Over half of all carbon monoxide (CO) emissions in 2002 were from highway vehicles; by 2016 that fell to 30%. The share of nitrogen oxides (NOx) emissions from highway vehicles declined from 43% of all NOx emissions in 2002 to 34% in 2016. The highway share of volatile organic compound (VOC) emissions declined by 9% during this same period. Highway vehicles contributed less than 3% of all particulate matter (PM) emissions.
Highway Share of All Pollutant Emissions, 2002-2016
Note: Particulate matter emissions include both fine particle matter less than 10 microns (PM-10) and fine particle matter less than 2.5 microns (PM-2.5).
Source: U.S. Environmental Protection Agency, National Emissions Inventory and Air Pollutant Emissions Trends Data.