Unraveling the fingerprints of NOx using stable isotopes: Implications for NOx source partitioning and oxidation chemistry

Wendell William Walters, Purdue University


The nitrogen (N) and oxygen (O) stable isotope composition (δ15N & δ18O) of nitrogen oxides (NOx )may be a useful tool for constraining NOx emission sources as well as for understanding the atmospheric oxidation pathways responsible for its removal if various NOx sources and sink processes exhibit characteristic isotopic compositions (“fingerprints”). However, this requires (1) an accurate and complete inventory of δ15N(NOx) values from major emission sources, (2) an assessment of the kinetic and equilibrium isotope effects that can impact δ15N and δ18O values of NOx, (3) and test these assumptions by conducting accurate in situ δ15N and δ18O measurements of atmospheric NOx. To this end, I have characterized the δ15N(NOx) signatures from various fossil-fuel NOx sources, including buses, trucks, lawn equipment, natural gasfired boilers, and airplanes. These δ15N(NOx) source characterization studies along with prior studies indicate that soil emission (nitrification/denitrification), “thermal” NOx producedfromfossil-fuelcombustion, and“source” NOx producedfromcoal-fired power plants have relative distinctive values. In addition, both my experimental and theoretical investigations on the isotope effects associated with NOx oxidation indicate that isotopes effects via equilibrium isotope exchange and kinetic isotope effects occurring during NOx oxidation reactions may influence the δ15N and δ18O values of atmospheric nitrate. Using these calculated isotope effects, I developed a simple model for the production of atmospheric nitrate through its three major pathways thatinclude(1)NO2 +•OH→HNO3, (2)N2O5 +surface→2HNO3, and(3)NO3+ R→•R. This model indicated that these pathways result in distinctive δ18O-δ15N relationships that tend to match reported literature values. Finally, in order to evaluate the influences of NOx emission sources and isotope effects on the isotope composition of NO2, which serves as precursor molecule to atmospheric nitrate, ambient NO2 was collected and analyzed for 15N and 18O . These results suggest that δ18O of NO2 has a distinctive diurnal profile reflecting the photochemical cycling of NOx while δ15N of NO2 tends to track with NOx sources with small but significant isotope effects altering daytime δ15N(NO2) by approximately 2-4%. Overall, this research has refined the “fingerprints” of atmospheric NOx and will be useful for future studies aimed at understanding regional and spatial distributions in NOx emission budgets and tracing NOx oxidation chemistry.




Michalski, Purdue University.

Subject Area

Atmospheric Chemistry|Biogeochemistry|Environmental science

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