Factors that influence UV actinic flux in and above Arctic snow

Photochemical rate constants (i.e., photolysis frequencies or j-values) are principle inputs to photochemical models used to illustrate atmospheric chemistry, both in the overlaying atmosphere and within the snowpack. We have measurements of the in situ actinic flux above and within the snowpack during most of the 2003 and 2004 Summit, Greenland measurement periods from which we calculated 39 different photolysis frequencies. Since the firn (or in-snow) actinic flux observations were made at a series of discrete depths while the firn air chemical measurements reflect variable subsurface volumes, it is necessary to test the various radiative transfer models that attempt to describe the actinic flux both in the atmosphere and snowpack environments. To do this we incorporate the available measurements and estimates of factors that impact the above-snow actinic flux, such as snow surface albedo, percent cloud cover, overhead ozone column and aerosol/blowing snow optical depth into an atmospheric radiative transfer model. We also investigate the impact of physical snow characteristics (e.g., density, grain size, etc.) on the penetration of UV radiation. We use our measurements of the UV actinic flux to test these radiative transfer models for a variety atmospheric and snow conditions to help identify which of these variables are the most important inputs to the models.