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

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 NOx in the snowpack interstitial air produced by photolysis of snowpack nitrate against wind pumping (tau(wind) (pumping)) from the snowpack, and compare this to the calculated lifetime of NOx against chemical conversion to HNO3 (tau(chemical)) to determine whether the NOx produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of tau(wind) (pumping) and tau(chemical) suggests efficient escape of photoproduced NOx 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 NOx from the snowpack. Calculated NOx fluxes of 4.4x10(8)-3.8x10(9) molecules cm(-2) s(-1) in remote polar locations and 3.2-8.2x10(8) 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 may be under-estimating the amount of NOx emitted from the clean polar snowpack.