Combustion processes emit a mixture of nitrogen oxides (NOX), primarily nitric oxide (NO) which is quickly oxidised in the atmosphere to nitrogen dioxide (NO2). Nitrogen dioxide has a variety of environmental and health impacts. It is a respiratory irritant which may exacerbate asthma and possibly increase susceptibility to infections. In the presence of sunlight, it reacts with hydrocarbons to produce photochemical pollutants such as ozone. NO2 can be further oxidised in air to acidic gases, which contribute towards the generation of acid rain.
(Definition taken from Defrea Glossary http://uk-air.defra.gov.uk/air-pollution/glossary.php?glossary_id=40#40 )