Source: EPA.
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 industrial activities, commercial energy sources and general climatic conditions (colder temperatures are linked with more NOx emissions).