New York State is expanding its Clean School Bus programme to install diesel oxidation catalysts (DOCs) in up to twice as many buses as had previously been planned.

Between 1,500 and 2,000 buses will now get the environmentally-friendly treatment, compared to the initial number of up to 1,000 commissioned in the $6 million programme.

DOCs are canisters which are installed on the exhaust pipe. They contain substrates coated with an active catalyst layer, usually containing a precious metal such as platinum or palladium.

As exhaust gases flow through the canister, the catalyst, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles oxidize, thereby reducing emissions.

The DOC reduces particulate matter emissions by 40 per cent, carbon monoxide by 70 per cent and unburned hydrocarbons by 85 per cent when used with ultra low sulphur diesel fuel.

The New York Power Authority (NYPA), which is supporting the initiative, will begin installing the DOCs next month, with the additional 500-1,000 buses to be fitted later in the year.

With the assistance of the city's education department, the NYPA says it is helping to significantly cut diesel emissions that can contribute to asthma and other respiratory illnesses.

The programme already provides ultra low sulphur diesel fuel for 2,800 school buses in New York City, more than two thirds of the school buses in operation.