The three stations which showed improvement were in Guangzhou and Foshan . The other 13, including the three stations in Hong Kong - in Tsuen Wan, at Tung Chung near the airport on Lantau and at Tap Mun, or Grass Island, in Mirs Bay off the north coast of Sai Kung - all had worse readings than a year earlier.

The government source ascribed the improvement in air quality in Foshan to the relocation of highly polluting ceramics factories. The worst air quality was recorded in March because winds were too light to dispel pollution, the source said. Coastal areas had better air quality because summer ocean winds dispersed pollutants. Regional air quality is graded from 1 (the best) to 5 (the worst).

Respirable suspended particulates were a bigger problem than the three other pollutants measured - ozone, nitrogen dioxide and sulfur dioxide.

In parts of Guangzhou, Foshan and Huizhou , the level of respirable suspended particles of a diameter of 10 microns or more (known as PM10) exceeded safe levels between 20 and 29 per cent of the time, or for 25 to 47 days in six months.

The monitoring network was set up by Guangdong's Environmental Protection Bureau and the then Environment, Transport and Works Bureau in Hong Kong three years ago.

Despite a lack of significant improvement in air quality since then, the source said the government was confident of achieving some of the 2010 targets jointly set for emissions reductions. As well as factories relocating, vehicles in some Guangdong cities had switched to cleaner fuels, the source said.

Alexis Lau Kai-hon, an atmospheric scientist at the Hong Kong University of Science and Technology, said his research showed there had been hardly any improvement in air quality in the region since 2004, although there was no clear sign either that it was getting worse.

He agreed that joint monitoring was beneficial, but urged the mainland authorities to release data on a daily basis instead of once every six months to allow for better research.

Hahn Chu Hon-keung, environmental affairs manager for campaign group Friends of the Earth, said the closure of some factories on the mainland and the relocation of others might have helped improve air quality in some places, but it was time governments stepped up efforts to reduce roadside pollution.

'One major source for PM10 is vehicular emissions. There is plenty of room for mainland authorities to reduce roadside pollution,' he said.

'For Hong Kong, power plants' emissions reductions have had an effect, but there's a need to replace old heavy diesel vehicles more quickly.'