ABSTRACT
A variety of different philosophies has been used to control air pollution. Historically, the philosophy in the UK has been based on emission standards, the assumption being that if one sets appropriate maximum emission concentrations then ambient concentration targets can be achieved. In Europe and the US, on the other hand, legislation has been framed in terms of Air Quality Standards (AQS) — potential new emitters have to show that their emissions will not cause AQS to be exceeded. Tailpipe emission standards specify the maximum amount of pollutants allowed in exhaust gases discharged from the engine. European Union standards for petrol-engined vehicles are divided by engine capacity categories into small (less than 1.4 liters), medium (1.4—2 liters) and large (over 2 liters). The national Tier 1 standards were phased in from 1994 to 1997. Tier 1 standards cover vehicles up to 8500 pounds gross vehicular weight rating. The Tier 2 standards bring significant emission reductions relative to Tier 1.
