Monte Carlo modeling and measurements of actinic flux levels in Summit, Greenland snowpack

Knowledge of actinic flux levels in snowpack is needed to find the influence of snowpack photochemical processes on atmospheric composition. Measurements show that while <0.2% of direct UV and visible light is transmitted through 0.7 cm of snowpack, downwelling actinic flux levels are at least 10% of incident levels at a depth of 10 cm within the snowpack. This results from the highly forward-scattering nature of the snowgrains within the snowpack. A 1-D Monte Carlo model of photon scattering from, and absorption in, snowgrains accurately simulates relative actinic flux levels in a horizontally homogeneous, vertically layered representation of the upper meter of Summit, Greenland snowpack. The resulting relative actinic flux levels may be used with other measurements, or with other radiative transfer models, to estimate absolute actinic flux levels within the snowpack at Summit. Results from the 1-D Monte Carlo model also demonstrate that buried radiometers which completely block upward scattered light from lower layers observe e-folding depths that may be more than an order of magnitude lower than actual values. Additional simulations with a 2- or 3-D Monte Carlo model are needed to quantify the magnitude of this effect for partial blocking of scattered light.