Air can be polluted through smoking or any fumes emitted in the air by cars or factories and combustion of coal. Land pollution is pollution of the Earth's natural land surface. It can be done through deforestation, construction debris, littering or oil refineries. Water pollution is evident through animal waste disposal and oil spills. You can get some more ways on http://www.greenstudentu.com/encyclopedia/pollution.
There is a lot of talk about how much carbon dioxide a tree scrubs from the atmosphere and how good that is in the reduction of pollutants. The carbon dioxide that is absorbed by trees in day light combines with essential nutrients and water through its root system to perform photosynthesis and out gasses oxygen. However, there is little to no talk about the "Inconvenient Truth" that is Natures cycle. Much of the carbon dioxide that is absorbed in day light is out gassed back into the environment over night. This is true of all green plants that use photosynthesis in their life cycle.