Nitrate (NO 3 − , including particulate (NO 3 − (p) ) and gas-phase nitric acid (HNO 3(g) )), the final product of nitrogen oxides (NO x , NO x = NO + NO 2 ), is ubiquitous in the atmosphere and plays a critical role in biogeochemical cycles (Galloway et al., 2008). Previous observations and model simulations suggested that atmospheric deposition contributes to up to one-third of external inputs of oceanic nitrogen, and increased nitrogen concentrations in ocean surface waters have been observed with the intensification of atmospheric NO 3 − deposition (Duce et al., 2008;Jickells et al., 2017;Kim et al., 2011). On the other hand, ocean emissions (e.g., alkyl nitrates (RONO 2 )) can also act as a source of atmospheric NO x , potentially influencing the budget of NO 3 − in the marine atmospheric boundary layer (MABL) (

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−.