Light penetration in the snowpack at Summit, Greenland: Part 1. Nitrite and hydrogen peroxide photolysis

Photochemical rate constants (j values) are crucial indicators for evaluating the importance of photochemical reactions in environmental systems. While measurement of aqueous j values via chemical actinometry is relatively straightforward under most conditions, problems arise with ambient conditions below freezing, such as at very high latitudes or altitudes. To address this problem, we have developed a new method for low temperature actinometry using solutions of acetonitrile (ACN) and water, which have freezing points down to - 44 ^{{ring operator}} C. In this method we measure the formation of phenol from the photolysis of an ^•OH-generating chromophore in the presence of benzene. Using results from laboratory tests we correct our phenol field results in ACN / H₂ O to rate constants for chromophore photolysis expected for water-ice (i.e., in the quasi-liquid layer of snow grains) under the same conditions. In part 1 of this study, we use this method at Summit, Greenland on the surface snow and to depths of ∼ 30 cm using hydrogen peroxide (HOOH) and nitrite (NO₂^-) as the chromophores. While the method works well for determining the rate constant for HOOH photolysis (j(HOOH)), we encountered problems using the technique with nitrite. However, measured PhOH formation rate constants for nitrite in acetonitrile, j_{NO2- → PhOH}^ACN, still provide an excellent means for calculating snowpack e-folding depths for NO₂^- photolysis (i.e., the depth over which the rate constant decreases by a factor of e). Values of j(HOOH) and j (NO₂^-) determined from measurements of actinic flux (above the snow) and irradiance (in snow) suggest that the value of j(HOOH) on the surface snow at midday was 8.6 × 10^{- 7} s^{- 1} in mid-March and increased by 300% by the start of May, while j_{NO2- → PhOH}^ACN midday surface values were consistently (1 - 3) × 10^{- 7} s^{- 1} throughout the season. Within the snowpack, average e-folding depths determined from chemical actinometry were 13.3 (± 0.88) cm for j(HOOH) and 16.3 (± 4.2) cm for j_{NO2- → PhOH}^ACN; e-folding depths determined from in-snow spectral radiometer measurements of irradiance were similar. The larger e-folding depth for nitrite is because this chromophore absorbs at longer wavelengths where there is less light extinction in the snow.