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 NOx 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 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). Copyright 2004 by the American Geophysical Union.
Journal of Geophysical Research D: Atmospheres