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 HNO3 (15N/14N and 18O/16O) from Summit, Greenland provide new observational constraints on postdepositional processing of snowpack nitrate. The δ15N of NO3− 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 N2. The δ18O of NO3−, however, is ∼40‰ higher in surface snow than air samples. Photochemical box modeling of HNO3 chemistry at Summit suggests that the δ18O of the air samples is derived predominantly from gas‐phase HNO3 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 HNO3 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 δ18O of nitrate buried in snow, while photolytic loss of snow nitrate will decrease the δ18O of buried nitrate by 0.9‰. Photolytic recycling and loss will increase the δ15N 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.