The province-wide concentrations of nitrogen dioxide and respirable suspended particulates also rose slightly.

Overall emissions of sulfur dioxide fell 2.1 per cent, the first such drop in at least a decade.

The Guangdong Environmental Protection Bureau admits in the report that 'the air quality in some urban regions has declined a bit compared with the year before', while saying it was 'still good in general'.

Green groups said the figures showed the province would have difficulty achieving its targets, agreed with Hong Kong, to slash levels of the four key pollutants by 2010. But scientists said Guangdong's air quality was still acceptable.

Hong Kong and Guangdong have agreed to cut emissions of sulfur dioxide by 40 per cent, nitrogen oxides by 20 per cent, and respirable suspended particulates and volatile organic compounds by 50 per cent from 1997 levels by 2010.

Carlos Lo Wing-hung, a Polytechnic University expert in environmental governance in the delta region, said that while the Guangdong government was working on the problem, 'apparently economic development is still a bigger concern'.

Albert Lai Kwong-tak, of the Conservancy Association, said the fact that air quality fell despite a drop in emissions meant nature could not clean up the volume of pollutants being released.

'There is a long, long way to go,' he said.

Greenpeace campaigner Edward Chan Yue-fai said the cost of cutting sulfur dioxide emissions might be too high for many factories to bear.

A Hong Kong government spokesman said the administration and the Guangdong government were determined to reach the 2010 targets.