What naturally occurring cleanser helps to remove pollutants from the atmosphere? Where does it come from? What other mechanisms also act to cleanse the atmosphere of pollutants? Describe the origins...

What naturally occurring cleanser helps to remove pollutants from the atmosphere? Where does it come from? What other mechanisms also act to cleanse the atmosphere of pollutants? Describe the origins of industrial smog, photochemical smog, and atmospheric brown clouds. What are the differences in the cause and appearance of each?