Nitrate Photochemistry at the Air–Ice Interface and in Other Ice Reservoirs

• Published in: Environmental science & technology 2018-05, Vol.52 (10), p.5710-5717
• Main Authors: McFall, Alexander S, Edwards, Kasey C, Anastasio, Cort
• Format: Article
• Language: English
• Subjects: Air-ice interface; Freezing; Ice; Light; Liquid nitrogen; Nitrates; Nitrites; Nitrogen dioxide; Oxidants; Oxidizing agents; Photochemistry; Photodegradation; Photolysis; Photoreceptors; Reactive nitrogen species; Remote regions; Reservoirs; Snowpack; Solid solutions
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.8b00095

The photolysis of snowpack nitrate (NO3 –) is an important source of gaseous reactive nitrogen species that affect atmospheric oxidants, particularly in remote regions. However, it is unclear whether nitrate photochemistry differs between the three solute reservoirs in/on ice: in liquid-like regions (LLRs) in the ice; within the solid ice matrix; and in a quasi-liquid layer (QLL) at the air–ice interface, where past work indicates photolysis is enhanced. In this work, we explore the photoformation of nitrite in these reservoirs using laboratory ices. Nitrite quantum yields, Φ­(NO2 –), at 313 nm for aqueous and LLR ice samples agree with previous values, e.g., 0.65 ± 0.07% at −10 °C. For ice samples made via flash-freezing solution in liquid nitrogen, where nitrate is possibly present as a solid solution, the nitrite quantum yield is 0.57 ± 0.05% at −10 °C, similar to the LLR results. In contrast, the quantum yield at the air–ice interface is enhanced by a factor of 3.7 relative to LLRs, with a value of 2.39 ± 0.24%. These results indicate nitrate photolysis is enhanced at the air–ice interface, although the importance of this enhancement in the environment depends on the amount of nitrate present at the interface.