In addition to the mixture of hydrocarbons, all fuels contain some impurities (with the possible exception of hydrogen
obtained from a fuel cell, and the lightest hydrocarbon fuels such as methane which are available with very low levels of
impurities). Sulphur is oxidised mostly to sulphur dioxide (SO2) on combustion, and sometimes to sulphate which can
assist in the nucleation of particles in the exhaust. Several other impurities such as vanadium in oil do not burn or have
combustion products that have a low vapour pressure and so contribute further to particle formation.