Aerosols play an important role in the atmosphere. Although they comprise less than .000001% of the atmosphere’s total volume, these tiny suspended liquid and solid particles play key roles in cloud and precipitation processes and Earth’s radiation budget. Aerosols range in size from very small to the “giant” sea salt aerosols that can be as large as 10µm in diameter. Aerosols larger than 10µm in diameter tend to settle out of the atmosphere very rapidly. Aerosol concentrations range from less than 1,000 per cubic centimeter over the open ocean to over 10,000 per cubic centimeter over polluted urban areas.

Natural Aerosols

There are many types of natural aerosols, many of them having varying concentrations in the atmosphere. Sand, dust, sea salt, pollen, sulfates emitted by oceanic phytoplankton, and aerosols resulting from volcanic action are all examples of natural aerosols.

Dust from the Sahara desert is often raised to heights higher than 10,000 feet. Caught in the tropical easterlies, this fine Saharan sand can make its way across the Atlantic Ocean as far west as North America. In addition to reflecting and scattering sunlight, this dust also absorbs some solar energy. The resulting mid-tropospheric warming and lower tropospheric cooling may suppress convection in the Western Atlantic Ocean, and even have a negative effect on the formation of tropical cyclones there.

courtesy of NASA Earth Observatory

Sea salt which is put into the atmosphere by the action of ocean waves tend to be very large aerosols, up to 10µm in diameter. They are very important in tropical convection where cumulus clouds can form and raindrops can grow to sufficient size to reach the ground without the aid of ice crystals.

Dust and pollen tend to be much smaller than sea salt particles, but provide condensation and ice nuclei over continental interiors. Solid particulate matter from volcanic eruptions tend to be composed of large particles that settle to Earth relatively quickly.

Mt. Pinatubo courtesy of NASA

Sulfates, from volcanic eruptions, however, react with water vapor in the atmosphere to form tiny suspended droplets of sulfuric acid. High in the stratosphere these droplets reflect some of the incoming solar radiation, resulting in a cooling at Earth's surface. After the eruption of Mount Pinatubo in 1991, much of Earth experienced a cooling trend for the following two years. These sulfuric acid aerosols also provide a surface for the reactions that result in the destruction of ozone to occur.

Human-made Aerosols

Aerosols produced by humans result from biomass burning like we see in the Amazonian rain forest, as well as the burning of fossil fuels. This burning results in particulate matter such as soot (black carbon) and other materials. Some (like soot) absorb solar energy, others reflect solar energy. Much of the human made aerosols come in the form of sulfates and nitrates. Sulfates absorb very little solar energy, but reflect a lot. Furthermore, gaseous oxides of sulfur and nitrogen react with other pollutants to form liquids like sulfuric acid.

URLs: Aerosols and Pollution

http://www.ogp.noaa.gov/ace-asia/aerosols/

http://oea.larc.nasa.gov/PAIS/Aerosols.html

http://earthobservatory.nasa.gov/Library/Aerosols/aerosol2.html

Pollution Decreases after Blackout
http://www.ukweatherworld.co.uk/forum/thread-view.asp?threadid=12862&posts=4

Volcanic Aerosols
http://www.epa.gov/ozone/science/aerosol.html

Dust Storms
http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/OCDST/asian_dust.html

< BACK TO PBL SCENARIO