Air quality in the Pearl River Delta improved last year, according to the latest regional air quality report, but concerns are mounting about the deterioration of roadside air in Hong Kong. The biggest improvement was in sulphur dioxide concentrations, which fell by an average of 25 per cent from 2011 levels. Respirable suspended particles - tiny specks of pollutants that can penetrate the lungs - fell 13 per cent year on year, while ozone dropped by 7 per cent and nitrogen dioxide by 5 per cent. Environment officials in Hong Kong attributed the decrease to "favourable meteorological conditions" as well as emission reductions. Last year Guangdong expanded the supply of cleaner petrol and boosted a clean-up at power plants and cement kilns, and in Hong Kong vehicle emission standards were tightened, officials said. According to the Hong Kong Observatory, last year there was 20 per cent less rainfall than normal, and 15 per cent less sunshine. The region also experienced its worst storm in years, when tropical cyclone Vicente swept into the city last July. Professor Wang Tao, an air pollution expert at Polytechnic University, said the news came as little surprise as pollution had been on a declining trend. He said the fall in sulphur levels could be due largely to a clean-up at mainland power plants. "It is very efficient to remove sulphur by as much as 90 per cent if scrubbers are installed," he said. Officials said accumulated improvements in air quality ranged from 17 to 62 per cent since 2006, the first full year air data became available under a cross-border monitoring network comprising 16 stations on building roofs. Despite the improvement, a local clean air advocacy group remained deeply worried about air quality at street level. "The continuing improvement to the regional air quality is in stark contrast to Hong Kong's deteriorating roadside air pollution, in particular nitrogen dioxide," said Kwong Sum-yin, of the Clean Air Network. The continuing improvement to the regional air quality is in stark contrast to Hong Kong's deteriorating roadside air pollution, in particular nitrogen dioxide Kwong Sum-yin of the Clean Air Network Earlier this month, Hong Kong recorded one of its worst roadside quality readings, with the air pollution index soaring to more than 210 in Central. Kwong urged the government to speed up a phase-out of dirty diesel trucks. Some HK$10 billion has been earmarked to compensate truck operators under a plan to remove up to 88,000 trucks from the streets by 2019.