Seasonal variations in N and O isotopes of nitrate in snow at Summit, Greenland: Implications for the study of nitrate in snow and ice cores

Nitrogen and oxygen isotopes of NO3- have been measured in snow and firn from Summit, Greenland. The N-15/N-14 and O-18/O-16 ratios of NO3- in recently fallen snow are similar to those of surface snow. Diurnal variation is observed in N-15/N-14 of NO3-, and possibly O-18/O-16, suggesting fractionating loss of NO3- from snow during the day, which is subsequently recovered at night. A larger seasonal variation is observed, with higher N-15/N-14 and lower O-18/O-16 of NO3- in summer than winter, which cannot be explained by postdepositional fractionation. The generally high O-18/O-16 of NO3- in Greenland snow (delta(18)O versus VSMOW = 65.2 to 79.6%) indicates that oxygen atoms from ozone have been incorporated into NOx that was subsequently deposited as HNO3. The lower mean delta(18)O of NO3- in summer snow relative to winter (68.9% in summer 2000 and 70.5% in summer 2001 versus 77.5% in winter 2000-01) is a result of summertime HNO3 production via NO2 reaction with hydroxyl radical (OH), which dilutes the high delta(18)O imparted on NO2 from ozone. The higher mean N-15/N-14 of NO3- observed in snow from spring (delta(15)N versus air N-2 = + 5.9parts per thousand in 2000 and -1.4parts per thousand in 2001) and summer (+0.1parts per thousand in 2000 and -0.8% in 2001) than fall (-9.2parts per thousand in 2000) and winter (-10.0parts per thousand in 2000-01) is more difficult to explain with seasonal photochemistry, given current knowledge. The seasonal N-15/N-14 change may reflect NOx sources, with a greater fall and wintertime contribution from fossil fuel emissions relative to other inputs of NOx (i.e., biogenic soil emissions, biomass burning, and lightning).