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 ( NOsubx/sub ) 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 ( Osub3/sub ) 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 Osub3/sub remained unchanged ( p=0.73 ) from 2007 to 2013, the observed Osub3/sub increased ( p ?1/sup driven by the rise in regionally transported Osub3/sub ( 1.77±0.04 ppbv?yr sup?1/sup ). Both the observed and locally produced Osub3/sub decreased ( p ?1/sup , respectively. However, a leveling-off ( p=0.32 ) was simulated for the regionally transported Osub3/sub during 2013–2016. The mitigation of autumn Osub3/sub 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 Osub3/sub production rate decreased remarkably ( p ?1/sup , leading to a reduction of the Osub3/sub production rate from 0.51±0.11 ppbv?h sup?1/sup in 2007 to 0.10±0.02 ppbv?h sup?1/sup in 2016. In addition, solvent usage made decreasing contributions to both VOCs (rate = - 2.29 ± 0.03 ppbv?yr sup?1/sup ) and local Osub3/sub production rate ( 1.22±0.17 and 0.14±0.05 ppbv?h sup?1/sup 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 Osub3/sub and the simulated Osub3/sub 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 Osub3/sub pollution in near future.
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