Air pollution in the Seoul metropolitan area (SMA) is 1.3-1.4 times higher than non-SMA regions in terms of nitrogen dioxide (NO2) and particulate matter (PM10). Vehicles are responsible for more than 65% of total PM10 emissions that cause adverse impacts on health and amenity. In order to improve the air quality of the SMA and bring it up to the standards of cities in the developed world within 10 years, the Korean government has promoted a “Master Plan for the Metropolitan Air Quality Management” based on the “Special Measures for Metropolitan Air Quality Improvement” and “Special Act on Metropolitan Air Quality Improvement”. As a part of the Master Plan, an emission reduction program for in-use diesel vehicles on road through enforcement of stricter emission standards to specific diesel vehicles running in the SMA has been implemened since 2005.