The influence of snow grain size and impurities on the vertical profiles of actinic flux and associated nitrogen oxide emissions on the Antarctic and Greenland ice sheets

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 the depth and wavelength dependent snowpack actinic flux that can be easily integrated into large scale (e.g. 3D) 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 (tau escape) from the snowpack via diffusion and wind pumping and compare this to the calculated lifetime of NOx against chemical conversion to HNO3 (tauchemical) to determine whether the NO x produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of tauescape and tauchemical suggests efficient escape of photoproduced NOx in the snowpack to the overylying 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 4.4x10 8-2.8x109 molecules cm-2 s-1 in remote polar locations and 3.2-8.2x108 molecules cm-2 s-1 near polar stations in January at Dome C and South Pole and in 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.