Isotopic Evidence for Changes in U.S. Energy Fuels and Impact on Particulate Nitrate

The successful implementation of the Clean Air Act has reduced anthropogenic nitrogen oxide (NO_x) emissions, significantly reducing acid rain and improving air quality in the U.S. However, particulate nitrate (pNO₃) concentrations have remained elevated, posing persistent challenges to link NO_x emission sources with pNO₃ formation, which limits the ability to evaluate the effectiveness of emission controls for particulate end points. Measured nitrogen isotopes (δ¹⁵N) in pNO₃ from a 2005-2015 aerosol record in the northeastern U.S. provide direct empirical confirmation of the effectiveness of emission regulations in reducing coal-derived NO_x, concurrent with an increasing influence from oil and natural gas combustion. These findings have broad implications for improving our understanding of how changes in NO_x emission sources influence pNO₃ formation and evaluating the success of past clean air policies.