PM and O 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) and ozone (O). In this work, the AOC and its relationships with PM and O 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 min in summer and ~ 6.4×10 min in winter. Hydroxyl (OH) radicals oxidation contributes 55-69% to the total AOC, followed by nitrate (NO) radicals and O (accounting for 19-34% and < 10%, respectively). The AOC attains a strong positive correlation with the O level in all seasons. However, it is weakly or insignificantly correlated with PM except in summer. Additionally, AOC×(SO + NO + volatile organic compound (VOC)) is well correlated with the PM 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 and O levels is affected by the AOC magnitude. PM and O are strongly correlated when the AOC is relatively high, but PM is independent of O under low-AOC (<6.6×10 min, typically in winter) conditions. This work reveals the dependence of PM-O relationships on the AOC, suggesting that joint PM and O reduction could be realized at moderate to high AOC levels, especially in spring and autumn when the cooccurrence of high O and PM events is frequently observed.