Nitrate photolysis in ice and snow: A critical review of its multiphase chemistry

Nitrate, a member of the oxidized nitrogen family (NO_y), is one of the primary species involved in the nitrogen cycle and thus plays a key role in ecosystem processes, globally. It exists as nitrate salts and as nitric acid (HNO₃) in both aerosols and the gas phase. It is formed from the NO₃ radical/N₂O₅ or directly from the oxidation of NO₂ and is lost by photolysis, OH oxidation, and deposition. In regions covered with snow/ice it has a significant impact on air quality, atmospheric oxidizing capacity, greenhouse gas concentrations, and paleoclimate/isotopic data. Snow/ice environments can, at seasonal maximum, comprise ∼30% of Earth's surface area while 10% is covered with ice/snow found at the polar cryosphere. Nitrate makes up 75–100% of the nitrogen budget deposited from the atmosphere and measured at the Arctic and Antarctica. Its concentrations in Greenland ice have risen by a factor of 2–3, reflecting the long-ranged transport of increased anthropogenic NO_x (NO + NO₂) emissions. The polar cryosphere is an active medium for the movement of traces gases, such as nitrate, between the snowpack/sea-ice and overlying atmosphere. Field, laboratory, and modeling efforts have quantitatively shown that the exchange of trace species between the snowpack and the air above is governed by photochemistry in combination with air moving gases between these two matrices. Polar tropospheric chemistry and dynamics immensely impact processes governing chemical composition, isotopic signatures, oxidizing capacity, and thus regional climate. This study presents a comprehensive review of laboratory and modeling efforts – contextualized by field measurements – that have elucidated physicochemical processes governing nitrate photochemistry and its impact on the polar snowpack. Specifically, after an Introduction to nitrate photochemistry in ice, we discuss the: 1) initial Arctic field measurements that sparked interest in ice photolysis in the polar regions; 2) suite of follow-up field studies that catalyzed laboratory and snow-chamber investigations that gave deeper understanding of the effects of snow/ice – air trace gas exchange due to nitrate photochemistry; 3) complementary laboratory, snow-chamber investigations; and 4) a detailed review of recent nitrate ice photolysis laboratory experiments and the potential impact of utilizing laboratory and computational models to study the role of nitrate in the nitrogen cycle.