March 6, 2013

Websites bellow: (accessed March 6, 2013)

 Aerosols

 Aerosols are naturally present microscopic airborne suspensions of liquid droplets or solid particles in a gas, usually air, which have tendencies to remain dispersed (floating) in the gas rather than to settle down.

When the airborne suspensions are solid, such as dust and sea salt, they are also commonly called particulate matter. A commonly seen natural event that spews out aerosols is an active volcano. Other examples of aerosols seen in Earth’s atmosphere are mist, smog, and fog.

In addition, aerosols include all types of artificially made containers that hold a suspension of liquid or solid particles within a gaseous propellant under pressure. Such a container-sometimes called an aerosol dispenser, but more commonly called an aerosol can-seals in the suspension and, through a valve, dispenses it in a foam, liquid, or spray stream. Common products packaged in aerosol cans are cosmetics, detergents, foods, insecticides, and paints.

Historical Background and Scientific Foundations

The term aerosol is derived from the Greek term aero, which means pertaining to air, and the term sol, which is defined as any scattering, or dispersion, of microscopic or sub-microscopic particles in a liquid.

Naturally produced aerosol particles are considered by scientists to have a maximum size of one micrometer (or one millionth [10-6] of a meter). They generally range from 4 x 10-8 to 4 x 10-5 in (10-7 to 10-4 cm) in diameter.

However, violently moving suspensions can contain aerosol particles that are over one-hundred times larger than normal, as is often the case with fog and cloud droplets and dust particles.

The climate of Earth has been affected by artificially produced aerosols. The combustion of fossil fuels is one detrimental way that sulfate aerosols are introduced into Earth’s atmosphere. Other sources of artificially produced aerosols include agriculture, deforestation, industry, mining, and transportation.

Artificially produced aerosols have been made from such particulate matter as asbestos, diesel fuel, and natural silicon dioxide (silica). However, these substances have been found to be harmful to humans, causing a variety of diseases such as anthracosis, a lung disease of coal miners commonly called black lung. It has been found that humans who work around such dangerous aerosols can be largely protected from health problems when they wear respirators.

Impacts and Issues

Aerosols are naturally found in Earth’s troposphere and the stratosphere. The particular layer in which each aerosol type is found depends on its size, chemical composition, origin, and other such physical factors. These factors can determine whether aerosol particles absorb or scatter solar radiation and, thus, how much radiation reaches Earth’s surface from the sun. Research is ongoing to learn more about the wide types of aerosols in the atmosphere because of their affect on global weather patterns and the climate in general.

The study of aerosols in the atmosphere is important due to the number of artificially produced aerosols that have been, and continue to be, produced by humans. For instance, when human’s burn fossil fuels, the aerosols produced by such activities absorb heat and, thus, contribute to global warming. Such aerosols are called atmosphere nuclei-that is, microscopic particles in the atmosphere that attract water droplets.

These aerosols tend to add more moisture to the atmosphere, which makes it more likely that precipitation will occur. Precipitation may occur outside of where the aerosols originated because it is easily transported by convection and wind currents. Consequently, problems around the world can occur due to localized aerosol use.

Many of these atmosphere nuclei come from industrial environments where combustion forms particles such as sulfur dioxide and sulfur trioxide.

Some scientific research has found that many artificially produced aerosols have a cooling affect on the atmosphere. However, scientists contend, based on preliminary research, that such cooling (what is sometimes called global dimming) has a minimal effect on Earth’s climate at best and does not counterbalance global warming, which is produced by greenhouse gases such as carbon dioxide, methane, and nitrous oxide.

Because of the increased presence of artificially produced aerosols in Earth’s atmosphere, many organizations are actively seeking more and better information about aerosols. Almost continuously since 1978, the U.S. National Aeronautics and Space Administration (NASA) have provided aerosol maps of the world from observations made by a series of satellites called the Total Ozone Mapping Spectrometer (TOMS). The TOMS map provides a global map of aerosol particles found on any particular day on and above Earth. NASA also provides another map to show the cloud cover at the exact time the aerosol particle map is made.

