Isotopic ratios in gas‐phase HNO₃ and snow nitrate at Summit, Greenland

Abstract

Measurements of the isotope ratios of snow nitrate and atmospheric gas‐phase HNO 3 ( 15 N/ 14 N and 18 O/ 16 O) from Summit, Greenland provide new observational constraints on postdepositional processing of snowpack nitrate. The δ 15 N of NO 3 − is similar in surface snow and in air sampled 1.5 m above the snow surface during Spring and Summer 2006, ranging between −15 and +6‰ versus N 2 . The δ 18 O of NO 3 − , however, is ∼40‰ higher in surface snow than air samples. Photochemical box modeling of HNO 3 chemistry at Summit suggests that the δ 18 O of the air samples is derived predominantly from gas‐phase HNO 3 originating from photolyzed snow nitrate. Measurements of isotopically labeled nitrate in surface snow confirm that photolytic recycling of snowpack nitrate, in which photolyzed nitrate products recombine to form HNO 3 that is subsequently redeposited to the snow surface, does occur at Summit. Calculations incorporating these measurements suggest that photolytic recycling of snow nitrate at Summit will have a maximum −7.5‰ change on the δ 18 O of nitrate buried in snow, while photolytic loss of snow nitrate will decrease the δ 18 O of buried nitrate by 0.9‰. Photolytic recycling and loss will increase the δ 15 N of buried snow nitrate, the degree to which depends on the nitrogen isotope fractionation of nitrate photolysis. Finally, a comparison of surface snow and snowpit samples demonstrates the preservation of the seasonal cycle in isotope ratios of snow nitrate.