Scientists from the United States and France are beginning to tease out how much of the aerosol pollution in the atmosphere is caused by humans, and how much comes from natural sources. The precise global maps produced by the researchers could help clarify the role that human pollution plays in the world’s weather and climate.
The researchers are using precise new satellite measurements and sophisticated new computer models to produce global maps of different types of air pollutants. In a review paper in the current issue of the journal “Nature,” the team reports that these global maps are an important breakthrough in the science of determining how much aerosol pollution comes from human activities.
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One way to discriminate between natural sources of aerosols and human made sources is to look at the location of aerosol plumes. Natural aerosols like salt particles from sea spray are typically widespread over larger areas and not particularly concentrated downwind of urban areas. Or, they are particularly concentrated downwind of obviously natural sources, such as the streams of dust originating from the Sahara Desert.
In general, smaller particles are more likely to be produced by human activities, while larger particles are more likely to have natural origins. To measure the size of different aerosol particles required the researchers to learn how aerosols reflect sunlight at key wavelengths of the solar spectrum.
Aerosol plumes comprised of smaller particles— less than one micrometer in diameter—reflect light at shorter wavelengths much more strongly than plumes comprised of larger particles—greater than one micrometer—which scatter and reflect light roughly equally at short and long wavelengths.
New instruments flying aboard NASA’s Terra and Aqua satellites can, for the first time ever, measure precisely the sunlight reflected by aerosols back to space every day over almost the entire planet at wavelengths spanning across the solar spectrum (from 0.41 to 2.2 micrometers).
The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard the NASA satellites read the light scattering data from different aerosol plumes. The researchers can use this data to show the size of the particles found in different areas.
The team then combines the particle size data with the information gathered about land uses and fire activities in advanced new computer aerosol models, producing maps of likely human pollution and probable natural aerosols. Aerosols produced by humans appear in punctuated bursts of thick and concentrated plumes comprised of small particles. They can also be found concentrated downwind of regions altered by human activities, such as deforested regions.
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Aerosols are tiny solid or liquid particles (1 µm to 0.001 µm in diametre), suspended in the atmosphere. There are a fraction of these aerosols that are hygroscopic i.e., they attract and absorb water molecules. These are called Cloud Condensation Nuclei (CCN). Natural sources include salt particles from sea spray, dust and clay particles as a result of weathering of rocks, both of which are carried upward by the wind. Aerosols can also originate as a result of human activities and are often considered pollutants.
Aerosols are important in the atmosphere as nuclei for the condensation of water droplets and ice crystals, as participants in various chemical cycles, and as absorbers and scatters of solar radiation, thereby influencing the radiation budget of the earth’s climate system.
They have been extensively studied because of the adverse health effects and poor visibility they cause. Once the aerosols are formed, they contribute to the background particulate matter concentrations affecting human health through respiratory effects.
Another new study places plankton, kelp and seaweed alongside cars, factories and planes as sources of aerosols that may affect Earth’s climate. Investigators from Ireland, Finland, Germany and the United States found sunlight can convert organic iodine vapors emitted by the aquatic organisms into secondary aerosols in sea air.
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If the transformation occurs on a large scale, it could significantly influence climate. “The discovery of a previously unrecognized source of aerosol particles is big news to atmospheric scientists,” said Charles Kolb, president of Aerodyne Research, Inc., in Billerica, Massachusetts.
Marine aerosols and their cloud-forming component can regulate climate by reflecting the Sun’s rays. Changes in the activities of marine biota can trigger a chain reaction altering emissions of iodine vapors, which in turn switches Earth’s “heat-shield,” said lead author Colin O’ Dowd, professor of physics at the National University of Ireland in Galway.
Depending on their makeup, aerosols can affect Earth’s “radioactive” balance by absorbing incoming solar radiation and leading to cooling, or warming. In less direct fashion, the particles can stimulate cloud production by inducing droplet formation in a cooling air mass.
“The smaller droplets produce brighter clouds, which might also be longer, lived because they are less likely to precipitate as rainfall,” Kolb said, “Indeed, certain observations indicate that aerosols from forest fires and urban pollution can suppress rain and snow fall.”
