Unraveling Urban NOx Emission Sources in Polluted Arctic Wintertime Using NO2 Nitrogen Isotopes

Nitrogen (N) isotopic fractionation during nitrogen oxides (NOx) cycling and conversion into atmospheric nitrate alters the original N isotopic composition (δ15N) of NOx emissions. Limited quantification of these isotopic effects in urban settings hampers the δ15N‐based identification and apportionment of NOx sources. δ15N of nitrogen dioxide (NO2) measured during winter in downtown Fairbanks, Alaska, displayed a large temporal variability, from −10.2 to 24.1‰. δ15N(NO2) records are found to be driven by equilibrium isotopic fractionation, at a rate in very close agreement with theoretical predictions. This result confirms that N isotopic partitioning between NO and NO2 can be accurately predicted over a wide range of conditions. This represents an important step for inferring NOx emission sources from isotopic composition measurement of reactive nitrogen species. After correcting our δ15N(NO2) measurements for N fractionation effects, a δ15N‐based source apportionment analysis identifies vehicle and space heating oil emissions as the dominant sources of breathing‐level NOx at this urban site. Despite their large NOx emissions, coal‐fired power plants with elevated chimney stacks (>26 m) appear to make a small contribution to surface NOx levels in downtown Fairbanks (likely less than 18% on average). The combined uncertainties of the δ15N of NOx from heating oil combustion and of the influence of low temperatures on the δ15N of NOx emitted by vehicle exhaust prevent a more detailed partitioning of surface NOx sources in Fairbanks. Plain Language Summary Nitrogen (N) stable isotopes measured in atmospheric reactive N (Nr) species can help trace emission sources of nitrogen oxides (NOx). However, large uncertainties subsist regarding the factors controlling the variability of N isotopes in Nr species, preventing a precise isotope‐based emission source apportionment. This study presents a comprehensive analysis of the enrichment of 15N–14N (δ15N) in atmospheric nitrogen dioxide (NO2) collected during the Alaskan Layered Pollution And Chemical Analysis (ALPACA) 2022 international winter campaign in Fairbanks, Alaska. Building on in situ meteorological and trace gas, the isotopic fractionation effect driving the significant δ15N variability of NO2 observed in downtown Fairbanks is quantified with great precision. The δ15N records corrected for equilibrium fractionation effects are discussed in light of a local NOx emission inventory. In particular, the influenc