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

Snow nitrate is vulnerable to photolytic loss that causes isotopic alteration, and thus its isotopes can potentially track the extent of snow nitrate photolysis and its impacts in environments where loss is significant. Large increases in δ¹⁵N-NO₃⁻ below the snow surface have been attributed to photolysis and this behavior is generally consistent amongst theoretical as well as lab and field studies. Oxygen isotope ratios are thought to be influenced by photolysis as well as secondary condensed-phase chemistry, but the competing effects have yet to be reconciled. Here we use a model that simulates nitrate burial, photolytic fractionation, and re-oxidation in snow to quantitatively assess these processes with the aim of developing a consistent framework for interpreting the photolytic effects of the complete nitrate isotopic composition (δ¹⁵N, δ¹⁸O, and Δ¹⁷O). This study reveals that isotopic effects of nitrate photolysis and aqueous-phase re-oxidation chemistry are important sources of uncertainties in modeling δ¹⁸O-NO₃⁻.