The data from TOMS are being used by scientists around the world to better understand the behavior of aerosol particles within Earth’s atmosphere. TOMS has been important to scientists because it was the first instrument to provide data of aerosol particles, especially as they move between land and water regions. Consequently, TOMS has provided valuable information about natural events such as forest fires and biomass burnings that affect the production of aerosol particles.

The TOMS satellites were replaced in January 1, 2006 by the Ozone Monitoring Instrument (OMI), which continues to provide even more advanced and detailed aerosol maps of the world as it orbits Earth onboard the Aero satellite, or Earth Observing Satellite (EOS) CH-1.

Words to Know

Biomass: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.

Convection: The rising of warm air from an object, such as the surface of Earth.

Deforestation: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

Fossil Fuels: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

Global Dimming: Decrease in amount of sunlight reaching Earth’s surface caused by light blockage by clouds and aerosols. Global dimming increased from 1960 to 1990, reducing sunlight reaching Earth’s surface by 4%, but this trend reversed after 1990 in most locations.

Greenhouse Gases: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth’s surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth’s atmosphere, causing global warming and global climate change.

Particulate Matter: Matter consisting of small particles. Particulate matter that is airborne forms aerosol pollution; particulate matter may also mix with water or lie on the surface of snow, ice, or ground. On snow or ice, small quantities of dark particulate matter (e.g., soot from fossil-fuel burning) can greatly accelerate melting.

Precipitation: Moisture that falls from clouds. Although clouds appear to float in the sky, they are always falling, their water droplets slowly being pulled down by gravity.

Because the water droplets are so small and light, it can take 21 days to fall 1,000 ft (305 m) and wind currents can easily interrupt their descent. Liquid water falls as rain or drizzle. All raindrops form around particles of salt or dust. (Some of this dust comes from tiny meteorites and even the tails of comets.) Water or ice droplets stick to these particles,then the drops attract more water and continue getting bigger until they are large enough to fall out of the cloud. Drizzle drops are smaller than raindrops. In many clouds, raindrops actually begin as tiny ice crystals that form when part or all of a cloud is below freezing. As the ice crystals fall inside the cloud, they may collide with water droplets that freeze onto them. The ice crystals continue to grow larger, until large enough to fall from the cloud. They pass through warm air, melt, and fall as raindrops.

Stratosphere: The region of Earth’s atmosphere ranging between about 9 and 30 mi (15 and 50 km) above Earth’s surface.

Suspension: A temporary mixture of a solid in a gas or liquid from which the solid will eventually settle out.

Troposphere: The lowest layer of Earth’s atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth’s surface.

Research also: Carbon Dioxide (CO2); Global Warming; Greenhouse Effect; Greenhouse Gases; Methane; Nitrous Oxide; Sulfate Aerosol; Volcanism.

Bibliography

Books:

Friedlander, Sheldon K. Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics. New York: Oxford University Press, 2000.

Hinds, William C. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles. Hoboken, NJ: Wiley-Interscience, 1999.

Web Sites:

‘‘Aerosols.’’ National Institute for Occupational Safety and Health (NIOSH), U.S. Centers for Disease Control and Prevention (CDC). http://www.cdc.gov/niosh/topics/aerosols/default.html  (accessed March 6, 2013).

‘‘Aura: Atmospheric Chemistry.’’ Goddard Space Flight Center, U.S. National Aeronautics and Space Administration (NASA). http://aura.gsfc.nasa.gov/index.html

 (accessed March 6, 2013).

‘‘Data Product: AEROSOL INDEX.’’ National Aeronautics and Space Administration (NASA), http://disc.sci.gsfc.nasa.gov/data-holdings/PIP/aerosol_index.shtml (accessed March 6, 2013).

‘‘Total Ozone Mapping Spectrometer.’’ National Aeronautics and Space Administration (NASA),. http://www.nasa.gov/centers/goddard/pdf/110908main_FS-2001-5-022-GSFC%20QuickTOMS.pdf  (accessed March 6, 2013).

‘‘Visible Earth: Aerosols.’’ National Aeronautics and Space Administration (NASA), http://visibleearth.nasa.gov/view_set.php?categoryID=109  (accessed March 6, 2013).

‘‘Welcome to AAAR.’’ American Association for Aerosol Research (AAAR). http://www.aaar.org/ (accessed March 6, 2013).