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 NO 3 − have been measured in snow and firn from Summit, Greenland. The 15 N/ 14 N and 18 O/ 16 O ratios of NO 3 − in recently fallen snow are similar to those of surface snow. Diurnal variation is observed in 15 N/ 14 N of NO 3 − , and possibly 18 O/ 16 O, suggesting fractionating loss of NO 3 − from snow during the day, which is subsequently recovered at night. A larger seasonal variation is observed, with higher 15 N/ 14 N and lower 18 O/ 16 O of NO 3 − in summer than winter, which cannot be explained by postdepositional fractionation. The generally high 18 O/ 16 O of NO 3 − in Greenland snow (δ 18 O versus VSMOW = 65.2 to 79.6‰) indicates that oxygen atoms from ozone have been incorporated into NO x that was subsequently deposited as HNO 3 . The lower mean δ 18 O of NO 3 − 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 HNO 3 production via NO 2 reaction with hydroxyl radical (OH), which dilutes the high δ 18 O imparted on NO 2 from ozone. The higher mean 15 N/ 14 N of NO 3 − observed in snow from spring (δ 15 N versus air N 2 = +5.9‰ in 2000 and −1.4‰ in 2001) and summer (+0.1‰ in 2000 and −0.8‰ in 2001) than fall (−9.2‰ in 2000) and winter (−10.0‰ in 2000–01) is more difficult to explain with seasonal photochemistry, given current knowledge. The seasonal 15 N/ 14 N change may reflect NO x sources, with a greater fall and wintertime contribution from fossil fuel emissions relative to other inputs of NO x (i.e., biogenic soil emissions, biomass burning, and lightning).