Intercomparison of O₃ formation and radical chemistry in the past decade at a suburban site in Hong Kong

Hong Kong, as one of the densely populated metropolises in East Asia, has been suffering from severe photochemical smog in the past decades, though the observed nitrogen oxides (NOx) and total volatile organic compounds (TVOCs) were significantly reduced. This study, based on the observation data in the autumns of 2007, 2013 and 2016, investigated the photochemical ozone (O3) formation and radical chemistry during the three sampling periods in Hong Kong with the aid of a photochemical box model incorporating the Master Chemical Mechanism (PBM–MCM). While the simulated locally produced O3 remained unchanged (p=0.73) from 2007 to 2013, the observed O3 increased (p < 0.05) at a rate of 1.78 ppbv yr−1 driven by the rise in regionally transported O3 (1.77±0.04 ppbv yr−1). Both the observed and locally produced O3 decreased (p < 0.05) from the VOC sampling days in 2013 to those in 2016 at a rate of -5.31±0.07 and -5.52±0.05 ppbv yr−1, respectively. However, a leveling-off (p=0.32) was simulated for the regionally transported O3 during 2013–2016. The mitigation of autumn O3 pollution in this region was further confirmed by the continuous monitoring data, which have never been reported. Benefiting from the air pollution control measures taken in Hong Kong, the local O3 production rate decreased remarkably (p < 0.05) from 2007 to 2016, along with the lowering of the recycling rate of the hydroxyl radical (OH). Specifically, VOCs emitted from the source of liquefied petroleum gas (LPG) usage and gasoline evaporation decreased in this decade at a rate of -2.61±0.03 ppbv yr−1, leading to a reduction of the O3 production rate from 0.51±0.11 ppbv h−1 in 2007 to 0.10±0.02 ppbv h−1 in 2016. In addition, solvent usage made decreasing contributions to both VOCs (rate =-2.29±0.03 ppbv yr−1) and local O3 production rate (1.22±0.17 and 0.14±0.05 ppbv h−1 in 2007 and 2016, respectively) in the same period. All the rates reported here were for the VOC sampling days in the three sampling campaigns. It is noteworthy that meteorological changes also play important roles in the inter-annual variations in the observed O3 and the simulated O3 production rates. Evaluations with more data in longer periods are therefore recommended. The analyses on the decadal changes of the local and regional photochemistry in Hong Kong in this study may be a reference for combating China's nationwide O3 pollution in near future.