The HCRA analysis finds that natural gas reduces emissions of fine particulates, those smaller than 2.5 microns. But natural gas may generate more ultra fine particles than diesel. Several studies indicate that ultrafine particles may have an even more dramatic impact on health than those in the fine category. The study finds that because natural gas is primarily methane, a relatively simple molecule, it combusts more completely than many fuels, producing fewer emissions of several types, particularly NOx, an important contributor to ground level ozone and the formation of fine particulates. The advantages of diesel come from its efficiency.
Diesel engines convert a large fraction of the available energy into useable work.
As a result, diesel engines consume less fuel overall than if they were converted to natural gas. The HCRA study suggests that converting heavy trucks and buses from diesel to natural gas would increase emissions of C02, a significant greenhouse gas. In addition, the study finds that more widespread use of natural gas would likely increase the escape of methane into the atmosphere. Methane is approximately 20 times more potent as a greenhouse gas than CO2. The study finds that European regulators seem to be favoring diesel fuel as part of their effort to comply with the Kyoto agreements to stabilize CO2 and other greenhouse gas emissions.