Air pollution occurs when the air contains gases, dust, smoke from fires, or fumes in harmful amounts. Tiny atmospheric particles - aerosols - are a subset of air pollution that are suspended in our atmosphere.
Aerosol can be both solid and liquid. Most are produced by natural processes such as erupting volcanoes, and some are from human industrial and agricultural activities.
Aerosols have a measurable effect on climate change. Light-colored aerosol particles can reflect incoming energy from the sun in cloud-free air and dark particles can absorb it. Over the historic period, the net effect globally was for aerosols to partially offset the rise in global mean surface temperature. Aerosols can modify how much energy clouds reflect and they can change atmospheric circulation patterns.
Black carbon aerosols, similar to the soot in a chimney, absorb sunlight rather than reflecting it. This warms the layer of the atmosphere carrying the black carbon, but also shades and cools the surface below.This portrait of global aerosols was produced by a GEOS-5 simulation at a 10-kilometer resolution. Dust (red) is lifted from the surface, sea salt (blue) swirls inside cyclones, smoke (green) rises from fires, and sulfate particles (white) stream from volcanoes and fossil fuel emissions
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. In moist air, clouds form when water vapor condenses onto these ‘cloud condensation nuclei’. These then grow into cloud drops, which eventually fall to the surface as rain or snow, depositing the particles on land or in the ocean.
A number of atmospheric probes are installed along the fuselage of NASA's DC-8 in preparation for the SEAC4RS study to learn more about how air pollution and natural emissions affect climate change.
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. 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, tiny aerosol particles interact with the solar beam. Particles containing little or no 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. 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. Since black carbon reflects the incoming sunlight, it also acts a shade and the ground surface below becomes cooler.
Black carbon aerosols, similar to the soot in a chimney, absorb sunlight rather than reflecting it. This warms the layer of the atmosphere carrying the black carbon, but also shades and cools the surface below.
These tiny particles also create cloud droplets in the lower troposphere. 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 the altitude of the cloud and whether the reflecting or trapping effect is strongest.
Because of many unknowns relating to aerosol particles, the magnitude of aerosol impacts is one of the major advancements in understanding that occurred between the fourth and fifth IPCC climate assessments. Particularly noteworthy is the higher confidence regarding aerosol radiation interactions and volcanic aerosols. NASA currently has several aerosol monitoring sites across the world, whose data are used for better understanding of climate and air quality, and for better pollution management.
