Differences between VOCs and NOx transport contributions, their impacts on O₃, and implications for O₃ pollution...

The relationship between O₃ and its precursors during urban polluted episodes remains unclear. In this study, the simultaneous source apportionment of VOCs, NO_x, and O₃ over the Yangtze River Delta (YRD) region during the O₃ 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_x, and their impacts on O₃ 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_x (31 %-43 %) and O₃ (16 %-33 %). NO_x and O₃ have strong transport abilities with high and close contribution percentages, which are highly correlated, mainly because oxygen atoms produced by the photolysis of NO₂ in the aged air mass combined rapidly with O₂ to form O₃ 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_x. Furthermore, owing to the sufficient aging of VOCs, those contributed by long-distance transport have little effect on O₃. To a certain extent, controlling one's NO_x 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_x among cities and even long-distance areas to alleviate regional O₃ pollution.