Nocturnal near-surface ozone increases events in Beijing-Tianjin-Hebei region, China during 2018–2023 and the associated mechanisms

The nocturnal ozone enhancement events (NOE events) have been observed in the Beijing-Tianjin-Hebei region (BTH), China over recent years with frequent ozone pollution during the daytime. However, the characteristics and mechanisms of NOE events in BTH is still insufficiently investigated. This study analyzes the spatiotemporal distribution of NOE events in the BTH region and explores the mechanisms of NOE events in different seasons based on the hourly ozone observation data from 2018 to 2023.The results show that there are 147 NOE events per year in the BTH region over 6 years, mainly occurring from March to September, between the local time 10 pm and 2 am of the next morning. Relative to the 6-year averages of nocturnal ozone concentrations, the majority of the nocturnal hourly ozone concentration increments are between 10 and 20 µg.m−3, accounting for 59%, while the extreme nocturnal ozone increments of exceeding 40 µg.m−3 only account for 7%. The frequent occurrences of NOE events over the BTH region are centered at the high-altitude station Zhangjiakou and the eastern coastal areas of the Northwest Pacific Ocean. The main mechanism of NOE events is identified with the horizontal transport in spring, the vertical transport in summer, vertical transport, horizontal and stratospheric intrusion in autumn, and stratospheric intrusion in winter. This study based on the NOE observation in regional environment over 2018–2023 provides a scientific support for the regional ozone pollution prevention.