Light penetration in the snowpack at Summit, Greenland: Part 2 Nitrate photolysis

Rates of photochemical reactions within the snowpack, both on snow grains and in the firn air, depend on how actinic flux is attenuated as a function of depth. This snowpack photon flux can either be measured directly (e.g., with spectral radiometers in the snow) or indirectly (e.g., by chemical actinometry where the rate of a photochemical reaction is measured). In this work we use both techniques to measure the rate constant for nitrate photolysis on water-ice, j (NO₃^-), on the snowpack surface and beneath at Summit, Greenland during spring and summer. The surface measurements from these two methods are generally similar ((1-2)×10^-7 s^-1 during midday near the summer solstice) and follow expected diurnal and annual trends. In addition, both methods show a similar effect of snow albedo on photolysis at the surface: rate constants measured on the surface snow were approximately 45% higher than values predicted for the surface based on regressions of in-snow rate constants. Average (±σ) j (NO₃^-) e-folding depths (the depth along which the rate constant decreases by a factor of e) during the 2 field seasons are 10.3(±2.8) cm from actinometry tubes and 8.4(±2.4) cm from in-snow spectral radiometers.