Under a GHG emissions trading scheme, permits to emit can initially either be given away for free, usually under a ‘grandfathering’ approach based on past emissions in a base year or an ‘updating’ approach based on the more recent emissions. The alternative is to auction permits in an initial market offering.

The release of substances (e.g. CO, NOx, PM, greenhouse gases incl. CO2, VOCs) into the atmosphere. (see Vehicle Emissions)

Refers to the air quality in a particular region or area. Most countries set vehicle emission and fuel quality requirements to meet ambient air quality targets. In the 1990’s the US established the Clean Air Act, which set Ambient Air Quality Targets covering the entire country, although the states do much of the work to carry out the Act. EPA calls these pollutants criteria air pollutants because the agency has regulated them by first developing health-based criteria (science-based guidelines) as the basis for setting permissible levels. One set of limits (primary standard) protects health; another set of limits (secondary standard) is intended to prevent environmental and property damage. A geographic area that meets or does better than the primary standard is called an attainment area; areas that don't meet the primary standard are called nonattainment areas.

In 1996 the European Union established a Framework Directive 96/62/EC on ambient air quality assessment and management. This Directive covers the revision of previously existing legislation and the introduction of new air quality standards for previously unregulated air pollutants, setting the timetable for the development of daughter directives on a range of pollutants. The list of atmospheric pollutants covered includes sulphur dioxide, nitrogen dioxide, particulate matter, lead and ozone – pollutants governed by already existing ambient air quality objectives- and benzene, carbon monoxide, poly-aromatic hydrocarbons, cadmium, arsenic, nickel and mercury.