Air pollution occurs when the air contains gases, dust, fumes or odor in harmful amounts—aerosols are a subset of air pollution that refers to the tiny particles suspended everywhere in our atmosphere. These particles can be both solid and liquid and are collectively referred to as ‘atmospheric aerosol particles.’ Most are produced by natural processes such as erupting volcanoes, and some are from human industrial and agricultural activities. Those particles in the lowest layer of the atmosphere, where our weather occurs, usually stay relatively close to the source of emissions and remain in the atmosphere only a few days to a week before they fall to the ground or are rained out; those higher up in the atmosphere travel farther and may linger in the atmosphere for a few years.
Light-colored aerosol particles can reflect incoming energy from the sun (heat) in cloud-free air and dark particles can absorb it. Aerosols can modify how much energy clouds reflect and they can change atmospheric circulation patterns—in short, aerosols can modify our climate.
Several climate engineering (so-called ‘geoengineering’) strategies for reducing global warming propose using atmospheric aerosol particles to reflect the sun’s energy away from Earth. Because aerosol particles do not stay in the atmosphere for very long—and global warming gases stay in the atmosphere for decades to centuries—accumulated heat-trapping gases will overpower any temporary cooling due to short-lived aerosol particles.
Aerosol sources, composition, and removal processes
Worldwide, most atmospheric aerosol particles are produced by ‘natural’ processes such as grinding and erosion of land surfaces resulting in dust, salt-spray formation in oceanic breaking waves, biological decay, forest fires, chemical reactions of atmospheric gases, and volcanic injection. Some particles, on the other hand, have human origins—industry, agriculture, transport (including aviation), and construction. The composition of atmospheric aerosol particles varies widely depending on their source—they may contain salts (predominantly sulfates), minerals (such as silicon), organic materials, and, in most cases, water.
The particles grow by absorbing water vapor and other gases. If the relative humidity is sufficiently high (usually about 80 percent or more), tiny water drops can form on some of the particles. A subset of these, called ‘cloud condensation nuclei,’ then grow into cloud drops, which eventually fall to the surface as rain or snow, depositing the particles on land or in the ocean. At higher altitudes, cloud ice particles form on some insoluble particles, such as dust.
Although dust plumes from the Sahara and Gobi deserts can be seen circling most of the globe in satellite pictures, aerosol particles in the lower troposphere (the lowest layer of the atmosphere where weather occurs) are usually removed from the atmosphere by settling and precipitation within several days to weeks after they were produced. Their impacts, then, are fairly localized. In the stratosphere (the atmosphere layer above the troposphere), chemical reactions of gases from volcanoes produce sulfate particles that can remain for one or more years, spreading over much of the globe.
Aerosol particles and climate
Although we are familiar with local particulate ‘air pollution’ due to human activities, the fact that atmospheric particles of both natural and human origin have substantial influence on our climate is less widely understood. The particles can play important climatic roles both outside and inside clouds.
In clear air, particles of sizes of approximately 0.1 to 1 micron (comparable to the dominant wavelengths in the solar spectrum) interact with the solar beam. Particles containing little carbon are effectively ‘white.’ They reflect solar radiation, making the air and Earth surface below them a bit cooler than they would otherwise be. Sulfate particles in the stratosphere from the Pinatubo volcanic eruption in 1991, for example, produced measurable cooling for two years over much of the globe (see global temperature figure in the human role FAQ). In contrast, particles containing substantial amounts of black carbon (e. g., soot, which is typically produced from combustion of fossil fuels, biofuels, and biomass burning) warm their surroundings by absorbing solar radiation before it reaches the ground.
When water vapor clings to water soluble particles in the same size range (~0.1 to 1 micron) it creates cloud droplets in the lower troposphere. In clean air, the concentrations of these droplets range from 10 to several 100 per cubic centimeter. At lower temperatures certain aerosol particles facilitate the formation of cloud ice. In and near urban areas, where the concentration of aerosol particles is high, the concentration of droplets can be as high as several thousand per centimeter cubed. The increased number of little drops causes the reflectivity of clouds to increase, so that, seen from above, clouds near polluted areas are often brighter than those above cleaner regions. Water droplets and ice particles are basically white, so they reflect solar radiation; on the other hand, the condensed water also traps and emits long wave radiation, producing heat. Thus clouds can have either cooling or warming effects on a local area, depending on whether the reflecting or trapping effect is strongest.
Because of many unknowns relating to aerosol particles, and in particular, to the possible effects of particles on cloud stability, the magnitude of aerosol impacts on climate remains among the most uncertain factors in climate projections.
How is human-caused air pollution changing our climate?
Human-caused particulate air pollution has a relatively minor—and likely decreasing—impact on our climate. Since aerosol particles of human origin both reflect and absorb solar energy as the solar beam travels down through the atmosphere, these particles can diminish the energy that arrives at the Earth’s surface as heat. Scientists estimate that particles produced by human activities have led to a net loss of solar energy (heat) at the ground (by as much as 8 percent in densely populated areas) over the past few decades. This effect, sometimes referred to as ‘solar dimming,’ may have masked some of the late 20th century global warming due to heat-trapping gases.
Human activities that result in production of both reflecting and absorbing aerosol particle have been curtailed by legislation and modern technology in many places. The ‘pea soup fogs’ that so bedeviled London in Sherlock Holmes’ day, for example, were caused by particles produced by incomplete combustion of coal. These ‘fogs’ are now a thing of the past, thanks to mandatory scrubbers and other advanced combustion techniques. Clean air regulations in the United States have also decreased particle concentrations considerably. Even today, though, haze clouds seen over urban regions give dramatic proof of the effects of human-induced particles in the United States, while atmospheric soot production is still very high in many parts of Asia.
Global warming is primarily caused by emissions of too much carbon dioxide (CO2) and other heat-trapping gases into the atmosphere when we burn fossil fuels to generate electricity, drive our cars, and power our lives. These heat-trapping gases spread worldwide and remain in the atmosphere for decades to centuries. Thus, as we continue to emit these gases, their atmospheric concentrations build up over time. In contrast, atmospheric aerosol particles are largely localized near their sources, and do not linger in the atmosphere for long so that, even if we continue to emit them at current rates, their atmospheric concentrations will not build up markedly over time. Thus the effect of long-lived global warming emissions will far outweigh the cooling effect of short-lived particles.
Can climate engineering with aerosols save us from global warming?
Because global warming is such a serious threat, some scientists and engineers have explored the idea of harnessing the reflective power of some aerosol particles to temporarily combat global warming while non fossil fuel energy sources are being more fully developed. The idea is to artificially increase the concentrations of ‘white’ atmospheric aerosol particles above the surface of the ocean and/or in the lower stratosphere in order to reflect more of the sun’s energy away from Earth. The field of climate engineering (so-called ‘geoengineering’), still in its infancy, has the potential to buy us some time in the attempt to maintain relatively slow warming rates. However, experimentation with our very complex climate system by dramatically increasing reflecting aerosols carries with it the potential for large unintended, and potentially dangerous side effects on ecosystems, agriculture, and human health. In particular, the human health consequences associated with further increases in particulate pollution are a grave concern. For example, efforts are underway to reduce harmful black carbon emissions in developing countries through improved cook stoves to improve human health as well as curb global warming.