Soot, also known as black carbon, is the second-leading cause of global warming after carbon dioxide, and it’s totally preventable. We already have the technology to avoid producing it; it’s just a matter of using it.
Soot is harmful to lungs, causes deaths, and heats up polar ice. It comes from diesels, smokestacks and cooking and heating stoves. However it’s easy to eliminate and only stays in the atmosphere for two weeks, so getting rid of it would have an immediate effect.
And don’t forget aircraft. I used to live near LAX, where cars get covered with soot in a matter of hours.