Differences between VOCs and NOx transport contributions, their impacts on O, and implications for O pollution mitigation based on CMAQ simulation over the Yangtze River Delta, China.

The relationship between O and its precursors during urban polluted episodes remains unclear. In this study, the simultaneous source apportionment of VOCs, NO, and O over the Yangtze River Delta (YRD) region during the O polluted episode on July 24-30, 2018, was performed based on the Integrated Source Apportionment Method (ISAM) embedded in the Community Multiscale Air Quality Modeling System (CMAQ). The results of the ISAM were compared with those of the Brute Force Method (BFM) and Positive Matrix Factorization (PMF). Furthermore, the differences between the transport contributions of VOCs and NO, and their impacts on O were analyzed. The results indicate that observations of VOCs species can be well captured by simulated VOCs, and the ISAM has a significant advantage in the source apportionment of VOCs, especially for sources emitting highly reactive species. In the clean and polluted periods, the local contribution percentages of VOCs in urban sites ranged from 60 % to 77 %, much higher than those of NO (31 %-43 %) and O (16 %-33 %). NO and O have strong transport abilities with high and close contribution percentages, which are highly correlated, mainly because oxygen atoms produced by the photolysis of NO in the aged air mass combined rapidly with O to form O during transport. The VOCs chemical loss caused by the oxidation of OH radicals during transport makes the ability of VOCs for long-distance transport much weaker than that of NO. Furthermore, owing to the sufficient aging of VOCs, those contributed by long-distance transport have little effect on O. To a certain extent, controlling one's NO emissions can help other cities more, while controlling one's VOCs emissions can help itself more. Therefore, it is recommended to attach enough importance to joint prevention and control of NO among cities and even long-distance areas to alleviate regional O pollution.