Environmental context. Regional atmospheric contaminants from both anthropogenic and natural events (industrial activities, biomass burning, dust events) can have large impacts on the aerosol properties of distant downwind sites. Data showing the influence of regional sources on air quality in Hong Hong are presented. In particular, the changes in aerosol properties (e.g. mass concentration and particle size distribution), and their effects on photochemical ozone formation, is discussed.

Abstract. Variations of PM₁₀ concentration and particle size distribution owing to the influence of industrial activities in mainland China, biomass burning in South East Asia and dust storms in north-west China, as well as the aerosol impact on surface ozone concentration, have been studied by a combination of analysis of air mass origin (by back-trajectory calculation), remote sensing techniques, regional chemical transport and photochemical box models. The PM₁₀ concentrations in Hong Kong were reduced by 24–57% owing to the industrial downtime during the Chinese New Year period. A photochemical box model coupled with a radiative transfer model has been employed to predict surface maximum ozone concentrations during high aerosol loading days. Increase of aerosol optical depth can lead to 7–32% reduction of surface maximum ozone concentration. Our results emphasise the need for regional cooperation in tackling the local air pollution of Hong Kong.