Detailing Atmospheric Nitrate Chemistry in the Northeastern US Under a Period of Changes in Emissions

The successful implementation of the US Clean Air Act has reduced nitrogen oxides (NO_x) emissions, with important improvements in air quality and acid deposition. However, predicting the impact of emission reductions on tropospheric chemistry remains challenging, as NO_x influences oxidant concentrations and aerosol formation. Stable oxygen isotopes (Δ¹⁷O and δ¹⁸O) of atmospheric nitrate (pNO₃) serve as powerful observational constraints for the understanding of oxidation processes. Here, we quantified Δ¹⁷O and δ¹⁸O in pNO₃ from a unique 10-year aerosol record in the northeastern US to constrain chemistry changes over a time of large reductions in NO_x (2005-2015). We observed a significant decreasing trend in δ¹⁸O(pNO₃) (-0.32‰yr^-1), which reflects a shift in oxidation chemistry driven by emission reductions. Air quality modeling indicated that the decrease in δ¹⁸O(pNO₃) between 2005 and 2015 was primarily driven by enhanced nighttime NO oxidation by ozone, with a relative reduction in N₂O₅ hydrolysis and increased NO₂ + OH, leading to more efficient nitrate formation in the recent past. This explains the continued elevated wintertime nitrate that plagues air quality in the eastern US despite significant NO_x decreases. The isotopic constraint on oxidizing chemistry greatly enhances our ability to test and predict the effectiveness of emissions reductions on oxidation chemistry.