Calculation of actinic fluxes with a coupled atmosphere-snow radiative transfer model

A one-dimensional atmospheric radiative transfer model has been extended to include scattering and absorption by multiple layers of snow. Empirical values for scattering coefficients for wet and dry snow and for absorption by impurities are derived from previous observations of snow albedo and irradiance attenuation depth, thus avoiding the uncertainties inherent in modeling the optical properties of ensembles of individual snow grains. Vertical profiles of spectral actinic flux within the model snowpack are computed and compared to those of down-welling spectral irradiance. The ratio of these two quantities is a strong function of solar zenith angle near the snow surface but rapidly approaches ∼0.25 (the isotropic limit) at depths greater than a few centimeters. Calculated rates of in-snow nitrate (NO₃^-) photolysis confirm that this process can explain at least part of the enhanced NO_x concentrations observed in the atmospheric boundary layer above snow.