Air pollution at street level has soared in the past four years while improving at the city's rooftops, calling into question assertions that Hong Kong's chronic air-quality problems have a regional more than local source.

Roadside monitoring stations recorded more than six times as many periods of health-threatening pollution levels in the first half of this year than in the same period in 2005.

The stations, in Central, Causeway Bay and Mong Kok, registered 1,066 hours - the equivalent of more than 44 days - during which the air pollution index rose above the "very high" 100 level. (A 100 level prompts a health warning to those with heart and respiratory conditions.)

But above street level, the number of such hours recorded by general monitoring stations - which track sulphur dioxide, nitrogen dioxide, ozone and respirable suspended particles - fell by more than half, to 56, according to an analysis of Environmental Protection Department data by the South China Morning Post (SEHK: 0583, announcements, news) .

There are 13 general monitoring stations across the city, located on building rooftops 11 to 25 metres above ground.

Scientists consulted by the Post say the improvement in air quality high above ground is due largely to Guangdong's efforts to install sulphur scrubbers in power plants and the closure of many factories because of the recession.

The worsening street-level air is likely the product of cars and trucks on the city's congested streets, the experts say.

"It is undeniably a local pollution problem at street level. All we need is a lot more and urgent measures to address vehicular pollution to protect public health," said Alexis Lau Kai-hon, an atmospheric scientist from the University of Science and Technology.

But the Environmental Protection Department blamed regional air pollution for deteriorating street-level air quality. It said three key pollutants emitted by motor vehicles had in fact fallen, while regionally generated ozone was combining with other pollutants to form nitrogen dioxide by the road, pushing up the figures.

The department said changes in pollutant concentration readings were more reliable than the index in reflecting air quality trends.

It credited emission-control measures introduced in the past decade for bringing down levels of sulphur dioxide, suspended particles and nitrogen oxides by about 20 per cent between 1999 and last year.

But another roadside pollutant, nitrogen dioxide, rose 9 per cent in the first six months of this year and has remained at 1999 levels.

"The increase is mainly due to the rise in ambient background ozone concentration, which has aggravated the conversion of nitrogen oxides from motor vehicles to nitrogen dioxide," a spokesman said.

Levels of ozone - the major component of smog - have risen 18 per cent in the first six months of the year and in recent years have been at higher levels than in 1999. Ozone can react with nitrogen oxides to form nitrogen dioxide.

But the government's explanation was not accepted unquestionably by scientist

"Without a high level of roadside nitrogen oxides from vehicles, the ozone would not have caused more serious secondary pollution of nitrogen dioxide," Professor Lau said.

Chan Chak-keung, a professor in chemical and biomolecular engineering at HKUST, said controlling ozone-inducing volatile organic compounds coming from a wide spectrum of sources such as vehicles, factories and products like paints was never an easy task. There are nearly 636,000 cars, buses and trucks on the road in Hong Kong, up 5.7 per cent from the end of 2005.

Roadside pollution is also linked to poor dispersion of pollutants, caused by an urban design that favours high-rise towers even in congested streets