ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-13-3547-2013 The influence of snow grain size and impurities on the vertical profiles of actinic flux and associated NO<sub>x</sub> emissions on the Antarctic and Greenland ice sheets Zatko M. C. 1 Grenfell T. C. 1 Alexander B. 1 Doherty S. J. 2 Thomas J. L. 3 4 Yang X. 5 6 Department of Atmospheric Sciences, Box 351640, University of Washington, Seattle, WA 98195, USA Joint Institute for the Study of Atmosphere and Ocean, 3737 Brooklyn Ave NE, Seattle, WA 98195, USA UPMC Univ. Paris 06, UMR8190, CNRS/INSU – Université Versailles St.-Quentin, LATMOS-IPSL, Paris, France Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA National Centre for Atmospheric Science (NCAS), Cambridge, CB2 1EW, UK Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK 02 04 2013 13 7 3547 3567 Copyright: © 2013 M. C. Zatko et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/13/3547/2013/acp-13-3547-2013.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/13/3547/2013/acp-13-3547-2013.pdf

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 snow impurity levels. We calculate the lifetime of NO<sub>x</sub> in the snowpack interstitial air produced by photolysis of snowpack nitrate against wind pumping (&tau;<sub>wind pumping</sub>) from the snowpack, and compare this to the calculated lifetime of NO<sub>x</sub> against chemical conversion to HNO<sub>3</sub> (&tau;<sub>chemical</sub>) to determine whether the NO<sub>x</sub> produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of &tau;<sub>wind pumping</sub> and &tau;<sub>chemical</sub> suggests efficient escape of photoproduced NO<sub>x</sub> in the snowpack to the overlying atmosphere throughout most of the photochemically active zone. Calculated vertical actinic flux profiles and observed snowpack nitrate concentrations are used to estimate the potential flux of NO<sub>x</sub> from the snowpack. Calculated NO<sub>x</sub> fluxes of 4.4 × 10<sup>8</sup>–3.8 × 10<sup>9</sup> molecules cm<sup>−2</sup> s<sup>−1</sup> in remote polar locations and 3.2–8.2 × 10<sup>8</sup> molecules cm<sup>−2</sup> s<sup>−1</sup> near polar stations for January at Dome C and South Pole and June at Summit suggest that NO<sub>x</sub> flux measurements near stations may be underestimating the amount of NO<sub>x</sub> emitted from the clean polar snowpack.

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