Long-term drivers of increasing diurnal NO₂ difference in representative cities of the Pearl River Delta

The increasing diurnal difference of nitrogen dioxide (NO₂) has a non-negligible promoting effect on the formation of next-day ozone and fine particulate matter. Therefore, a thorough analysis of the driving mechanisms behind this diurnal variation is crucial for achieving precise pollution control. Here, we investigate the long-term evolution of NO₂ diurnal differences and their meteorological contributions in the Pearl River Delta (PRD) from 2014 to 2023. Three primary clusters are identified using K-means clustering: Cluster 1 represents regional background concentrations, Cluster 2 shows significant nighttime accumulation with a large diurnal difference, and Cluster 3 shows all-day high NO₂ concentrations. The increasing trend in diurnal differences was mainly driven by the rising number of days classified as Clusters 2 and 1. Anthropogenic emission contributions to NO₂ concentrations in the PRD decreased by 34.2 % from 2014 to 2023, reflecting the effectiveness of emission reduction measures. However, meteorological impacts vary significantly across clusters. In Cluster 1, meteorological conditions reduced NO₂ concentrations by 4.4-11.0 μg/m³; in Cluster 2, contributions were positive in central cities but negative in surrounding cities; and in Cluster 3, conditions increased NO₂ concentrations by 9.9-22.0 μg/m³. Boundary layer height, temperature, and atmospheric pressure were identified as key meteorological drivers of NO₂ diurnal variation, with their contributions varying significantly across clusters, between pollution and non-pollution days, and at different times of the day. This study highlights the critical role of specific meteorological patterns in amplifying NO₂ diurnal variation.