Description of Bromine Activation and Recycling Within Arctic Snowpack

1. Description of Bromine Activation and Recycling Within Arctic Snowpack K. Murray1 , V. Rao Kotamarthi2 , P.V. Doskey1,4, C. Toro1 , B.A. Van Dam3 A13F-0436 Introduction Exchange of reactive gases like O3, NOx, NOy, halogens, peroxides, and aldehydes with snowpack is regulated by physical characteristics of the snow, photochemistry, and heterogeneous chemistry of reactive species in the snow column. Bromine (Br) compounds affect O3 as follows: ------------------------------------ Depletion of O3 by reaction with Br is more prominent in snow-covered regions due to the large reservoir of Br in the snow and elevated photolysis rates related to high albedo. The levels of Br compounds within and above snowpack are regulated by micrometeorology, surface microtopography, e-folding depth, and physicochemical parameters that define the snow. Here we present the development of a high-resolution, 1-D snowpack model with parameterizations of chemical and physical interactions between the snowpack interstitial air (IA), the quasi-liquid layer (QLL), and the atmospheric boundary layer (ABL). The sensitivity of Br chemistry to snow density and the e-folding depths of photolysis rates will be evaluated. Background Photochemical rates of reaction Photolysis rates are calculated online based on locations, time of year, elevation, absorption cross sections, and quantum yields. The j-values within snow are calculated as follows (Thomas et al., 2010): Where z is the depth and is the e-folding depth. Photolysis E-fold Depth Analysis Results Quasi-liquid layer (QLL) of snow and volumetric ratios The QLL is a thin, wet film that exists on the surface of the snow between the bulk of ice and the atmosphere and is the location of enhanced chemical activity. In the 1-D model, the thickness of the QLL (d), the radius of a snow flake (r), and the densities of snow (snow), air (air), and ice (ice) are used to estimate volumetric ratios of liquid and air in the snowpack, wl and wa, respectively. From the mass balance on 1 m3 of the snowpack and by assuming snowflakes are spherical and IA is much less than ice and snow, the ratios are as follows: Methods Snow densities were varied from 0.15 g cm-3 (fresh snow) to 0.60 g cm-3 (wind-packed snow) and values of  were varied from 10-30 cm to evaluate effects on Br chemistry. Meteorological conditions are typical of an early summer day and initial concentrations of halogen species and ozone are presented in the following table. Initial concentrations of other species are similar to conditions set by Thomas et al. Photolysis E-fold Depth Analysis Results E-fold – 0.10 cm E-fold – 0.30 cm E-fold – 0.10 cm E-fold – 0.30 cm Bromide Bromine Monoxide Bromide Bromine Monoxide Snow Density Analysis Results Density – 0.15 g/cm3 Density – 0.60 g/cm3 Bromide Bromine Monoxide Acknowledgements I would like to give special thanks to V. RaoKotamarthiof Argonne National Laboratory for his time and effort in development of the 1-D process-scale snow model, my advisor Paul Doskey of Michigan Technological University for his wisdom and late night discussions, and the National Science Foundation for funding our research (project #ARC-0713992). References Thomas, J. L., Stutz, J., Lefer, B., Huey, L.G., Toyota, K., Dibb, J.E., von Glasow, R. (2011). "Modeling chemistry in and above snow at Summit, Greenland - Part 1: Model description and results." Atmos. Chem. Phys.11. pg. 4903 • Conclusions • Modification of the photolysis e-fold depth and the density of snow resulted in significant changes in concentrations of bromine species in the QLL and IA, and near the surface of the snowpack. • Ionic species in the QLL like bromide do not exchange with the IA and are affected most significantly. • Levels of bromine species in the QLL and near the surface of the snowpack are more sensitive to snow density than photolysis e-fold depth. • Measurements of photolysis e-fold depth and snow density would reduce uncertainties in model simulations of observations.