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Aerosols and Their Importance

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What are aerosols?

Aerosols are small particles suspended in the atmosphere. They are often not or barely visible to the human eye, yet their impact on climate, weather, health, and ecology are significant. This page introduces the various major types of aerosols, and explains why researching them is important.

Aerosols range in size from a few tens of nanometers—less than the width of the smallest viruses—to several tens of micrometers—about the diameter of human hair. The size and composition of aerosol particles affects how far they can travel around the world, their interactions with solar and thermal radiation, and their potential effects on health. Aerosols injected into the atmosphere directly are known as 'primary aerosols'. Sea spray, mineral dust, smoke, and volcanic ash are all primary aerosols. Secondary aerosols are aerosols which were emitted in another form (e.g. gases), then become aerosol particles after going through chemical reactions in the atmosphere, such as sulfate aerosols from volcanoes or industrial emissions. All aerosols can also undergo further chemical changes, referred to as ‘aging effects’. Some more information about these various aerosol types is given below.

 

Mineral Dust

Sand dune field over Northern Mali, shrouded by a deep dust haze, fairly typical of the Sahara in summer. Photo taken from an aircraft on 17th June 2012 by Dr. J. R. Banks, Imperial College London, used with permission.

Mineral dust is emitted when wind blows over deserts or otherwise dry soils, lifting the particles get carried off into the atmosphere. Mineral dust is one of the most abundant aerosol types, and dust particles are also very large compared to other aerosols, often a size of several micrometers in diameter. One micrometer is 0.000001 meters.

Approximately two billion metric tons of mineral dust are emitted per year, with wide-ranging effects. For example, mineral dust from the Sahara can affect the formation of hurricanes in the Atlantic, and fertilize the Amazon basin. Some desert source regions of mineral dust include the Sahara, Sahel, Gobi, Taklamakan, Namib, Salar de Uyun, central Australia, and the United States’ Great Basin. Dust can also be emitted from dry, barren soils, which can be exacerbated by deforestation and overgrazing. North African dust storms occur year-round, while Asian dust storms are most frequent during the spring. The satellite images below show two dust storms on quite difference scales.

A dust storm rages over Baja California Sur, Mexico.

MODIS Aqua image of a dust storm over Baja California Sur, Mexico on November 27, 2011.

A dust storm rages across Western Sahara/Mauritania and the Atlantic.

MODIS Terra image of a dust storm across Western Sahara/Mauritania and the Atlantic on March 2, 2003.

Sea Spray

Sea spray is largely composed of sea salt, but also consists of organic matter such as dissolved organic carbon, or even bacteria, phytoplankton, and microalgae. It is commonly formed through the bursting of air bubbles over the ocean surface. The properties of sea spray are chiefly dependent on wind speed, near-surface relative humidity, and sea surface temperature.

 

Smoke

Smoke from fires burning in Alberta, Canada seen over the Mid-Atlantic, United States.

MODIS Aqua image of smoke from fires burning in Alberta, Canada, seen over the Mid-Atlantic United States on June 10, 2015.

Smoke is emitted from fires, both natural wildfires and human-caused (for e.g. agricultural practices such as land clearing and waste incineration). These are often also referred to as 'biomass burning' aerosols, and are composed of organic (brown) and black carbon (soot). The composition of smoke is strongly dependent on the fuel source burning, and atmospheric conditions (e.g. moisture) at the time.

Fires and smoke in Canada

Airborne photo of fires and smoke above boreal forest in Saskatchewan, Canada, collected on 2 July 2008 during the ARCTAS field campaign. Image courtesy Dr. C. Gatebe (NASA GSFC/USRA), used with permission.

Organic and black carbon absorb light to varying extents, which warms the atmosphere locally while simultaneously shading and cooling the surface below. When black carbon is deposited on snow and ice, it decreases their reflectivity of those surfaces, which can warm them and speed up their melting. Short-term and long-term exposure to black carbon can result in visibility impairment, respiratory and cardiovascular effects, and even premature death.

