Coal naturally has traces of mercury in it. Half of mercury emissions come from power plants, but installing pollution control devices can prevent this.
Remember that primary pollutants affect the atmosphere directly, and secondary pollutants always involve chemical reactions before going into the atmosphere.
Earth Science Concepts dealing with weather explain how wind, mountainous regions, and temperature inversions affect clouds. Think about how these factors can apply to pollution as well.
Think of saving water as saving money. There is only a finite supply, so spending it sparingly can extend the time you have water (money) so you can spend it when you need to.