Aerosols are suspensions of solid or liquid particles in gas. Humans release aerosols when they burn wood or fossil fuels certain chemical pollutants into the atmosphere. Toxic aerosols may be released by work operations and processes into the working environment. Aerosols can also be released naturally, through fine dust, volcanic ash and sea salt.
Atmospheric aerosols have at least two important implications for the environment: The acid rain it produces harms both terrestrial and marine life, and they alter the earth's climate by reflecting solar radiation and thereby cooling the part of the earth beneath the haze. Aerosols combine with heat-trapping green house gases to induce climatic change. Greenhouse gases tend to warm the earth while aerosols tend to cool some regions. Greenhouse gases tend to remain in the atmosphere for decades or even centuries while aerosols can rain out of the atmosphere in a few days.
The aerosol "haze" over the Indian Ocean (1999) has surprised scientists by its extent and its thickness, covering about 9.9 million square kilometers, roughly the combined area of 50 USA states. Aerosols over Europe and America are confined to within 600 meters of the earth's surface, those observed above the Indian Ocean rose to about 3,050 metres. Preliminary measurements indicated that the amount of solar radiation reflected was enough to reduce the amount absorbed by the ocean by 10%. This means that less water will evaporate from the sea producing less rainfall. Aerosols sucked up by thunderstorms and mixed with rain falls as acid rain affecting marine life.
Due to incomplete combustion "haze" has an unusually high proportion of soot which being dark absorbs solar radiation. This would counteract the effect of cooling by the aerosols that reflect sunlight. So it is just too early to say whether the net effect is one of cooling or warming. Environmental controls are reducing aerosols over North America and Europe where industry has traditionally been located, but they are on the increase in Asia as it's industry grows.