There are less particles from CNG, sure. But it is still a fossil fuel that adds CO2. So not a huge advantage compared to dirsel.Oh, you use diesel buses in Norway? Ouch. The cities I know in the U.S. switch to CNG years ago, and I had assumed as much in Norway. Diesel would change the equation, as you point out.
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Maine transit agency spurs wider adoption of alt fuels - Metro Magazine
Here in Portland we're even shifting over the school bus fleet to CNG:
Portland transitions to compressed natural gas school buses to save money, lungs BDN Maine
unless the CNG is made from biomass of course. But that is not very common.