Cause of nocturnal surface ozone enhancement in the North China Plain.

The North China Plain (NCP), known for its dense population, extensive urbanization, and developed industry and agriculture, faces one of the foremost ozone (O) pollution issues nationwide and even globally. Currently, most studies focus on daytime peak O levels, with insufficient understanding of the increase in nighttime O concentrations. Based on data from 204 national atmospheric composition monitoring sites in the NCP from 2015 to 2023, we investigated the characteristics of nocturnal surface O enhancement (NSOE) events and explored potential formation mechanisms. The mean annual frequencies of single-site and regional NSOE event in the NCP between 2015 and 2023 are 42 % and 21 %, respectively. The daytime peak O concentrations before and after NSOE events exceeded those during the corresponding periods of non-NSOE events by 84 ± 19 and 32 ± 15 μg/m, respectively. The overall effect of the NSOE events was to decelerate the rate of decline in nighttime O concentrations and resulted in a reduction of NO and CO concentrations from 22:00 onwards. Low level jet (LLJ) and vertical mixing were the main factors affecting NSOE events in the NCP. The proportion of NSOE events affected by LLJ in four representative cities ranged from 57.6 % to 79.5 %. Furthermore, the high concentration of O in the residual layer before the NSOE event and the reduction of atmospheric stability during the NSOE event favored downward mixing of upper layer O. The primary weather systems influencing the four most severe regional NSOE events were LLJ, typhoon, and cold fronts. The first two events were dominated by vertical mixing of O, while the latter two events were mainly affected by horizontal transport. Our findings provide the first overview of NSOE events in the NCP from characteristics to mechanisms, emphasizing the necessity for future detailed studies based on nocturnal vertical O observations.