Aerosols

     Aerosols are solid or liquid particles suspended in a gas, usually air. There are many types of aerosols, lots of which are man-made and likely what the average person thinks of when hearing the term--hairspray, air freshener, and the like. But the vast majority of aerosols are natural, ranging from the water found in clouds to dust found in dry regions of the world to sea salt from ocean spray. These are part of a planet-wide system of weather patterns which have always existed and which significantly shape the climate of every part of the earth. Many others are natural in origin, but caused by human behaviors--huge amounts of sulfate from burning fossil fuels and black carbon and organic carbon from "biomass burning" (clearing land or getting rid of farm waste).

     The primary ways these particles affect the climate is by changing the levels of solar radiation which reach the earth, either by reflecting radiation back into space or by preventing it from leaving the atmosphere once it enters. These changes can have the net effect of cooling the earth and of heating it, respectively. This makes the process of predicting climate change difficult, because while some atmospheric variables, like greenhouse gases, are well-understood and fairly consistent, the effects of the changing levels of man-made aerosols are highly varied and difficult to predict.

     This means that the long-term effect of aerosols on earth's climate is difficult to pinpoint. There are, however, other ways that they affect daily life. A change in the level of aerosols in a given area can affect visibility, as in city smog or the recently noted haze over places like the Grand Canyon. Certain man-made aerosols can be damaging when breathed in. And some of the "propellants", or base gases, once used in canned aerosol sprays contributed significantly to the thinning of the ozone layer. These propellants, called fluorocarbons, have been almost universally banned, but their effects are still being felt today.

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sources:
http://www-das.uwyo.edu/~geerts/cwx/notes/chap02/aerosol&climate.html
http://www.eoearth.org/article/Aerosols#gen4
http://earthobservatory.nasa.gov/Features/Aerosols/
http://en.wikipedia.org/wiki/Particulates
http://en.wikipedia.org/wiki/Aerosol