Aerosols are defined as microscopic liquid or solid particles that enter the atmosphere through natural and man-made processes. Aerosols are more complicated than the typical greenhouse gas. There are some aerosols that cause cooling while others cause warming. Surprisingly, humans are only responsible for about 10% of the current concentration of aerosols, as of 2009.
|Figure A: Aerosol Impacts on Clouds
|Image from NASA
Aerosols come from volcanoes, dust storms, fires, vegetation, sea spray, burning of fossil fuels and land use. Warming aerosols include black carbon and dark soot. Cooling aerosols include dust, sulfate particles and sea spray.
Aerosols can affect the climate in two ways, through direct or indirect processes. A direct process is the immediate effect on radiation absorption. If the aerosol is light in color, it generally reflects solar radiation and causes cooling by reducing incoming energy. If the aerosol is dark it absorbs solar radiation and directly affects the climate through warming. Some aerosols can also encourage the growth of cloud and fog droplets if they have water-attracting properties. However, an abundance of aerosols can make cloud droplets smaller by spreading out the water vapor between many particles. This causes clouds to be more optically dense. They reflect more sunlight, last longer and donâ€™t produce as much precipitation. Increased reflectivity of solar radiation and longer lifespan leads to increased cooling. The release of silver iodide particles into clouds ("cloud seeding") can help clouds to form water droplets and lead to enhanced rainfall, but too many particles or the wrong type of cloud may actually reduce the chance of rain, and so cloud seeding can be a risky proposition.
It is generally agreed upon that the overall effect of aerosols is to cool the climate. It is not agreed upon how much cooling they are responsible for and if they will significantly offset the warming caused by other greenhouse gases. However, many scientists believe that the cooling in the United States in the middle of the 20th century was due to both an increase in volcanic particles and an increase in man-made aerosols due to the burning of fossil fuels in the days before the Clean Air Act was in effect. Once the Clean Air Act was passed, the concentration of aerosols in the atmosphere decreased.
Aerosols also have adverse effects on humans and animals. When the small particles are inhaled, they can irritate the eyes and the lining of the lungs and cause watering, coughing and asthma attacks. Some aerosols caused by the burning of fossil fuels are also carcinogenic and can help lead to the formation of smog and pollution in urban areas.