Does CNG buses reduce the poly aromatic hydrocarbons?
Yes. Buses typically run on diesel fuel, which has long-chain hydrocarbons (chains of maybe 15-30 carbon atoms). Diesel may also contain trace amounts of aromatic compounds (like benzene, toluene, xylene or even more complex aromatics). Compressed natural gas (CNG) consists mostly of very short-chain hydrocarbons (ethane, propane, butane), and almost no aromatic hydrocarbons. Combustion of diesel can easily create small quantities of polyaromatic hydrocarbons (PAH), which are molecules consisting of two or more aromatic compounds joined together (like benz-a-pyrene). It is much harder (although not impossible) for combustion of CNG to form PAHs. All this *is* important because PAHs are air pollutants and suspected cancer-causing chemicals.
