Abstract
Photochemical reactions of nitrate in snow release reactive nitrogen species via two channels, which produce (1) nitrogen dioxide (NO2) and hydroxyl radical (•OH) and (2) nitrite (NO2-) and oxygen atom (O(3P)). Quantum yields (Φ) for these channels are generally well characterized, except for channel 2 in ice. In this study, we quantify Φ(NO2-) in water ices and examine the impacts of pH and organic scavengers of •OH. Compared to solution results, we find that nitrite quantum yields in ice are more sensitive to pH and that •OH scavengers are less effective, although 2-propanol appears to work well. The temperature dependence (-30 to 25 °C) of Φ(NO2-) in samples containing 2-propanol is well described by a single regression line, ln(Φ(NO2-)) = -(1330 ± 100)(1/T(K)) + (0.09 ± 0.39). At -10 °C, the resulting quantum yield is 4.6 times larger than the previously reported (and recommended) value without an •OH scavenger. Although some reports suggest nitrite is a minor product from nitrate photolysis, based on our current and past results, rates of photoproduction of NO2- and NO2 are similar at room temperature, while NO2- production dominates at lower temperatures in solution and ice.
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