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
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abstract
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.;
