Radiation decay in snowpack

This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract Snow photochemical processes drive production of chemical trace gases, including nitrogen oxides (NO and NO 2), and HO x radicals in snowpacks which are then released to the lower atmosphere. Coupled atmosphere–snow modelling on global scales requires simple parameterisations of actinic flux in snow to reduce computational cost. 5 The disagreement between a physical radiative transfer method and a method based upon the e-folding depth of light-in snow is evaluated. In particular for the photolysis of the nitrate anion (NO − 3), the nitrite anion (NO − 2) and hydrogen peroxide (H 2 O 2) within snow and photolysis of gas-phase nitrogen dioxide (NO 2) within the snowpack intersti-tial air are considered. 10 The emission flux from the snowpack is estimated as the depth-integrated photolysis rate, v, calculated (a) explicitly with a physical radiative transfer model (TUV), v TUV and (b) with a simple parameterisation based on e-folding depth, v z e. The evaluation is based upon the deviation of the ratio of depth-integrated photolysis rate determined by the two methods, v TUV v z e , from unity. The disagreement in depth-integrated photolysis 15 rate between the RT model and e-folding depth parameterisation depends primarily on the photolysis action spectrum of chemical species, solar zenith angle and optical properties of the snowpack, (scattering cross-section and a weak dependence on light absorbing impurity (black carbon) and density). For photolysis of NO 2 , the NO − 2 anion, the NO − 3 anion and H 2 O 2 the ratio v TUV v z e varies within the range of 0.82–1.35, 0.88–1.28 20 and 0.92–1.27 respectively. The e-folding depth parameterisation underestimates for small solar zenith angles and overestimates at solar zenith angles around 60 •. A simple algorithm has been developed to improve the parameterisation which reduced the ratio v TUV v z e