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

• Main Authors: Jiang, Zhuang, Savarino, Joel, Alexander, Becky, Erbland, Joseph, Jaffrezo, Jean-Luc, Geng, Lei
• Format: Text
• Language: English
• Published: 2021
• Subjects: Greenland; ice core
• Online Access: https://doi.org/10.5194/tc-2021-355; https://tc.copernicus.org/preprints/tc-2021-355/

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 δ 15 N(NO 3 –) 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 δ 15 N(NO 3 –) variations observed in snowpack. The spring average aerosol δ 15 N(NO 3 –) was (–17.9 ± 8.3) ‰, significantly depleted compared to snowpack spring average of (4.6 ± 2.1) ‰, with surface snow δ 15 N(NO 3 –) of (–6.8 ± 0.5) ‰ that is in between. The differences in aerosol, surface snow and snowpack δ 15 N(NO 3 –) are best explained by the photo-driven post–depositional processing of snow nitrate, with potential contributions from fractionation during nitrate deposition. In contrast to δ 15 N(NO 3 –), the atmospheric Δ 17 O(NO 3 –) was of similar seasonal pattern and magnitude of change to that in snowpack, suggesting little to no changes in Δ 17 O(NO 3 –) from photolysis, consistent with previous modeling results. The atmospheric δ 18 O(NO 3 –) varied similarly as atmospheric Δ 17 O(NO 3 –), with summer low and winter high values. However, the difference between atmospheric and snow δ 18 O(NO 3 –) was larger than that of Δ 17 O(NO 3 –), and the linear relationships between δ 18 O/Δ 17 O(NO 3 –) 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 δ 18 O(NO 3 –) is larger than that of Δ 17 O(NO 3 –) given that photolysis is a mass-dependent process.