Even air that appears pristine contains many tiny particles suspended within it. These particles, known as aerosols, particulates or particulate matter, can either be solid particles or liquid droplets. The majority of aerosols are naturally occurring, such as dust, pollen, sea salt, volcanic ash, and carbon particles from natural fires. Anthropogenic aerosols from combustion sources in urban areas include soot, carbon particles, ammonia, sulfate and nitrate compounds.
Aerosols can be found suspended in the air at different heights over the varied surfaces of the Earth and are divided into two types; primary and secondary. Primary aerosols are atmospheric particles that are emitted or injected directly into the atmosphere. Secondary aerosols are formed in the atmosphere by chemical reactions on the primary aerosols, such as the creation of sulfuric acid droplets and sulfate particles from an initial injection of sulfur dioxide gas from volcanic eruptions.
Smog is another example of a secondary aerosol formed as a result of chemical reactions in the lower part of the atmosphere, less than 5km above the ground. Smog is a combination of airborne particulate matter such as soot, and invisible toxic gases including ozone, carbon monoxide and sulfur dioxide. Although some of these compounds such as carbon monoxide are emitted directly from vehicle emissions, ground level ozone is formed in the air from nitrogen oxide emissions. Surface level ozone (unlike stratospheric ozone which protects the surface from harmful solar rays) is hazardous to human health. Both primary and secondary aerosols have either natural or anthropogenic sources, or a combination of both.
Aerosols can remain suspended in the Earth’s atmosphere from high up in the stratosphere to just above the Earth’s surface, and can be carried huge distances around the globe by atmospheric winds. Their size, which determines their lifetime in the atmosphere, varies from a few nanometers (less than the width of the smallest viruses) to tens of micrometers (about the diameter of human hair).
Often, the varying types of aerosol groups clump together to form hybrid particles of both natural and anthropogenic origin. On average, one could expect to inhale 160,000 particles per cubic centimeter (primarily soot) in polluted urban air, and 3000 particles per cubic centimeter in ‘clean’ air. Typically, the larger particle aerosols injected into the troposphere (the layer of atmosphere closest to the surface of the Earth, up to 12-16km), are washed out by rain within a matter of days or are deposited at the Earth’s surface due to gravitational force.
Aerosols play an important role in climate and climate change depending on their structure, size, composition and location. Since they are transported by air currents, aerosols injected or formed in the atmosphere in one location can affect the climate in regions that are hundreds or even thousands of miles away.
Light colored aerosols are generally thought to reduce the amount of solar radiation reaching the Earth’s surface, by reflecting the incoming solar rays out to space. Volcanic eruptions, for example release huge amounts of sulfur dioxide gas high into the stratosphere, where they react with atmospheric molecules to produce sulfate aerosols. Since sulfates are fine particles that remain suspended for longer periods of time and are more easily distributed through wind currents, these highly reflective particles are effective at cooling the entire planet, not just in the region in which they occur. Further, they remain in the stratosphere much longer (timescales of years) than aerosols nearer the surface of the Earth since they remain above the clouds and therefore cannot easily be washed out by rain. Studies have shown that past volcanic eruptions, such as Mount Pinatubo in 1991 in the Philippines, led to global temperature decreases of up to half a degree Celsius, although this temperature decrease was temporary and lasted no more than three years.
However, not all aerosol particles reflect incoming solar rays. Darker colored particles such as black carbon (soot), absorb the incoming solar radiation leading to localized warming of the atmosphere. Further, the eventual deposition of these darker particles onto the Earth’s surface, either due to gravity or through rain can alter the albedo (reflectivity) of the Earth, particularly if they are deposited on surfaces that are usually light-colored such as ice, snow or desert. Recent studies have investigated the particularly damaging effects of black soot particles deposited in the Arctic region. Due to atmospheric circulations, these black carbon particles tend to accumulate at lower altitudes in the Arctic region and form dark deposits on snow and ice. This reduction in albedo leads to greater surface heating and hence more rapid melting.
Aerosols also play a critical role in the formation of clouds. Clouds form in the presence of both water vapor and aerosol particles that serve as the tiny ‘seeds’ (known as cloud condensation nuclei), which virtually all clouds need to form. Aerosol effects on cloud formation lead to both global cooling and heating, depending on their type.
Natural aerosols—often sulfates, sea salt or ammonium salts—are the most common condensation nuclei in clean environments, particularly over oceans. Polluted urban air usually contains much higher concentrations of particles, such as nitrates or soot, which means pollution-rich clouds tend to have more numerous, but smaller droplets as a result of the increased number of seed particles. These smaller droplets make polluted clouds look brighter than they would otherwise be. As an example, crushed ice appears brighter than a solid cube of ice, since the number of reflective surfaces has increased.
Additionally, since more aerosols mean more cloud droplets, the clouds can be bigger and form over larger surfaces. Since the water vapor available in the cloud is divided into a larger number of smaller droplets, the cloud will scatter more light and become more reflective. A more reflective cloud would lead to surface cooling, since the incoming solar rays are reflected before they reach the surface.
On the other hand, studies have shown that metallic and mineral dust from industrial processes, act as ‘seeds’ for the creation of high altitude cirrus clouds. Cirrus clouds are thought to have an overall warming effect on the surface of the Earth. Aircraft condensation trails (known as contrails) can also create cirrus clouds, as a result of aerosol particles in the exhaust fumes. Contrail cirrus clouds are clearly visible in satellite data.
Finally, aerosols have been shown to affect regional precipitation in a number of complex ways. Scientists generally believe that aerosols have a tendency to suppress precipitation. This is because the addition of aerosols not only tends to increase the numbers of water droplets in the cloud, but to decrease the size of the droplets themselves, since there is only a finite amount of water vapor for distribution in any one cloud. These smaller individual droplets do not reach the necessary size to fall as raindrops under gravitational forces. Under some environmental conditions, aerosols can lead to taller clouds, since the individual water droplets are smaller, lighter and more readily lifted to greater altitudes than larger, heavier, water droplets. These taller, longer-lasting clouds, while creating cooling during the day, retain heat at night which can lead to measurable increases in local temperatures.
In some cases, aerosols within a cloud cause can even cause the individual cloud droplets to evaporate. Whereas reflective aerosols such as sulfates tend to brighten clouds and make them last longer, soot and darker colored particles can have the opposite effect. Studies of soot particles and other particulates from industrial processes over the Indian Ocean, and biomass burning smoke in the Amazon, have shown that dark particles warm the surrounding atmosphere leading to the evaporation of the droplets. This process turns clouds into a smoky haze that also suppresses precipitation.
Over the last forty years or so, since scientists first identified that aerosols could affect climate, large quantities of data from an array of satellite, aircraft and ground-based instruments that measure so-called aerosol optical depth has become available. As a result of studies showing the global cooling effects of volcanic eruptions, a small group of scientists has suggested ways in which the climate can be artificially engineered (climate or geo-engineering). An example of this includes the introduction of man-made sulfate particles into the stratosphere to offset some of the warming that the earth has experienced. Given the migratory nature of aerosols, and their potential for wide-scale disruption to precipitation, the geopolitical implications of such schemes would clearly need to be examined closely.
Scientists generally agree that to-date aerosols have offset some of the warming that the Earth has experienced in recent decades. Despite progress however, questions remain about the competing effects of aerosols on climate. Differentiating between the different types of particles, particularly hybrid particles, remains a challenge. As measurements and computer modeling continue to improve, global climate models which currently are limited to modeling primarily simple particle structures, can more accurately predict how the climate will continue to change.