Snow Nitrate Isotopes in Central Antarctica Record the Prolonged Period of Stratospheric Ozone Depletion From ∼1960 to 2000

Abstract

Interpretation of NO3− variability recorded in ice cores remains challenging as it can be lost from snow. Here, we present 60‐year records of NO3− and its isotopic composition (δ15N, δ18O, and Δ17O) in snow in central Antarctica, Dome A. In the upper ∼90 cm snowpack, variations in concentration and isotopic composition of NO3− are dominated by photolytic loss, and δ18O and Δ17O of NO3− are associated with the recycling of NOx to NO3− in the condensed phase driven by photolysis. In the deeper snowpack (∼1960–2000), we observe prolonged trends in concentration and isotopic composition of NO3−, which are best explained as enhanced snow NO3− photolysis due to long‐term decreasing total column ozone (TCO). That is, the prolonged period of trends in NO3− and its isotopes in extremely low snow accumulation sites such as Dome A relay information on variations in TCO and consequently surface solar ultraviolet radiation over time.

Modeling the Complete Nitrogen and Oxygen Isotopic Imprint of Nitrate Photolysis in Snow

“…To test the simulation accuracy of the model in other sites, snow pits from Dome A (DA 13; Shi, Hu, et al., 2022) and Dome C (DC07; Frey et al., 2009) were used for modeling. Using the best‐fit conditions of DA11, the two snow pit profiles can be generally reproduced by the model (Figure S6 in Supporting Information ), although the predicted Δ 17 O of DC07 is ∼4‰ on average lower than the observations.…”

“…We compare our results to the DA11 snow pit sampled at Dome A (80°22′51″S, 77°27′23″E; 4093 m) in January 2011. DA11 was sampled for all three isotopic parameters (δ 15 N, δ 18 O, and Δ 17 O) and [NO 3 − ] at a resolution of ∼2.0 cm to a depth of 300 cm (Text S1 in Supporting Information S1), and the upper ∼90 cm likely fully spans the photic zone at Dome A (Shi, Hu, et al, 2022).…”

“…As described below, this does not influence our overall findings and we therefore primarily use the Δ(ZPE) method to maintain consistency in estimating both ε 15 N and ε 18 O. At Dome A, snow accumulation (∼7 cm snow yr −1 and a snow density of 0.35 g cm −3 ; Shi, Hu, et al (2022)) is considered as distributed evenly with each time step as NO 3 − is deposited. The primary aim of the model is to assess isotopic change over the course of burial and thus concentration and isotopic composition in a new snow layer reflects that at the top of DA11 in our initial runs ([NO 3 − ] = 227 ng g −1 ; δ 15 N = 107‰; δ 18 O = 58‰; Δ 17 O = 31‰).…”

“…As the layer is buried, photolysis proceeds with the fractionation factors and rate constant varying throughout the day and with each month. Actinic flux decreases using a ∼30 cm e ‐folding depth (EFD) (Shi, Hu, et al., 2022).…”

“…Additional runs consider initial concentration, δ 18 O, and Δ 17 O of NO 3 − inputs varying throughout the year based upon skin layer snow and atmosphere measurements on the East Antarctic plateau (Table S1 in Supporting Information ). The δ 15 N is reported to be relatively constant except during spring‐summer when lower values may reflect snow‐sourced NO x at Dome C (Erbland et al., 2013), which features very similar environmental conditions to Dome A (Shi, Hu, et al., 2022). It is therefore difficult to assess seasonal changes in the “original” δ 15 N input, and the initial values are held constant in these simulations, which is typical in previous model simulations (Erbland et al., 2015; Jiang et al., 2022).…”