Meteorological conditions drive divergent responses of co-occurring PM_2.5 and O₃ pollution to emission reductions and role of aerosol feedback in Beijing–Tianjin–Hebei-Shandong

Abstract. Co‑occurring PM_2.5 and O₃ pollution (Double High Pollution, DHP) presents a growing air quality concern, yet its response to emission controls under different meteorological conditions is poorly quantified. Previous studies have focused on synoptic‑scale or single meteorological variable with limited consideration of aerosol-meteorology feedback. This study investigated the DHP response to NOx–VOC emission reductions and the role of aerosol-meteorology feedback in Beijing–Tianjin–Hebei-Shandong in July 2020 using the WRF-Chem model. DHP occurred under warm (23–31 °C), moderately humid (45–80 %), and shallow boundary layer (0.5–1.1 km) conditions. These typical meteorological conditions were classified into five types: Convective, Stable, WarmHumid, DryHot, and Moderate based on boundary layer height, temperature and humidity. PM_2.5 was highest under Stable and WarmHumid (~46 μg m^-3) and lowest under DryHot (~40 μg m^-3); O₃ was highest under DryHot and Moderate (~83–84 ppb) and lowest under WarmHumid (~76 ppb). O₃ responded most strongly to emission controls under WarmHumid (-25.2 % at 50 %/50 % NOx/VOC) and weakest under DryHot (-23.3 %) with stronger NO_x sensitivity under WarmHumid and Convective; PM_2.5 reductions were largest under Stable and WarmHumid (-20.9 % and -20.6 %) and smallest under DryHot (-15.2 %). Aerosol feedback enhanced PM_2.5 reduction most under Stable and WarmHumid but weakened O₃ reduction under Stable when NO_x cuts were large without VOC co-reduction, driven by the most lasting PBLH increase and strong OH amplification. These results demonstrate that meteorological condition-specific strategies, particularly coordinated NOx–VOC control under stagnant and humid conditions, are essential for effective DHP mitigation.