Atmos. Chem. Phys. Discuss., 12, 15743-15799, 2012
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The influence of snow grain size and impurities on the vertical profiles of actinic flux and associated NOx emissions on the Antarctic and Greenland ice sheets
M. C. Zatko1, T. C. Grenfell1, B. Alexander1, S. J. Doherty2, J. L. Thomas3,4, and X. Yang5,6
1Department of Atmospheric Sciences, Box 351640, University of Washington, Seattle, WA 98195, USA
2Joint Institute for the Study of Atmosphere and Ocean, 3737 Brooklyn Ave NE, Seattle, WA 98195, USA
3UPMC Univ. Paris 06, UMR8190, CNRS/INSU – Université Versailles St.-Quentin, LATMOS-IPSL, Paris, France
4University of California, Los Angeles; Department of Atmospheric and Oceanic Sciences, Los Angeles, CA 90095, USA
5National Centre for Atmospheric Science (NCAS), Cambridge, CB2 1EW, UK
6Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK

Abstract. We use observations of the absorption properties of black carbon and non-black-carbon impurities in near-surface snow collected near the research stations at South Pole and Dome C, Antarctica and Summit, Greenland combined with a snowpack actinic flux parameterization to estimate the vertical profile and e-folding depth of ultraviolet/near-visible (UV/near-vis) actinic flux in the snowpack at each location. We have developed a simple and broadly applicable parameterization to calculate depth and wavelength dependent snowpack actinic flux that can be easily integrated into large scale (e.g. 3-D) models of the atmosphere. The calculated e-folding depths of actinic flux at 305 nm, the peak wavelength of nitrate photolysis in the snowpack, are 8–12 cm near the stations and 15–31 cm away (>11 km) from the stations. We find that the e-folding depth is strongly dependent on impurity content and wavelength in the UV/near-vis region, which explains the relatively shallow e-folding depths near stations where local activities lead to higher impurity levels. We calculate the lifetime of NOx in the snowpack interstitial air produced by photolysis of snowpack nitrate against escape (τescape) from the snowpack via diffusion and windpumping and compare this to the calculated lifetime of NOx against chemical conversion to HNO3 (τchemical) to determine whether the NOx produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of τescape and τchemical suggests efficient escape of photoproduced NOx in the snowpack to the overlying atmosphere. Calculated vertical actinic flux profiles and observed snowpack nitrate concentrations are used to determine the flux of NOx from the snowpack. Calculated NOx fluxes of 4.4 × 108–2.8 × 109 molecules cm−2 s7−1 in remote polar locations and 3.2–8.2 × 108 molecules cm−2 s−1 near polar stations for January at Dome C and South Pole and June at Summit suggest that NOx flux measurements near stations are likely underestimating the amount of NOx emitted from the clean, polar snowpack by a factor of 1.4–2.4.

Citation: Zatko, M. C., Grenfell, T. C., Alexander, B., Doherty, S. J., Thomas, J. L., and Yang, X.: The influence of snow grain size and impurities on the vertical profiles of actinic flux and associated NOx emissions on the Antarctic and Greenland ice sheets, Atmos. Chem. Phys. Discuss., 12, 15743-15799, doi:10.5194/acpd-12-15743-2012, 2012.
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