Emissions from transport are the main contributor to air pollution. This indicator analyses the transport emissions of NOx, NMVOCs and particulate matter (PM10), with data provided annually starting from 1990. The indicator is an index to year 2000 (index 2000 = 100). Emissions, expressed in Gg (1000 tonnes), are officially reported under the UNECE's Convention on Long-Range Transboundary Air Pollution (CLRTAP) to the EMEP Programme (Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air pollutants in Europe). The pollutants can be grouped into acidifying substances, particulates and ozone precursors. NOx, the leading pollutant from transport, contributes to acidification, formation of ground level ozone and particulate formation. No specific emission reduction target or objective exists for transport-related emissions. However, emission ceiling targets are specified in both the EU National Emission Ceilings Directive (NECD) and the Gothenburg protocol under the CLRTAP (UNECE 1999). Following the Directive on ambient air quality and cleaner air for Europe (2008/50/EC), a number of limit values (e.g. hourly limit values) have been set for the atmospheric concentrations of main pollutants, including SOx, NOx, air borne particulate matter (PM10, PM2.5), lead, CO, benzene and ozone. (i) More information can be found in the European Union emission inventory reports to the United Nations Economic Commission for Europe (UNECE) Convention on Long-range Transboundary Air Pollution (CLRTAP). Such reports are published annually and compiled by the European Environment Agency (EEA) in cooperation with the EU Member States.