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The aerosol radiative effect can be direct or indirect. The direct effect is due to scattering and adsorption of solar radiation. The indirect effect comes from the action of CCN and from their participation in heterogeneous chemical reaction (HCR). Their action as CCN can increase the occurrence of clouds and change their properties.
The participation of aerosol particles in HCR can affect the concentration of trace species in the atmosphere, such as greenhouse gases and stratospheric ozone depleting compounds (stratospheric ozone loss has been estimated to have a cooling effect). CCN are numerous, typically between 100 (marine environment) to 100,000 (dirty continental air) per cubic metre.
Aerosol particles that consists of compounds of sulphur formed by the interaction of sulphur dioxide and sulphur trioxide with other, compounds in the atmosphere. Sulphate aerosols are injected into the atmosphere from the combustion of fossil fuels and the eruption of volcanoes like Mt. Pinatubo. Recent theories suggest that sulphate aerosols may lower the earth’s temperature.
The work builds on previous field measurements along the coast of Ireland that pointed to a source of new marine aerosols not accounted for by known mechanisms or models. In the new study, O’Dowd and his colleagues reproduced coastal conditions in a state-of-the-art atmospheric smog chamber at the California Institute of Technology in Pasadena, California. Using a suite of advanced instruments that measure particle sizes and numbers, they traced copious numbers of aerosol particles to minute, concentrations of the iodine compound, well within levels often present along the coast.
Aerosols come in two main forms. Primary aerosols, made infamous by the likes of smoke from fires, soot and ash from factories, emissions from motor vehicles, boats and planes and airborne dust, spew their poison directly into the atmosphere. Secondary aerosols, generated from gaseous pollutants in exhaust outpourings and emissions from land vegetation and marine organisms, arise in the atmosphere itself.
Photochemical processes in urban smog are known to produce high levels of such secondary particles. Over the oceans, the main chemical source of these aerosols was thought to be the co-condensation of sulphuric acid vapor and water vapor. Now, it seems that iodine oxides may also belong to the list.
A recent study published in the Journal Science suggests that large amounts of black carbon or soot particles and other pollutants are causing changes in precipitation and temperatures over China. The researchers explained that black carbon can affect regional climate by absorbing sunlight, heating the air, and altering large scale atmospheric circulation and the hydrological cycle.
Black carbon or soot is generated from industrial pollution, traffic, outdoor fires and household burning of coal, wood and other biomass fuels. Soot is produced when these fuels are not burned completely. China and India both produce large amounts of soot because much of their cooking and heating is done with wood, agricultural trash, cow dung and coal, at a low temperature that does not allow for complete combustion.
These dark soot particles absorb sunlight, heating the air and reducing the amount of sunlight reaching the ground. The heated air makes the atmosphere more unstable, creating rising air or convection, which forms clouds and brings rainfall to heavily polluted regions.
When soot blocks the sun’s energy from reaching the ground, it can also reduce crop yields. The increase of rising air in southern China is balanced by an increase of sinking air or subsidence in northern China. When air sinks, clouds and rain cannot form, creating dry conditions.
In recent years, northern China has suffered from an increased severity of dust storms, while northern China has had increased rainfall that is thought to be the largest change in precipitation trends since the year 950 A.D. The scientists believe that human made sunlight absorbing soot particles may be responsible for these changes.
As soot heats the lower atmosphere over China some of this warm air can get transported to the other regions of the world, causing surface warming in distant locations. But the roles that soot plays in global climate change may be much more complex than this. A research paper presented by William Chameides and Michael Bergin of the Georgia Institute of Technology, the differences between black carbon soot and greenhouse gases have been pointed out.
For instance, soot particles are removed from the atmosphere on time scales of weeks to months, while carbon dioxide lingers for hundreds of years. The authors argue that this finding could point toward a better near term control strategy for global warming than attempts to reduce carbon dioxide emissions.
Little is known about the worldwide impact of soot emissions or even how best to measure them. A key uncertainty is the amount of soot going into the atmosphere. Localized studies in China and India, where crop wastes are burned for heating and cooking, show very high levels. In developed nations, elevated soot levels are found in urban areas, which have often been excluded from climate studies to avoid confusing global climate change with local urban heat island effects.