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 (NOx) 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 NOx 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 NOx, concurrent with an increasing influence from oil and natural gas combustion. These findings have broad implications for improving our understanding of how changes in NOx emission sources influence pNO3 formation and evaluating the success of past clean air policies.