Many of Hong Kong's oldest buses - the most visible source of the roadside pollution that still plagues the city even as regional air quality improves - will disappear from the roads this year.
Kowloon Motor Bus, which runs the biggest fleet, will retire 288 buses introduced before Hong Kong adopted European emission standards for diesel buses in 1992 as part of a big upgrade.
But the operator has reservations over another government proposal to improve the air: fitting its fleet with selective catalytic reduction (SCR) equipment to help reduce nitrogen oxide emissions.
It says there is not enough space in the older buses to accommodate the device and that it might compromise the safety and efficiency of pollution control devices already installed.
The Environmental Protection Department last week reported a significant improvement in air quality in the Pearl River Delta but said roadside pollution in Hong Kong was getting worse.
KMB said the 288 buses being retired as they reached the end of their lives had been fitted with equipment to meet the lowest Euro I standards for suspended particles - one of the emissions from vehicle exhausts. But it did not say whether these buses met the same standards for other pollutants.
A Euro I model can spew out 18 times the amount of nitrogen dioxide emitted by the Euro V model that the company began to introduce in October 2008.
Edwin Lau Che-feng, director of Friends of the Earth, questioned why KMB gave only selective information about emissions. 'It is misleading to report only particulate matter as if the buses have fully met the European emission standards,' he said.
In 2008, Euro I buses accounted for a third of the suspended particles and nitrogen dioxide emitted by the whole bus fleet in 2008.
The other two franchised bus firms, New World First Bus and sister company CityBus, also operate about 440 pre-Euro and Euro I buses in a 1,600-strong fleet. They have pledged to phase out the pre-Euro models by no later than 2012.
KMB has 3,906 buses of which about a third are pre-Euro and Euro I models, although they have all been upgraded to reduce emissions of particulates.
Kane Shum Yuet-hung, KMB's principal engineer, said the company would introduce up to 300 new Euro IV and V models this year to replace the oldest buses.
Shum said the fleet upgrade, the use of cleaner diesel and driving system adjustments had brought average particulate emission levels down by nearly 86 per cent from the 1992 levels. 'I will say our emissions are not as bad as in the past but there is always room for further improvement,' he said.
While KMB is still in talks with the government on whether it is possible to replace older vehicles earlier and a pilot trial to test SCR equipment, Shum said priority should be given to the reliability and efficiency of the bus fleet.
Shum also said the company was closely watching the development of the hybrid diesel-electric double-decker bus in London.
He believed it would not be ready for Hong Kong in two to three years' time. The existing hybrid model, which was generally smaller than buses running in Hong Kong, did not have air conditioning.