Comment on tc-2021-355

doi:10.5194/tc-2021-355-rc2

Abstract

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.

Highlights

Previous studies suggest there were several processes occurring at the air–snow interface related to nitrate deposition and preservation that could lead to nitrogen fractionation, including (i) fractionations during snow nitrate photolysis and physical release (Berhanu et al, 2014; Erbland et al, 2013; Frey et al, 2009; Jiang et al, 2021; Shi et al, 2019), and (ii) the proposed fractionation
The gradual enrichments in δ15N(NO3–) from atmospheric nitrate to surface snow nitrate, and to snowpack nitrate can only be explained by the effect of the photo-driven post-depositional processing, and the enrichment after deposition can be 705 quantitatively explained by the photo-induced effect (PIE)
We proposed a simplified method for estimating PIE that can quickly assess the degree of δ15N(NO3–) enrichment from the time of deposition to preservation in snow beneath the snow photic zone

Summary

Introduction

Ice-core nitrate and its isotopes are potential proxies to constrain atmospheric variability of NOx and oxidants concentrations in past atmospheres. This can be compromised by the impacts of post-depositional processing on nitrate 50 concentrations and isotopes (i.e., δ15N, δ18O and Δ17O, where Δ17O = δ17O – 0.52 × δ18O) (Alexander et al, 2019; Savarino et al, 2016; Wolff et al, 2008). Follow–up studies further indicate changes in the isotopic compositions of snow nitrate at depth in the snowpack, i.e., increases in δ15N and decreases in δ18O/Δ17O 60 (Blunier et al, 2005; Curtis et al, 2018; Erbland et al, 2013; Frey et al, 2009; Shi et al, 2015). Processes leading to such changes were referred to as post–depositional processing, and δ15N of the archived nitrate was used to reflect the degree of post– depositional processing due to its high sensitivity to these processes (Erbland et al., 2013; Frey et al, 2009; Geng et al, 2015; Jiang et al, 2021; Shi et al, 2015; Winton et 65 al., 2020)

Results

Discussion

Conclusion