Particulate nitrate pollution control in North China Plain and Northeastern United States: A comparison of formation mechanism and its response to gaseous pollutants.

Particulate nitrate (NO3-) is an increasingly important component of fine particulate matter (PM2.5) during winter in both the North China Plain (NCP) and Northeastern United States (NE US). In this study, we conducted a comprehensive assessment of NO3- formation and its response to gaseous precursors in the NCP and NE US using extended ground-level measurements and model simulations during the wintertime. Different emission control policies implemented in two regions affect the relationship between NO3- and NO2. In the NCP, despite large decreases in anthropogenic NOX emissions, the NO3- concentrations remained stable and were much higher than those in the NE US. Besides, the linear slope of NO3--NO2 was approximately ten times higher than that in the NE US, likely driven by higher atmospheric oxidation capacity and lower aerosol acidity, which favour nitrate formation and its partitioning from the gas to particulate phase. The sensitivity results indicate that, under current conditions, controlling NH3 emissions is the most effective strategy for reducing NO3- in the NE US. In contrast, for the NCP, further reductions in NOX emission beyond current levels are more effective. The control of NO3- in the NCP is modulated by variations in both NO2 and photochemical oxidants, whereas the response of NO3- to NH3 emission control is highly nonlinear and becomes effective only at a thermodynamic sweet spot (pH < 3). To prevent further conversion from NO2 to NO3- in the NCP, future control strategies for particulate nitrate pollution should implement coordinated reductions in NOX and VOCs emissions.