Sensitivity of PM_2.5 to NO_x emissions and meteorology in North China based on observations

This study examines the sensitivity of daily PM_2.5 to NO_x emissions and meteorology using in situ observations from main cities of North China (NC). NC cities are divided into low-, medium-, and high-emission groups by the ranking of their 4-year mean NO₂. For each emission group, daily NO₂ levels are used to divide the days into good-, medium-, and bad-meteorological conditions. Regardless of their emission levels, all cities reveal significant decreases (96%-172%) in daily PM_2.5 levels from bad to good meteorological conditions. The largest difference in PM_2.5 concentrations between the emissions groups is found under bad meteorological conditions, with 56% higher PM_2.5 in high-emission cities than low-emission cities, indicating PM_2.5 under bad meteorological conditions has the largest sensitivity to emissions. The high-emission, bad-meteorology group saw a 24% decrease in mean daily PM_2.5 levels from 2017, a high-emission year, to 2019, a low-emission year. However, under good meteorological conditions, the high-emissions group shows an increase of 8.8 μg/m³ in mean daily PM_2.5 from 2017 to 2019 with a 2.6% increase in the possibility of high PM_2.5. These results suggest the current emission reduction measures are more effective in controlling PM_2.5 in high-emission cities under bad meteorological conditions than under other meteorological conditions.