Sensitivity analysis of PM_2.5 and O₃ co-pollution in Beijing based on GRAPES-CUACE adjoint model

In recent years, incidents of simultaneous exceedance of PM_2.5 and O₃ concentrations, termed PM_2.5 and O₃ co-pollution events, have frequently occurred in China. This study conducted atmospheric circulation analysis on two typical co-pollution events in Beijing, occurring from July 22 to July 28, 2019, and from April 25 to May 2, 2020. These events were categorized into pre-trough southerly airflow type (Type 1) and post-trough northwest flow type (Type 2). Subsequently, sensitivity analyses using the GRAPES-CUACE adjoint model were performed to quantify the contributions of precursor emissions from Beijing and surrounding areas to PM_2.5 and O₃ concentrations in Beijing for two types of co-pollution. The results indicated that the spatiotemporal distribution of sensitive source region varied among different circulation types. Primary PM_2.5 (PPM_2.5) emissions from Hebei contributed the most to the 24-hour average PM_2.5 (24-h PM_2.5) peak concentration (41.6 %-45.4 %), followed by Beijing emissions (31 %-35.7 %). The maximum daily 8-hour average ozone peak concentration was primarily influenced by the emissions from Hebei and Beijing, with contribution ratios respectively of 32.8 %-44.8 % and 29 %-42.1 %. Additionally, NO_x emissions were the main contributors in Type 1, while both NO_x and VOCs emissions contributed similarly in Type 2. The iterative emission reduction experiments for two types of co-pollution indicated that Type 1 required emission reductions in NO_x (52.4 %-71.8 %) and VOCs (14.1 %-33.8 %) only. In contrast, Type 2 required combined emission reductions in NO_x (37.0 %-65.1 %), VOCs (30.7 %-56.2 %), and PPM_2.5 (31 %-46.9 %). This study provided a reference for controlling co-pollution events and improving air quality in Beijing.