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Transportation in the United States emitted *** million tons of nitrogen oxides (NOx) in 2023. Since 2006, NOx emissions from transportation have fallen by more than ** percent. Nitrogen oxides are a group of gases that include nitrogen dioxide. The U.S. Clean Air Act aims to improve air quality by limiting these harmful emissions.

Road vehicle emissions in the U.S.

On-road vehicles contributed to ** percent of transportation-related NOx emissions in the U.S. in 2023. These vehicles – which include light-duty passenger vehicles, heavy-duty trucks and buses, and motorcycles – are also major contributors to fine particulate matter pollution in the North American country. Tiny particles like PM2.5 can be harmful to human health in high concentrations.

Passenger car emissions

In addition to being a significant source of air pollutants, passenger cars are also major greenhouse gas emitters. Passenger cars are responsible for more than ** percent of transportation sector greenhouse gas emissions in the United States.