Nitrogen Stable Isotope Composition of Various Fossil-fuel Combustion Nitrogen Oxide Sources

Abstract:

Nitrogen oxides (NO_x = NO + NO₂) are important trace gases that impact atmospheric chemistry, air quality, and climate. In order to help constrain NO_x source contributions, the nitrogen (N) stable isotope composition of NO_x (δ¹⁵N-NO_x) may be a useful indicator for NO_x source partitioning. However, despite anthropogenic emissions being the most prevalent source of NO_x, there is still large uncertainty in the δ¹⁵N-NO_x values for anthropogenic sources. To this end, this study provides a detailed analysis of several fossil-fuel combustion NO_x sources and their δ¹⁵N-NO_x values. To accomplish this, exhaust or flue samples from several fossil-fuel combustion sources were sampled and analyzed for their δ¹⁵N-NO_x that included airplanes, gasoline-powered vehicles not equipped with a catalytic converter, gasoline-powered lawn tools and utility vehicles, diesel-electric buses, diesel semi-trucks, and natural gas-burning home furnace and power plant. A relatively large range of δ¹⁵N-NO_x values were measured from -28.1 to 0.3‰ for individual exhaust/flue samples with cold started diesel-electric buses contributing on average the lowest δ¹⁵N-NO_x values at -20.9‰, and warm-started diesel-electric buses contributing on average the highest values of -1.7‰. The NO_x sources analyzed in this study primarily originated from the “thermal production” of NO_x and generally emitted negative δ¹⁵N-NO_x values, likely due to the kinetic isotope effect associated with its production. It was found that there is a negative correlation between NO_x concentrations and δ¹⁵N-NO_x for fossil-fuel combustion sources equipped with catalytic NO_x reduction technology, suggesting that the catalytic reduction of NO_x may have an influence on δ¹⁵N-NO_x values. Based on the δ¹⁵N-NO_x values reported in this study and in previous studies, a δ¹⁵N-NO_x regional and seasonal isoscape was constructed for the contiguous United States. The constructed isoscape demonstrates the seasonal importance of various NO_x sources to particular regions and will be helpful in evaluating the N isotopes in nitrate deposition studies.