Atmospheric and snow nitrate isotope systematics at Summit, Greenland: the reality of the post-depositional effect

The effect of post–depositional processing on the preservation of snow nitrate isotopes at Summit, Greenland remains a subject of debate which hinders the interpretations of ice–core nitrate concentrations and isotope records. Here we present the first year–round observations of atmospheric aerosol nitrate and its isotopic compositions at Summit, and compare them with published surface snow and snowpack observations. The atmospheric δ15N(NO3–) remained negative throughout the year, ranging from –3.1 ‰ to –47.9 ‰ with a mean of (–14.8 ± 7.3) ‰, and displayed no apparent seasonality that is different from the distinct seasonal δ15N(NO3–) variations observed in snowpack. The spring average aerosol δ15N(NO3–) was (–17.9 ± 8.3) ‰, significantly depleted compared to snowpack spring average of (4.6 ± 2.1) ‰, with surface snow δ15N(NO3–) of (–6.8 ± 0.5) ‰ that is in between. The differences in aerosol, surface snow and snowpack δ15N(NO3–) are best explained by the photo-driven post–depositional processing of snow nitrate, with potential contributions from fractionation during nitrate deposition. In contrast to δ15N(NO3–), the atmospheric Δ17O(NO3–) was of similar seasonal pattern and magnitude of change to that in snowpack, suggesting little to no changes in Δ17O(NO3–) from photolysis, consistent with previous modeling results. The atmospheric δ18O(NO3–) varied similarly as atmospheric Δ17O(NO3–), with summer low and winter high values. However, the difference between atmospheric and snow δ18O(NO3–) was larger than that of Δ17O(NO3–), and the linear relationships between δ18O/Δ17O(NO3–) were different for atmospheric and snowpack samples. This suggests the oxygen isotopes are also affected before preservation in the snow at Summit, but the degree of change for δ18O(NO3–) is larger than that of Δ17O(NO3–) given that photolysis is a mass-dependent process.