Nighttime ozone excess days and the associated meteorological conditions in the North China Plain from 2015 to 2022

We examined the spatial and temporal variations of nighttime ozone (O3) excess days (NOEDs) in the North China Plain (NCP) during 2015-2022 by using the observed O3 concentrations from the China National Environmental Monitoring Center network. The NOEDs were defined as those with any 1-hour average O3 concentration at night (20:00 to 06:00 local time) exceeding 200 μg m-3. The observations showed that the annual frequencies of NOEDs averaged over the 57 cities in the NCP were 1.56, 1.72, 6.23, 6.53, 8.84, 5.93, 5.07, and 6.86 days in 2015, 2016, 2017, 2018, 2019, 2020, 2021, and 2022, respectively, with a linear increasing trend of 0.66 d y-1. Using the K-means clustering method, two typical regional NOEDs types in the NCP were identified. Type1 had the highest frequencies of NOEDs in the southwest of NCP, while Type2 had the highest frequencies in the central NCP region. These two types contributed to 53.5% and 46.5% of the regional NOEDs in NCP, respectively. Compared with the regional non-NOEDs (with nighttime O3 did not exceed but daytime O3 exceeded O3 air quality standard), both Type1 and Type2 NOEDs generally had higher temperature, lower relative humidity, and higher planetary boundary height in both afternoon and night, favoring the production and accumulation of O3 in the afternoon as well as the persistent high O3 at night. Corresponding weather patterns were also identified. These results have important implications for emission control strategies during NOEDs. Emission control measures can be applied once the dominant circulation patterns have been predicted.