The global motor vehicle population has grown very rapidly in the past half century and is expected to continue to grow rapidly for the next several decades, especially in developing countries. As a result, vehicles are a major source of urban air pollution in many cities and are the fastest-growing source of greenhouse emissions. Strategies exist to reduce both problems, but many countries emphasize one over the other rather than pursuing strategies that reduce both concerns.

Hydroxyl radicals are often called the "detergent" of the atmosphere because they control the atmosphere's capacity to cleanse itself of pollutants. In this article the researchers show that the reaction of electronically excited nitrogen dioxide with water can be an important source of tropospheric hydroxyl radicals.

More than a hundred people live in cities that fail to meet international standards for air quality. Efforts to improve conditions in these urban areas have usually focussed on reducing emissions of reactive hydrocarbons, nitrogen oxide free radicals and primary and secondary sources of particulate matter.

A case study is done in Salem City, India for ambient air quality for major air pollutants for a period of one year from Oct 2003 to Sep 2004 at the six selected stations. (2007)

Chennai Metropolitan Area extends over 1180 sq. km and has a population of more than 8.5 million. Rapid increase in urbanization with vehicle congestion has increased menacingly on the roads of Chennai. As a result of this, gaseous pollutants and respirable and suspended particulate matter pollutants are continuously increasing in the ambient air of Chennai city. These pollutants have been assessed using high volume sampler at residential and traffic intersection sampling stations in Chennai City.