Unraveling Urban NOx Emission Sources in Polluted Arctic Wintertime Using NO2 Nitrogen Isotopes
Résumé
Nitrogen (N) isotopic fractionation during nitrogen oxides (NO x ) cycling and conversion into atmospheric nitrate alters the original N isotopic composition (δ 15 N) of NO x emissions. Limited quantification of these isotopic effects in urban settings hampers the δ 15 N-based identification and apportionment of NO x sources. δ 15 N of nitrogen dioxide (NO 2 ) measured during winter in downtown Fairbanks, Alaska, displayed a large temporal variability, from 10.2 to 24.1‰. δ 15 N(NO 2 ) 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 NO 2 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 δ 15 N(NO 2 ) measurements for N fractionation effects, a δ 15 Nbased 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 δ 15 N of NO x from heating oil combustion and of the influence of low temperatures on the δ 15 N of NO x emitted by vehicle exhaust prevent a more detailed partitioning of surface NO x sources in Fairbanks.
Domaines
Sciences de l'environnement| Origine | Fichiers éditeurs autorisés sur une archive ouverte |
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