Hong Kong's Environmental Protection Department says it recorded very high Air Pollution Indices of 102 and 132 at the Yuen Long and Tung Chung stations, respectively, on October 5 due to ozone concentrations and expect rates to remain high for the coming days.

Under the combined effects of a continental airstream and Typhoon Krosa located in the western Pacific Ocean, the weather over South China region was sunny and hot. Pollutants are often trapped under the light wind, and the sunny and hot weather conditions favored the photochemical formation of ozone in the region, resulting in elevated ozone levels.

With elevated ozone concentrations, the nitrogen oxide emitted from motor vehicles will be oxidized to form nitrogen dioxide quickly. Due to higher nitrogen dioxide levels and poor dispersion under the light wind, the roadside API readings are also expected to increase steadily and reach the very high range later.

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