Nitrogen Isotope Composition of Thermally Produced NOx from Various Fossil-Fuel Combustion Sources

The nitrogen stable isotope composition of NOx (delta N-15-NOx) may be a useful indicator for NOx source partitioning, which would help constrain NOx source contributions in nitrogen deposition studies. However, there is large uncertainty in the delta N-15-NOx, values for anthropogenic sources other than on-road vehicles and coal-fired energy generating units. To this end, this study presents a broad analysis of delta N-15-NOx, from several fossil-fuel combustion sources that includes: airplanes, gasoline-powered vehicles not equipped with a three-way catalytic converter, lawn equipment, utility vehicles, urban buses, semitrucks, residential gas furnaces, and natural-gas-fired power plants. A relatively large range of delta N-15-NOx values was measured from -28.1 parts per thousand to 8.5 parts per thousand for individual exhaust/flue samples that generally tended to be negative due to the kinetic isotope effect associated with thermal NOx production. A negative correlation between NOx concentrations and delta N-15-NOx, for fossil-fuel combustion sources equipped with selective catalytic reducers was observed, suggesting that the catalytic reduction of NOx increases delta N-15-NOx values relative to the NOx produced through fossil-fuel combustion processes. Combining the delta N-15-NOx, measured in this study with previous published values, a delta N-15-NOx regional and seasonal isoscape was constructed for the contiguous U.S., which demonstrates seasonal and regional importance of various NOx sources.