Despite the alleviation of particulate matter (PM), the ambient ozone (O3) concentration is continuously increasing in Hunan province where the investigation of O3 pollution has been rarely reported. Accordingly, the spatio-temporal evolution of O3 pollution was first analyzed based on hourly air quality data observed by national monitoring stations from 2015 to 2020 over 14 cities in Hunan province. Afterwards, the combination of meteorological data from the European Center for Medium-range Weather Forecast (ECMWF) and the generalized additive model (GAM) was applied to investigate the driving factors of the O3 long-term trend during this period. The results presented obvious diurnal, monthly, and seasonal characteristics of O3 variations. High O3 concentrations occurred in May and September monthly, and the peak O3 season was autumn. Furthermore, the 90th percentile O3 increased at a rate of 4.7 μg·(m3·a)-1 temporally, and high O3 values mainly occurred in the north-eastern region spatially, in contrast to the low O3 values in the western region. The modeling results indicated that the increase in O3 was mainly ascribed to precursor emissions. Furthermore, meteorology promoted a rise in O3 with the impact magnitude of 1 μg·(m3·a)-1. Remarkably, meteorology accelerated the O3 increases in spring, summer, and the eastern region, whereas it restrained increases in autumn, winter, and the northwest. The effect of meteorology on PM10 was different from O3 during this period. Overall, this study highlighted the importance of meteorological impacts when regulating emission reduction measures for O3 abatement. It required greater effort regarding O3 mitigation to offset the side-effect from meteorology in meteorology-sensitive seasons and regions. Additionally, the regional corporation is indispensable to reduce O3 transportation from upwind.
Keywords: driving factor; long-term trend; meteorological adjustment; ozone (O3); spatial-temporal evolution.