Thousand Years of Nitrogen Oxide Sources in Western Europe: Evidence From Nitrogen Stable Isotopes ( δ 15 N) of Nitrate in a Mont Blanc Ice Core

Abstract Nitrogen stable isotopes ( δ 15 N) of ice core nitrate (NO 3 − ) are often subject to diverse interpretations associated with changes in nitrogen oxide (NO x ) sourcing, atmospheric reactions, and/or post‐depositional processes. Here, an ice core from Mont Blanc (French Alps) was analyzed to investigate the δ 15 N(NO 3 − ) record over the past 1,000 years. Atmospheric isotopic fractionation, including gas‐particle partitioning, introduces a limited and quantifiable effect on δ 15 N(NO 3 − )—up to 2.4 ‰ in extreme cases. Thus, the glacially‐archived NO 3 − reliably reflects a record of Western European NO x emissions by comparing δ 15 N and concentrations of NO 3 − , and isotope–concentration mixing relationships (Keeling plot) with historical emission inventories. An increase in δ 15 N(NO 3 − ) in the 1800s reflects the dominance of NO x emissions from coal combustion. During the 20th century, the δ 15 N(NO 3 − ) value substantially decreased, from (4.7 ± 1.5) ‰ in 1900 to (−1.9 ± 1.4) ‰ in 1990, and Keeling plot interpretation attributes this decrease to increasing oil combustion emissions. Between 1750 and 2016, the ice core's δ 15 N(NO 3 − ) record generally aligns with existing NO x inventories for Western Europe. However, during the early 20th‐century, low ice core δ 15 N(NO 3 − ) values suggest that the inventories may have underestimated NO x emissions resulting from agriculture. Since 2000, the decreasing NO 3 − concentrations and δ 15 N(NO 3 − ) values highlight the success of mitigation policies in reducing fossil fuel‐induced NO x emissions, albeit with a delay of 20 years relative to emission inventories, that can be attributed to gas‐particle partitioning and mis‐quantification of NO x sources. This work reaffirms the value of alpine ice cores for understanding aerosol sources.