Long-Term Changes in Summertime Nitrate Chemistry in the Top Boundary Layer of North China

Reducing fine particulate nitrate (pNO₃^–) is critical for further mitigating PM_2.5 pollution in China. However, previous NO_x emission reductions have failed to achieve the expected pNO₃^– decreases. The present study reports that pNO₃^– concentration in summer increased by 55.8% and 5.6% at North China Peak (1534 m a.s.l.) from 2007 to 2014 and 2014 to 2021, respectively. pNO₃^– formation enhancement was caused mainly by decreased aerosol acidity due to notable SO₄^2– reduction. pNO₃^– formation changed from a process limited by NH₄⁺ to one colimited by NO₂ and NH₄⁺, suggesting an increased effect of NO_x reduction on decreasing pNO₃^– production. Vertical transport represents a significant source of pNO₃^– near the surface, illustrating a percentage as high as 98% recorded during daytime hours and a proportion of 34% in the dark over North China in the simulation scenario during summer 2020. The scheme to reduce NO_x emissions by 10% from 2020 to 2025 is predicted to slowly decrease aloft pNO₃^– over North China, which may facilitate further reductions in pNO₃^– concentrations near the surface via vertical transport. The inflection of nitrate chemistry in the top boundary layer suggests an opportunity to accelerate PM_2.5 reduction under projected further emission reductions.