Source: EPA.
The most important transport emissions are related to carbon monoxide (CO), Volatile Organic Compounds (VOC), and nitrogen oxides (NOx). Still, transportation plays a marginal role in particulates and sulfur oxide emissions. Due to better engine technology and more stringent standards, the amount of pollutants released on highways has declined substantially. Contributions patterns are fairly consistent among advanced economies, 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). The environmental profile of transportation, particularly regarding the emission of harmful pollutants, has improved substantially even if the number of vehicles circulating has increased.