The sources of pollutants as seen on our overview page under "Pollutants" were assembled into 6 categories for the following graphs. The major contributors for CO and NOx are consistently highway vehicles and wildfires, while SO2’s largest contributor is fuel combustion from electric utilities. VOCs sources are more mixed with the major sources being stationary sources, while the PM2.5 sources are primarily miscellaneous and wildland fires. There is a general downward trend shown here for all emissions from 2008 through 2018.