Abstract

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 ... Show moreRates 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(NO&#x2083), 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(NO&#x2083) 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. Show less