The Impact of Parameterising Light Penetration Into Snow on the Photochemical Production of NOx and OH Radicals in Snow

Abstract:

Snow photochemical processes drive production of chemical trace gases in snowpacks, including nitrogen oxides (NO_x = NO + NO₂) and hydrogen oxide radicals (HO_x = OH + HO₂), which are then released to the lower atmosphere. Coupled atmosphere–snow modelling of theses processes on global scales requires simple parameterisations of actinic flux in snow to reduce computational cost. The disagreement between a physical radiative-transfer model and a parameterisation based upon the e-folding depth of actinic flux in snow is evaluated. In particular, the photolysis of nitrate (NO₃^-), nitrite (NO₂^-) and hydrogen peroxide (H₂O₂) in snow and nitrogen dioxide (NO₂) in the snowpack interstitial air are considered. The emission flux from the snowpack is estimated as the product of the depth-integrated photolysis rate coefficient and the concentration of photolysis precursors in the snow. The depth-integrated photolysis rate coefficient is calculated (a) explicitly with a radiative-transfer model (RT), and (b) with a simple parameterisation based on e-folding depth. The metric for the evaluation is based upon the deviation of the ratio of the depth-integrated photolysis rate coefficient (R) determined by the two methods from unity. R depends primarily on the position of the peak in the photolysis action spectrum of chemical species, solar zenith angle and physical properties of the snowpack, i.e. strong dependence on the light-scattering cross section and the mass ratio of the light-absorbing impurity (i.e. black carbon and HULIS), but only a weak dependence on density. For the photolysis of NO₂, NO₂^-, NO₃^- and H₂O₂ the ratio R varies within the range of 0.82–1.35, 0.88–1.28, 0.93–1.27 and 0.91–1.28 respectively. The e-folding depth parameterisation underestimates for small solar zenith angles and overestimates at solar zenith angles around 60º compared to the RT method. A simple algorithm has been developed to improve the parameterisation which reduces values of R to 0.97–1.02, 0.99–1.02, 0.99–1.03 and 0.98–1.06 for photolysis of NO₂, NO₂^-, NO₃^- and H₂O₂, respectively. The e-folding depth parameterisation may give acceptable results for the photolysis of NO₃^- and H₂O₂ in cold polar snow with large solar zenith angles, but it can be improved by a correction based on solar zenith angle and for cloudy skies.