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

Measurements of the isotope ratios of snow nitrate and atmospheric gas-phase HNO3 (N-15/N-14 and O-18/O-16) from Summit, Greenland provide new observational constraints on postdepositional processing of snowpack nitrate. The delta N-15 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 parts per thousand versus N-2. The delta O-18 of NO3-, however, is similar to 40 parts per thousand higher in surface snow than air samples. Photochemical box modeling of HNO3 chemistry at Summit suggests that the delta O-18 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 parts per thousand change on the delta O-18 of nitrate buried in snow, while photolytic loss of snow nitrate will decrease the delta O-18 of buried nitrate by 0.9 parts per thousand. Photolytic recycling and loss will increase the delta N-15 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.