Environment officials should focus on replacing the city's oldest and most polluting vehicles first to make the best use of their resources, a green group said.
Friends of the Earth questioned if officials should include the Euro III commercial diesel trucks - some of which are 10 years old or less - under the government's HK$10 billion proposal to phase out about 88,000 diesel trucks by 2019.
The proposal will also set a 15-year life span for all new trucks.
The environmental group said it was relatively less cost effective to get the Euro III trucks off the roads at this stage, compared with the benefit of removing the pre-Euro and Euro I trucks that are not only the oldest vehicles but also those with the poorest emission standards.
Estimates by the group show the money spent removing about 28,000 Euro III trucks might be around HK$3.7 billion, while their share of contributions to roadside pollution might be between 11 and 21 per cent.
Yet, owners of these vehicles, given their higher residual values than older trucks, are entitled to a replacement allowance equal to 30 per cent of a new truck.
Meanwhile, owners of the dirtiest trucks, despite accounting for 26 to 50 per cent of the roadside pollution, would only receive up to 21 per cent of the replacement cost.
Friends of the Earth estimated the bill would be about HK$2.6 billion for about 31,000 vehicles. Environmental affairs manager Melonie Chau Yuet-cheung said there were problems with the existing design of the proposal, which she said officials had not thought through. "Could we just deal with the Euro IIIs at a later stage and give more resources to the pre-Euro or Euro I?" she said.
Yuen Cheung-fung from the Motor Transport Workers General Union said the union wanted the government to offer different allowances for different types of trucks. He said the rate should be between 40 and 60 per cent.
He agreed it would be "easier" for officials to make a deal with operators if they left Euro III vehicles out of the plan and lifted allowances for older trucks.
An Environmental Protection Department spokeswoman said it was still consulting stakeholders on the scheme, which it said was based on vehicles' ages.
The department released more accurate estimates that showed if all 18,000 trucks less than 10 years old - most being Euro III - were removed, it would cost about HK$2.8 billion. The cost of phasing out 35,000 preEuro or Euro I trucks more than 16 years old was HK$3.2 billion.
On Monday, the city had one of its worst days for pollution, with readings in Central hitting 210. At 5pm yesterday, the reading was still at a "very high" level of more than 150 in Central.
The EPD expects the situation to improve today as the wind picks up.