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Why Are Aerosols Important?

Radiation Effects

Direct [Sunlight and Temperature]

Aerosol-radiation interactions occur directly through sunlight and temperature. A sizable portion of sunlight that comes in contact with the Earth’s atmosphere get reflected back into space by aerosols. Aerosols either absorb or scatter sunlight depending on the type and color of the aerosol. Darker aerosols tend to reflect more of the Sun’s energy while lighter aerosols tend to absorb more of the Sun’s energy. Aerosols that scatter sunlight cool the atmosphere. Aerosols that absorb sunlight warm the atmosphere but cool the Earth’s surface.

The Earth’s reflectivity, or albedo, can be altered by aerosols. When dark aerosols land on snow or ice, they hasten the snow and ice’s melting due to the dark aerosols’ ability to absorb heat into the ground beneath it as a surface. The cooling ability of aerosols may have countered around half of greenhouse gases’ global warming[1], but aerosols do not last in the atmosphere for anywhere near as long as greenhouses gases do.

Fresh snow reflects 80-90% of the sunlight that falls on it. Dusty snow, however, only reflects 50-60%, absorbing the rest. These Landsat 8 images show clean (top) and dusty (bottom) snow on the mountains near Telluride, Colorado, on May 3 and June 20, 2013, respectively. Landsat image by Robert Simmon, NASA Earth Observatory.

Aerosols can also have an effect on regional temperatures, and sometimes global temperatures. For example, when the volcano Pinatubo erupted, so many reflective sulfate aerosols were released into the stratosphere that the temperature dropped 0.6°C due to all the aerosols reflecting the world’s sunlight[2].

Photo taken aboard Space Shuttle Atlantis in August 1991, showing a layer of aerosols from the June 1991 eruption of Mount Pinatubo. NASA photo ID STS043-22-12.

Indirect [Clouds and Precipitation]

Aerosol-radiation interactions occur indirectly through clouds and precipitation. Aerosols are largely responsible for the creation of clouds by acting as a cloud condensation nuclei, or a sort of foundation for clouds to accumulate water on. This is why contrail-like clouds form behind ships. Clouds formed on natural aerosols such as sulfates tend to have a small number of large water droplets in them, giving the clouds a gray and wispy look. Clouds formed on polluted aerosols such as black carbon tend to have a large number of small water droplets in them, giving the clouds a white and opaque look due to the polluted particles’ extra reflectiveness from their higher concentrations of water-soluble particles. The brighter polluted clouds shade the Earth excessively, producing a cloud albedo effect which results in a net cooling of the planet.

Indirect Effect Diagram

Clouds in clean air are composed of a relatively small number of large droplets (left). Higher concentrations of aerosols can lead to a large number of small droplets (right). These clouds are typically more reflective (brighter). Original by Robert Simmon, NASA Earth Observatory.

Additionally, the type of aerosol a cloud is formed on can dramatically affect precipitation. In most places, a cloud formed on polluted aerosols releases less precipitation due to its smaller droplet size. However, in other areas, a polluted aerosol cloud would be taller and produced heavier precipitation as well as electrical storms. In those areas it was even possible to match the frequency of thunderstorms with the amount of aerosols produced in that region at the time.

Clouds over Kuwait

This photograph, snapped by an astronaut aboard the International Space Station on December 12, 2013, shows a white flash of lightning amidst the yellow city lights of Kuwait and Saudi Arabia. (Photograph by the ISS)

Health

Aerosols have the capacity to inflict serious damages to human health. Sulphates and ash can irritate the lungs, and in high enough concentrations can cause permanent respiratory damage and even death. Chronic exposure to fine particulate matter is associated with adverse health impacts such as decreased life expectancy and higher likelihoods of lung cancer.[3][4] Fine particulate air pollution has also been determined to have adverse effects on cardiopulmonary health.[5]

Extreme haze and low visibility in Shanghai, China.

Heavy smog in industrial countries like China can cause harmful health conditions for its residents. (Photograph by Wikimedia user Galaxyharrylion)

 

Ecology

Aerosols with ammonium, sulfate, and nitrate contribute significantly to acid deposition, or acid rain. Acid deposition  can be seriously damaging to lakes, streams, and forests, as well as all the plants and animals in those ecosystems. It can cause slowed growth, injury, or even death to trees in forests, and acidifies lakes and streams, killing fish and reducing biodiversity.

Mineral dust, the most visible aerosol, can be moved from one continent to a different continent or region to be used as soil and nutrients. It has been discovered that the majority of the Amazon rainforest’s fertilizer came from mineral dust from the Bodele Depression, a single valley in the Sahara. Mineral dust can also transport bacteria and viruses living within it. When the Aral Sea was emptied, harmful bacteria were exposed to the wind and caused the region’s inhabitants to become ill when they breathed in the bacteria that were now in the mineral dust aerosols floating around.

Nitrate, a common aerosol, is also a major soil nutrient. However, too much nutrient concentration in soil and groundwater often has consequences. This issue is called eutrophication, and is another adverse effect of aerosols on the environment. Eutrophication often leads to diversity loss. It can also instigate the excessive growth of algae, which can block sunlight from other plants and destroy ecosystems.[6]

False-color image showing algal blooms off the coast of Argentina on Dec 2, 2014. Airborne dust, iron-rich currents from the south, and upwelling deep currents provide a bounty of nutrients for the grass of the sea—phytoplankton. In turn, those floating sunlight harvesters become food for some of the richest fisheries in the world. Bands of color reveal the location of plankton and the eddies and currents that carry them. VIIRS image by Norman Kuring, NASA Ocean Color Group.

 

 

Solar Power

Domestic solar panels. Image courtesy J. Limbacher, NASA GSFC/SSAI, used with permission.

The efficiency of solar power generation systems is threatened by aerosols. As shown by light scattering data, the presence of aerosols results in the reduction of direct sunlight. Aerosols reduce direct sunlight by about 4 W for every watt reflected to outer space[7], resulting in only diffused sunlight for solar panels to use. Since the locations and frequency of various aerosols must be considered when placing solar power generation systems, more data on aerosols and their patterns is a necessity. In addition, when considering the deliberate enhancement of the stratospheric aerosol layer, the detrimental effect on solar power must be recognized.[8]