Nitrogen oxide emissions
The term "nitrogen oxides" (NOx) embraces nitrogen dioxide (NO2) and nitrogen monoxide (NO). Nitrogen oxides form in the combustion of transport and heating/process fuels, especially when combustion is at high temperatures. Road transport is the main source. NOx is an important precursor in the formation of acid rain, secondary aerosols and - in combination with volatile organic compounds - photo-oxidants (ozone/summer smog).
Emissions are determined by multiplying the annual human activity rates by pollutant-specific emission factors. Data are gathered by means of surveys, modelling and calculations. As NO oxidises quickly to NO2, total emissions are stated as nitrogen dioxide (NO2) equivalents.