PM2.5 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 (PM2.5) and ozone (O3). In this work, the AOC and its relationships with PM2.5 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 PM2.5 except in summer. Additionally, AOC×(SO2 + NO2 + volatile organic compound (VOC)) is well correlated with the PM2.5 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 PM2.5 and O3 levels is affected by the AOC magnitude. PM2.5 and O3 are strongly correlated when the AOC is relatively high, but PM2.5 is independent of O3 under low-AOC (<6.6×10-5 min-1, typically in winter) conditions. This work reveals the dependence of PM2.5-O3 relationships on the AOC, suggesting that joint PM2.5 and O3 reduction could be realized at moderate to high AOC levels, especially in spring and autumn when the cooccurrence of high O3 and PM2.5 events is frequently observed.