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

Nitrogen (N) isotopic fractionation during nitrogen oxides (NO_x) cycling and conversion into atmospheric nitrate alters the original N isotopic composition (δ¹⁵N) of NO_x emissions. Limited quantification of these isotopic effects in urban settings hampers the δ¹⁵N-based identification and apportionment of NO_x 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 NO_x 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 NO_x at this urban site. Despite their large NO_x emissions, coal-fired power plants with elevated chimney stacks (>26 m) appear to make a small contribution to surface NO_x levels in downtown Fairbanks (likely less than 18% on average). The combined uncertainties of the δ¹⁵N of NO_x from heating oil combustion and of the influence of low temperatures on the δ¹⁵N of NO_x emitted by vehicle exhaust prevent a more detailed partitioning of surface NO_x sources in Fairbanks.