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 ¹⁵N/ ¹⁴N and ¹⁸O/¹⁶O ratios of NO ₃^- in recently fallen snow are similar to those of surface snow. Diurnal variation is observed in ¹⁵N/ ¹⁴N of NO₃^-, and possibly ¹⁸O/¹⁶O, suggesting fractionating loss of NO₃^- from snow during the day, which is subsequently recovered at night. A larger seasonal variation is observed, with higher ¹⁵N/¹⁴N and lower ¹⁸O/¹⁶O of NO₃^- in summer than winter, which cannot be explained by postdepositional fractionation. The generally high ¹⁸O/¹⁶O of NO ₃^- in Greenland snow (δ¹⁸O versus VSMOW = 65.2 to 79.6‰) indicates that oxygen atoms from ozone have been incorporated into NO_x that was subsequently deposited as HNO₃. The lower mean δ¹⁸0 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 HNO ₃ production via NO₂ reaction with hydroxyl radical (OH), which dilutes the high δ¹⁸O imparted on NO₂ from ozone. The higher mean ¹⁵N/¹⁴N of NO₃^- observed in snow from spring (δ ¹⁵N versus air N₂ = +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 ¹⁵N/¹⁴N 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).