 

Industrial aerosols

Since the start of the Industrial Revolution, the amount of aerosols emitted by human activities has increased greatly. These industrial aerosols may have a wide variety of compositions. Some industrial aerosols include:

  • Sulfates, which are produced when sulfur dioxide (SO2) reacts with water vapor and other gases in the atmosphere. One source of this is the burning of coal and oil.
  • Nitrates, which are often formed when combustion engines (such as vehicles or power plants) release nitrogen oxides (NOx).
  • Organic and black carbon, again also from combustion.

Together, this combination can be responsible for a visible smog, and heavy exposure can have harmful effects to plant and animal life, although air quality regulations have improved the situation in recent years over some parts of the world. For example, cities such as London and Los Angeles used to be infamous for their smog, although conditions in these cities have improved greatly. However, air quality in many other parts of the world is still a concern.

Satellite view of haze and clouds over Eastern China.

MODIS Aqua image of a heavy pall of pollution over parts of eastern China and the Yellow Sea on January 14, 2013. Clouds and snow are also present.

Volcanic aerosols

Volcanoes produce two main aerosols: ash and sulfate. Ash is a dark, large aerosol produced by the pulverization of crystallized magma and contains minerals such as silica and feldspar. The ash clouds can have significant impacts on human health and safety. They pose a significant threat to air traffic, as ash can destroy the engines of planes. Ash deposits, if thick enough, can damage buildings. Volcanic ash also irritates the lungs, and can cause acute respiratory damage or even death if enough is inhaled.

Volcanic clouds on the Aleutian Islands off the coast of Alaska, USA.

MODIS Terra image of a volcanic ash plume from the Aleutian Islands off the coast of Alaska, USA. 

Through eruptions and during passive degassing, volcanoes emit SO2, which is oxidized in the atmosphere to sulfate aerosols. Large eruptions can cause widespread cooling as a result of these sulfate aerosols. For instance, Pinatubo injected 20 million tons of SO2 into the stratosphere, causing an abrupt half-degree (0.6°C) drop in global temperatures. This lasted two years, ending as sufate finally dropped out of the atmosphere. Volcanic ash is also rich in nutrients, such as iron, that are used by ocean-dwelling organisms such as phytoplankton. An eruption therefore can be a natural fertilizer for areas of the ocean with insufficient levels of iron, such as the northeast Pacific. This area experienced a large phytoplankton bloom shortly after the eruption of the volcano Kasatochi in the Aleutian Islands in 2008.

Vog, or volcanic fog, off the coast of Hawai'i Island, Hawai'i.

MODIS Terra image of vog off the coast of Hawai'i Island, Hawai'i (August 8, 2008). When a volcano releases SO2, the SO2 reacts in the atmosphere to form volcanic fog, aka "vog".

Biogenic aerosols

Not all aerosols fall into the above categories. Biogenic aerosols are those which come from living things. This can include organic chemicals such as limonene, which are emitted by plants and react in the atmosphere to form aerosols, as well as other debris such as pollen, spores, and microbes.

 

Aerosol transport and lifetime

Aerosol lifetimes range from hours to years, dependent primarily on the size of the particles and the height at which they are injected into the atmosphere. For fine aerosols injected near the surface, the lifetime is typically several days. This increases to weeks or months for particles injected into or transported to the upper troposphere, and fine aerosols in the stratosphere (e.g. volcanic sulfate) can remain there for years. Because of these short lifetimes, many therefore are not usually transported far from their sources. For example, the highest concentrations of organic carbon, black carbon, and nitrate aerosols are found over North America, Europe, and East Asia, which are also the most prolific producers of industrial emissions. However, there are many examples of long range transport, such as:

Aerosols are removed from the atmosphere by either "dry" or "wet" processes:

