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 (NOx) emissions, with important improvements in air quality and acid deposition. However, predicting the impact of emission reductions on tropospheric chemistry remains challenging, as NOx 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 NOx (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 NOx decreases. The isotopic constraint on oxidizing chemistry greatly enhances our ability to test and predict the effectiveness of emissions reductions on oxidation chemistry.