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

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 snow impurity levels. We calculate the lifetime of NO_x in the snowpack interstitial air produced by photolysis of snowpack nitrate against wind pumping (τ_{wind pumping}) from the snowpack, and compare this to the calculated lifetime of NO_x against chemical conversion to HNO₃ (τ_chemical) to determine whether the NO_x produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of τ_{wind pumping} and τ_chemical suggests efficient escape of photoproduced NO_x 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_x from the snowpack. Calculated NO_x fluxes of 4.4 × 10⁸–3.8 × 10⁹ molecules cm⁻² s⁻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 NO_x flux measurements near stations may be underestimating the amount of NO_x emitted from the clean polar snowpack.