  • Dry removal is when particles are deposited on the surface by turbulence or gravity. Movement of small particles is dominated by Brownian motion (random movement), while heavy particles feel the effects of gravitational settling.
  • Wet removal processes occur when the aerosol is removed in precipitation (water, fog or ice). Aerosol particles may be rained out by collision with falling raindrops ('inertial removal', most effective for heavy particles), or through diffusion of aerosols into falling drops (most efficient for very small particles). The total aerosol amount removed through wet processes does, of course, depend on how much moisture there is. Large aerosol particles are able to act as cloud condensation nuclei (CCN), meaning cloud droplets form by condensing on the aerosol particles. Clouds can form and rain within an hour, and the raindrops remove both the aerosols they formed on and any caught by wet removal processes. Wet removal processes are illustrated below:

An animation depicting one method of the wet removal process. A raindrop forms around the aerosol, and brings it down.

One wet removal method. A cloud droplet forms around an aerosol, grows to the size where is falls as rain, then drags it down.

An animation depicting another method of the wet removal process. A falling raindrop hits an aerosol, dragging it down.

Another wet removal method. A falling raindrop hits an aerosol, then drags it down with it.

Why are we interested in aerosols?

There are many reasons why we study and monitor aerosol levels. Some of these are discussed below.

Radiation Effects

Aerosols scatter (reflect) a portion of the Sun's incoming light, and, dependent on type, may also absorb some. These are known as 'direct radiative effects' of aerosols. The scattering has a local cooling effect on the surface below, while absorption has a local warming effect on the atmosphere where the aerosols are located. Sometimes these effects can be large and more widespread. For example, when the volcano Pinatubo erupted in 1991, so many reflective sulfate aerosols were released into the stratosphere that the temperature dropped around 0.6°C.

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.

 

Another set of effects are known as 'indirect effects' or 'aerosol-cloud interactions'. Aerosols are largely responsible for the creation of clouds by acting as cloud condensation 'nuclei, or a sort of foundation for clouds to accumulate water on. Increasing the amount of aerosols in the atmosphere can influence factors such as how many clouds there are, how large the cloud droplets are, how high the clouds are, and when or how heavy rainfall is. These are known as 'indirect radiative effects' of aerosols. You can watch a video about these indirect effects from volcanic sulfate aerosols on YouTube here. There is also a `semi-direct effect' whereby local heating of the atmosphere by absorbing aerosols like black carbon can influence whether clouds form at all, or lead to the evaporation of existing clouds.

 

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.

 

One well-known manifestation of aerosol effects on clouds is seen in ship tracks, where aerosols emitted from ship exhaust lead to a decrease in cloud particle size, causing a bright streak of cloud to follow the ship.

MODIS Terra image of ship tracks

MODIS Terra image of ship tracks off the Pacific coast of the USA. Original by Jeff Schmaltz, MODIS Rapid Response Team.

 

When absorbing aerosols such as soot or dust are deposited on snow or ice, they decrease its reflectivity ('albedo') and cause the surface to absorb more light, which has a warming effect. This can lead to, for example, faster retreat of glaciers.

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.

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Health

Aerosols have the capacity to cause damage to plants and animals, including humans. Aerosol particles can irritate the lungs, and in high enough concentrations 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. Fine particulate air pollution has also been determined to have adverse effects on cardiovascular health.

 

Ecology

Mineral dust can be blown across and between continents by the wind. For example, about half of the dust arriving at the Amazon rainforest, which fertilizes plant growth, comes from the Bodele Depression in the Sahara Desert. Nutrients from dust deposited in the oceans can also lead to algal blooms, as seen below.

 

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.

 

 

Mineral dust can also transport bacteria and viruses living within it. For example, the shrinking of the Aral Sea since the second half of the 20th century has led to health problems for those living in the region. 

 

Solar Power

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

The efficiency of solar power generation systems is affected by aerosols. Aerosol scattering and absorption reduces the amount of direct sunlight reaching solar panels, by about 4 W for every watt reflected to outer space, decreasing their potential yield.

 

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This page was prepared by A. Chen, B. Howl, and A. Sidel during their summer 2015 internship with the Deep Blue group at NASA GSFC.