Rolling coal is the practice of modifying a diesel engine to deliberately emit large amounts of black or grey diesel exhaust, containing soot and incompletely combusted diesel. Rolling coal is used as a form of anti-environmentalism protest.
A 2020 EPA report said 15 percent of diesel trucks in the U.S. are rigged to “delete” emissions controls. According to government estimates, the practice can increase nitrogen oxide emissions as much as 310 times, non-methane hydrocarbons 1,400 times, and carbon monoxide 120 times.
Unfun fact: Rolling coal modification: https://en.wikipedia.org/wiki/Rolling_coal