Investigation of the multi-year trend of surface ozone and ozone-precursor relationship in Hong Kong

As a highly urbanized city in East Asia, Hong Kong has faced severe photochemical ozone (O-3) pollution for years. A comprehensive understanding of pollution characteristics is crucial for effective mitigation of the O-3 problem. This study systematically investigated the multi-year trends of surface O-3 over the last decade using measurements from 11 selected Air Quality Monitoring Stations (AQMS). The analysis reveals upward trends in the annual average and daily maximum 8-h average (DMA8) O-3 from 2011 to 2022, with rates of 0.34 ppb year(-1) (p < 0.05) and 0.22 ppb year(-1) (p = 0.19), respectively. Though upward trends of DMA8 O-3 were observed at urban stations (0.10-0.88 ppb year(-1)), the significant decreases in NO2 levels at all stations (-0.18 to -1.75 ppb year(-1)) led to a decline of total oxidants (O-x, O-3+NO2). In addition to the daytime O-3 peak increase, a clear nighttime O-3 peak was present with a significant increasing trend, which was associated with the regional transport of O-3-rich plumes and weakened NO titration effect. The relative contributions of local production and regional transport were investigated, and results indicated that local control efforts have predictably alleviated the locally produced O-x, while the regional background O-x still contributes significantly but appears to decline. Furthermore, an observation-based model was employed to examine the evolution of O-3-precursors relationships. The results indicate that O-3 formation was still VOC-limited in Hong Kong, and the suppressive effect of NOx on O-3 formation has decreased in recent years. In terms of abatement strategies, apart from aromatics, we emphasize the significance of carbonyl compounds due to their high reactivity to O-3 production.