Estimated Air Pollutants Emitted by Highway Transportation in the
United States, 1970-2014
The most important transport emissions are related to carbon monoxide,
Volatile Organic Compounds (VOC) and nitrogen oxides, while transportation
plays a marginal role for particulates and sulfur oxides emissions.
Due to better engine technology and more stringent standards, the amount
of pollutants released has declined substantially. Contributions patterns
are fairly consistent among different developed countries, but differences
exist. They are attributed to modal choice, the composition of
commercial energy sources and general climatic conditions (colder
temperatures are linked with more NOx emissions).