Abstract
Abstract. Episodes of high bromine levels and surface ozone depletion in the springtime Arctic are simulated by an online air-quality model, GEM-AQ, with gas-phase and heterogeneous reactions of inorganic bromine species and a simple scheme of air-snowpack chemical interactions implemented for this study. Snowpack on sea ice is assumed to be the only source of bromine to the atmosphere and to be capable of converting relatively stable bromine species to photolabile Br2 via air-snowpack interactions. A set of sensitivity model runs are performed for April 2001 at a horizontal resolution of approximately 100 km×100 km in the Arctic, to provide insights into the effects of temperature and the age (first-year, FY, versus multi-year, MY) of sea ice on the release of reactive bromine to the atmosphere. The model simulations capture much of the temporal variations in surface ozone mixing ratios as observed at stations in the high Arctic and the synoptic-scale evolution of areas with enhanced BrO column amount ("BrO clouds") as estimated from satellite observations. The simulated "BrO clouds" are in modestly better agreement with the satellite measurements when the FY sea ice is assumed to be more efficient at releasing reactive bromine to the atmosphere than on the MY sea ice. Surface ozone data from coastal stations used in this study are not sufficient to evaluate unambiguously the difference between the FY sea ice and the MY sea ice as a source of bromine. The results strongly suggest that reactive bromine is released ubiquitously from the snow on the sea ice during the Arctic spring while the timing and location of the bromine release are largely controlled by meteorological factors. It appears that a rapid advection and an enhanced turbulent diffusion associated with strong boundary-layer winds drive transport and dispersion of ozone to the near-surface air over the sea ice, increasing the oxidation rate of bromide (Br−) in the surface snow. Also, if indeed the surface snowpack does supply most of the reactive bromine in the Arctic boundary layer, it appears to be capable of releasing reactive bromine at temperatures as high as −10 °C, particularly on the sea ice in the central and eastern Arctic Ocean. Dynamically-induced BrO column variability in the lowermost stratosphere appears to interfere with the use of satellite BrO column measurements for interpreting BrO variability in the lower troposphere but probably not to the extent of totally obscuring "BrO clouds" that originate from the surface snow/ice source of bromine in the high Arctic. A budget analysis of the simulated air-surface exchange of bromine compounds suggests that a "bromine explosion" occurs in the interstitial air of the snowpack and/or is accelerated by heterogeneous reactions on the surface of wind-blown snow in ambient air, both of which are not represented explicitly in our simple model but could have been approximated by a parameter adjustment for the yield of Br2 from the trigger.
Highlights
During the spring after complete darkness in the winter, boundary-layer air over Arctic sea ice and its surrounding coastal areas experiences a frequent occurrence of ozone depletion events (ODEs) from background levels (∼30– 40 nmol mol−1) to below 5–10 nmol mol−1 and sometimes even below experimental detection limits ( 1 nmol mol−1) (Oltmans, 1981; Bottenheim et al, 1986, 2002, 2009; Solberg et al, 1996; Hopper et al, 1998; Tarasick and Bottenheim, 2002)
We look at a sensitivity of the simulation results on the choice of temperature below which reactive bromine release from the snow/ice surface is turned on in the model, to provide some insights into chemistry occurring in the snow across the Arctic
We used hourly data of surface ozone measurements at Alert, Barrow and Zeppelin where the link between springtime ODEs and reactive bromine chemistry has been established by previous field studies (Simpson et al, 2007b)
Summary
During the spring after complete darkness in the winter, boundary-layer air over Arctic sea ice and its surrounding coastal areas experiences a frequent occurrence of ozone depletion events (ODEs) from background levels (∼30– 40 nmol mol−1) to below 5–10 nmol mol−1 and sometimes even below experimental detection limits ( 1 nmol mol−1) (Oltmans, 1981; Bottenheim et al, 1986, 2002, 2009; Solberg et al, 1996; Hopper et al, 1998; Tarasick and Bottenheim, 2002). On the other hand, Adams et al (2002) experimentally showed that the reactive uptake of HOBr onto frozen NaBr/NaCl solution results in the Br2 and/or BrCl release below −20 ◦C even if the substrate is alkaline at room temperature before frozen Another important question that remains to be fully answered is how halide anions such as Br− are supplied to the surface snow and made accessible from the atmosphere across the polar regions. Kaleschke et al (2004) proposed an algorithm to diagnose the potential coverage of frost flowers (called “potential frost flower”, or PFF) on sea ice across the polar regions by using satellite data of sea ice concentrations and objective analyses of large-scale surface air temperatures. We look at a sensitivity of the simulation results on the choice of temperature below which reactive bromine release from the snow/ice surface is turned on in the model, to provide some insights into chemistry occurring in the snow across the Arctic
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