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.
In industrial hygiene surveys of air contaminants in the working environment a distinction is made between dusts, fumes, smokes, and mists and fogs. Dusts are generally formed by disintegration processes, such as in mining and ore-reduction operations. Examples are silica and asbestos dusts. Fumes usually result from chemical reactions, such as oxidation, or from sublimation or distillation processes followed by condensation. Examples are oxides of iron and copper. Smokes result from the combustion of fossil fuels, asphaltic materials, and wood. Smokes consist of soot, liquid droplets and, as in the case of wood and coal, a significant material-ash fraction. Mists and fogs consist of liquid droplets produced by atomization or condensation processes. Examples are oil mists from cutting and grinding operations, mists from spraying operations.
One way to discriminate between natural sources of aerosols and human made sources is to look at the location of aerosol plumes. Natural aerosols like salt particles from sea spray are typically widespread over larger areas and not particularly concentrated downwind of urban areas. Or they are particularly concentrated downwind of obviously natural sources, such as the streams of dust originating from the Sahara Desert. Another clue to a plume's origin can be found in the size of the aerosol particles it carries. In general, smaller particles are more likely to be produced by human activities, while larger particles are more likely to have natural origins.
Aerosols produced by humans appear in punctuated bursts of thick and concentrated plumes comprised of small particles. They can also be found concentrated downwind of regions altered by human activities, such as deforested regions. Plumes of smoke and regional pollution are distinguished by their large concentrations of small particles (less than one micrometer) downwind of biomass burning sites and urban areas. These particles are important because, depending upon the type of particles produced, human pollution can either have a warming or cooling influence on climate, and they can either increase or decrease regional rainfall.
Examining global satellite images in concert with global scale models and globally distributed ground based pollution measurements gives scientists the best tools they have ever had to estimate the effects of aerosols on climate and weather patterns around the world. The next step is to quantify more precisely the roles human aerosol pollution plays in Earth's weather and climate systems. NASA plans to further expand global aerosol research with the launch of satellite based light imaging radars, known as lidars, that send bursts of light to Earth and, like a radar signal, provide a measure of the altitude and vertical structure of aerosol plumes and clouds.
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.