Impacts of meteorology and precursor emission change on PM and O and identification of synergistic emission reduction pathway: A case of combined pollution event in Beijing, China.

Given the leveling off of PM, the coordinated control of PM and O has become a critical challenge in the Beijing-Tianjin-Hebei (BTH) region. Here, we leveraged long-term observation data spanning 2013 to 2023 to reveal spatiotemporal heterogeneity, provincial spatial correlation, and driving factors influencing PM and O integrating Moran's Index and correlation analysis. Additionally, sensitivity experiments on precursor emission reduction focusing on both overall and key sources were conducted by combining WRF-CAMx models and the empirical kinetic modeling approach (EKMA), and synergistic emission reduction pathways were proposed during periods of combined pollution. The findings indicated that PM and MDA8 O exhibited strong positive spatial autocorrelation, with high-high agglomeration region predominantly distributed in the BTH region and central China. PM and O concentrations and their corresponding dominant days have exhibited declining and fluctuating upward trends from 2013 to 2023, respectively. While combined pollution days have significantly alleviated in 2020-2023, severe cases have still occurred in various cities. The correlation between PM, O, and meteorological factors varied markedly across cities and seasons, with noteworthy differences in concentrations, exceedance rates, and meteorological thresholds. During the combined pollution episode, the EKMA revealed that O formation in Beijing was in a VOC-limited regime. By comparing PM, O and sAQI variations response to the different emission reduction pathways, both the "VOCs only" and "NOx only" path has inevitable limitations. Furthermore, the VOC/NOx ratio = 2:1 and 3:1 path was required to ensure that both PM and O are below 75 μg/m and 160 μg/m, respectively, with 82.7% and 92.2% reduction of total VOCs and NOx emissions. For the key source, focusing on reducing industrial VOCs proved more effective in improving air quality. These findings highlighted the importance of strengthening NOx emission reduction to shift O-precursor sensitivity toward the NOx-limited regime for PM and O co-control.