Nocturnal ozone enhancement in Shandong Province, China, in 2020-2022: Spatiotemporal distribution and formation mechanisms.

Nighttime ozone enhancement (NOE) can increase the oxidation capacity of the atmosphere by stimulating nitrate radical formation and subsequently facilitating the formation of secondary pollutants, thereby affecting air quality in the following days. Previous studies have demonstrated that when nocturnal ozone (O) concentrations exceed 80 μg/m, it leads to water loss and reduction of plant yields. In this study, the characteristics and mechanisms of NOE over Shandong Province as well as its 16 cities were analyzed based on observed hourly O concentrations from 2020 to 2022. The analysis results show that NOE predominantly occurred in the periods of 0:00-3:00 (41 %). The annual mean frequency of NOE events was ~64 days/year, approximately 4-7 days per month. The average concentration of nocturnal O peak (NOP) was ~72.6 μg/m. Notably, high NOP was observed in the period from April to September with the maximum in June. Coastal cities experienced more NOE events. Typical NOE events characterized by high NOP concentrations in the coastal cities of QingDao, WeiHai and YanTai in June 2021 were selected for detailed analysis with a regional chemical transport model. The results showed that high levels of O in eastern coastal cities during NOE events primarily originate from horizontal transport over the sea, followed by vertical transport. During the daytime, O and its precursors are transported to the Yellow Sea by westerly winds, leading to the accumulation of O near the sea and coastline. Consequently, under the influence of prevailing winds, the movement of O pollution belts from the sea to land causes rapid increases in near-surface O levels. Meanwhile, vertical transport can also contribute to NOE in coastal areas. The high-level O in the upper atmosphere generally originates from long-distance transport and turbulent transport of O produced near the ground during the daytime. At night, the absence of chemicals that consume O in the upper air and descending air flow carries O to the near-surface. The impacts of other O-depletion processes (such as dry deposition) on NOE are less pronounced than those of transport processes.