Nitrogen isotopes: Tracers of origin and processes affecting PM10 in the atmosphere of Paris

Nitrogen in atmospheric particles in an urban environment is the result of complex primary and secondary processes, which renders identifying its origin somewhat complicated. Using the example of PM10 in the atmosphere of Paris (France), it is shown that the use of stable nitrogen-isotope compositions (δ15N) alleviates this difficulty and provides clear information on the sources of primary and possibly of secondary nitrogen. Characterization of emissions of the different types of emitters in the city (road traffic, waste incinerators and heating sources) shows that these are clearly discriminated by specific isotope signatures. δ15N is particularly useful in showing that a substantial portion of the nitrogen is the result of secondary reactions, reactions that are different in summer and winter, as are the corresponding pollution sources. While it is unclear, among point sources, what the winter source of primary nitrogen is, road traffic appear to be the source of primary nitrogen in summer. Identification of the sources of the secondary nitrogen strongly depends on the nitrogen isotope fractionations (Δ15N) associated to atmospheric conversion of NOx to nitrate, but hypothesises presented here hint at the possible corresponding pollution sources.