Beginning in October 1993; CARB implemented a 10 volume percent limit on the aromatics content of vehicular diesel fuel as a means to achieve further reductions in diesel engine particulate matter (PM) and nitrous oxide (NOx) emissions. In California, vehicular diesel fuel is the fuel which must be used in both on-highway vehicles and in non-road use (e.g. agriculture and construction equipment). Locomotive and marine vehicles are not included in the definition of vehicular use.br /br /
Subsequent to the implementation of the 10 volume percent aromatics limit, CARB adopted a provision for alternative low aromatics diesel (ALAD) formulations, whereby a refiner/producer could apply for certification to produce an alternative diesel fuel formulation if they could demonstrate through a specified testing protocol that their formulation provided equivalent or better emissions performance when compared to a 10 volume percent aromatics reference fuel. Most diesel fuel in California today is produced to a certified ALAD formula, with typical fuels having about 160 ppm sulfur, 21% aromatics, and a 53 cetane number.