temperatures in some places in the eastern United States, Russia and China were as much as half to two-thirds of a degree (.3 to .37 degrees Celsius) warmer. That’s due to less soot and sulfate particles from car exhaust and burning coal, which normally cool the atmosphere temporarily by reflecting the sun’s heat

Overall, the planet was about .05 degrees (.03 degrees Celsius) warmer for the year because the air had fewer cooling aerosols, which unlike carbon dioxide is pollution you can see, the study found.

“Cleaning up the air can actually warm the planet because that (soot and sulfate) pollution results in cooling” which climate scientists have long known

I was taught in school that these particles reflect the Sun's heat which get's trapped in the atmosphere and warms the planet in a "run away greenhouse effect." Now they are saying these same particles are now cooling the planet.