production mechanisms of Br 2 and Cl 2 , as well as on the potential impact of iodine in the High Arctic.

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Section: Comparison Of Modeled and Observed Mole Ratios For Select Spmentioning

confidence: 99%

Interactions of bromine, chlorine, and iodine photochemistry during ozone depletions in Barrow, Alaska

et al. 2015

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“…Photolysis rate constants (J coefficients) for many of the species included were calculated during OASIS using the Tropospheric Ultraviolet and Visible Radiation model from measurements of down-welling actinic flux conducted throughout the campaign (Shetter and Müller, 1999;Stephens et al, 2012). Estimates of J max in the Arctic for OClO were taken from Pöhler et al (2010), for HOCl from Lehrer et al (2004), and for CHBr 3 from Papanastasiou et al (2014).…”

Section: Model Descriptionmentioning

confidence: 99%

Bromine atom production and chain propagation during springtime Arctic ozone depletion events in Barrow, Alaska

et al. 2017

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Abstract. Ozone depletion events (ODEs) in the Arctic are primarily controlled by a bromine radical-catalyzed destruction mechanism that depends on the efficient production and recycling of Br atoms. Numerous laboratory and modeling studies have suggested the importance of heterogeneous recycling of Br through HOBr reaction with bromide on saline surfaces. On the other hand, the gas-phase regeneration of bromine atoms through BrO-BrO radical reactions has been assumed to be an efficient, if not dominant, pathway for Br reformation and thus ozone destruction. Indeed, it has been estimated that the rate of ozone depletion is approximately equal to twice the rate of the BrO self-reaction. Here, we use a zero-dimensional, photochemical model, largely constrained to observations of stable atmospheric species from the 2009 Ocean-Atmosphere-Sea Ice-Snowpack (OA-SIS) campaign in Barrow, Alaska, to investigate gas-phase bromine radical propagation and recycling mechanisms of bromine atoms for a 7-day period during late March. This work is a continuation of that presented in and utilizes the same model construct. Here, we use the gas-phase radical chain length as a metric for objectively quantifying the efficiency of gas-phase recycling of bromine atoms. The gas-phase bromine chain length is determined to be quite small, at < 1.5, and highly dependent on ambient O 3 concentrations. Furthermore, we find that Br atom production from photolysis of Br 2 and BrCl, which is predominately emitted from snow and/or aerosol surfaces, can account for between 30 and 90 % of total Br atom production. This analysis suggests that condensed-phase production of bromine is at least as important as, and at times greater than, gas-phase recycling for the occurrence of Arctic ODEs. Therefore, the rate of the BrO self-reaction is not a sufficient estimate for the rate of O 3 depletion.

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“…Therefore, when ozone goes below a few nanomoles per mole (nmol mol −1 ), reactive bromine (BrO x ) may be present, but not all of that BrO x is spectroscopically visible as BrO. BrO x is instead present as Br atoms, which rapidly oxidize mercury (Holmes et al, 2006(Holmes et al, , 2009Stephens et al, 2012;, affecting the fate of this pollutant. This low-ozoneinduced BrO x repartitioning has been observed as decreased surface BrO during very low surface ozone periods in multiple field studies (Simpson et al, 2007a;Helmig et al, 2012;Peterson et al, 2015) and is a common feature of chemical modeling studies (Sander et al, 1997;Evans, 2003;Thomas et al, 2011;Toyota et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

Horizontal and vertical structure of reactive bromine events probed by bromine monoxide MAX-DOAS

et al. 2017

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Abstract. Heterogeneous photochemistry converts bromide (Br − ) to reactive bromine species (Br atoms and bromine monoxide, BrO) that dominate Arctic springtime chemistry. This phenomenon has many impacts such as boundarylayer ozone depletion, mercury oxidation and deposition, and modification of the fate of hydrocarbon species. To study environmental controls on reactive bromine events, the BRomine, Ozone, and Mercury EXperiment (BROMEX) was carried out from early March to mid-April 2012 near Barrow (Utqiaġvik), Alaska. We measured horizontal and vertical gradients in BrO with multiple-axis differential optical absorption spectroscopy (MAX-DOAS) instrumentation at three sites, two mobile and one fixed. During the campaign, a large crack in the sea ice (an open lead) formed pushing one instrument package ∼ 250 km downwind from Barrow (Utqiaġvik). Convection associated with the open lead converted the BrO vertical structure from a surfacebased event to a lofted event downwind of the lead influence. The column abundance of BrO downwind of the refreezing lead was comparable to upwind amounts, indicating direct reactions on frost flowers or open seawater was not a major reactive bromine source. When these three sites were separated by ∼ 30 km length scales of unbroken sea ice, the BrO amount and vertical distributions were highly correlated for most of the time, indicating the horizontal length scales of BrO events were typically larger than ∼ 30 km in the absence of sea ice features. Although BrO amount and vertical distribution were similar between sites most of the time, rapid changes in BrO with edges significantly smaller than this ∼ 30 km length scale episodically transported between the sites, indicating BrO events were large but with sharp edge contrasts. BrO was often found in shallow layers that recycled reactive bromine via heterogeneous reactions on snowpack. Episodically, these surface-based events propagated aloft when aerosol extinction was higher (> 0.1 km −1 ); however, the presence of aerosol particles aloft was not sufficient to produce BrO aloft. Highly depleted ozone (< 1 nmol mol −1 ) repartitioned reactive bromine away from BrO and drove BrO events aloft in cases. This work demonstrates the interplay between atmospheric mixing and heterogeneous chemistry that affects the vertical structure and horizontal extent of reactive bromine events.

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The relative importance of chlorine and bromine radicals in the oxidation of atmospheric mercury at Barrow, Alaska

Cited by 73 publications

References 135 publications

Interactions of bromine, chlorine, and iodine photochemistry during ozone depletions in Barrow, Alaska

Interactions of bromine, chlorine, and iodine photochemistry during ozone depletions in Barrow, Alaska

Bromine atom production and chain propagation during springtime Arctic ozone depletion events in Barrow, Alaska

Horizontal and vertical structure of reactive bromine events probed by bromine monoxide MAX-DOAS

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