PM_2.5 and O₃ relationships affected by the atmospheric oxidizing capacity in the Yangtze River Delta, China

The atmospheric oxidizing capacity (AOC), reflecting the self-cleansing capacity of the atmosphere, plays an important role in the chemical evolution of secondary fine particulate matter (PM_2.5) and ozone (O₃). In this work, the AOC and its relationships with PM_2.5 and O₃ were investigated with a chemical transport model (CTM) in the Yangtze River Delta (YRD) region during the four seasons of 2017. The region-wide average AOC is ~4.5×10^-4 min^-1 in summer and ~ 6.4×10^-5 min^-1 in winter. Hydroxyl (OH) radicals oxidation contributes 55-69% to the total AOC, followed by nitrate (NO₃) radicals and O₃ (accounting for 19-34% and < 10%, respectively). The AOC attains a strong positive correlation with the O₃ level in all seasons. However, it is weakly or insignificantly correlated with PM_2.5 except in summer. Additionally, AOC×(SO₂ + NO₂ + volatile organic compound (VOC)) is well correlated with the PM_2.5 level, and high levels of precursors counteract lower AOC values in cold seasons. Collectively, the results indicate that the abundance of precursors could drive secondary aerosol formation in winter, and aqueous or heterogeneous reactions (not considered in the AOC estimates) are likely of importance at high aerosol loadings in the YRD. The relationship between the daily PM_2.5 and O₃ levels is affected by the AOC magnitude. PM_2.5 and O₃ are strongly correlated when the AOC is relatively high, but PM_2.5 is independent of O₃ under low-AOC (<6.6×10^-5 min^-1, typically in winter) conditions. This work reveals the dependence of PM_2.5-O₃ relationships on the AOC, suggesting that joint PM_2.5 and O₃ reduction could be realized at moderate to high AOC levels, especially in spring and autumn when the cooccurrence of high O₃ and PM_2.5 events is frequently observed.