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ScienceWeek
ATMOSPHERIC SCIENCE: ON THE NUCLEATION OF AEROSOL PARTICLES
The following points are made by Markku Kulmala (Science 2003 302:1000):
1) Aerosol particles are ubiquitous in Earth's atmosphere. They influence the quality of life in many ways: for example, through their climatic and health effects and by affecting visibility. Better understanding of these effects, especially their role in climate change, requires knowledge of the mechanism by which new particles nucleate and grow in the atmosphere.
2) Particle nucleation in the atmosphere has exercised the minds of scientists since John Aitken (1839-1919) built the first apparatus to measure the number of dust and fog particles in the late 19th century. Today, size distributions of nanometer-scale particles and concentrations of gases participating in particle nucleation can be measured directly in the atmosphere (1). However, although several mechanisms for particle nucleation have been proposed (2-4), it remains unclear which of them dominates in the atmosphere.
3) In recent years, the nucleation and growth of nanometer-scale atmospheric aerosol particles have been observed with state-of-the-art technology in many different atmospheric environments, including the lower stratosphere, the free troposphere, the continental boundary layer just above Earth's surface, and coastal environments (5). Some continental sampling sites can serve as natural laboratories. For example, at the SMEAR I station in Lapland, clean Arctic air is frequently replaced by polluted plumes from the Kola Peninsula, allowing different conditions for particle nucleation and growth to be studied in one location.
4) Particles typically form during the late morning and then grow throughout the day, reaching growth rates of 1 to 20 nm per hour. In the boundary layer, measured nucleation rates of particles with a diameter of 3 nm typically lie between 0.01 and 10 particles per cubic centimeters per second during regional particle nucleation events. In contrast, rates as high as 104 to 105 particles per cubic centimeters per second have been reported in coastal environments and industrial plumes (5).
5) Atmospheric aerosol nucleation is tied strongly with chemistry, particularly for sulfuric acid (2-4) and other condensable gases (vapors) of low volatility, such as organic species in the continental boundary layer or iodide compounds in coastal environments. However, existing aerosol particles act as a sink for these vapors and for nucleated clusters, thus inhibiting the nucleation of new atmospheric particles. Nucleation seems to be also affected by the magnitude of solar radiation (3,4) and by atmospheric mixing processes, for example, in the continental boundary layer or at the boundary between the stratosphere and the troposphere.
6) Four main nucleation mechanisms have been proposed. Homogeneous nucleation involving binary mixtures of water and sulfuric acid may, for example, occur in industrial plumes. Homogeneous ternary (water-sulfuric acid-ammonia) nucleation may take place in the continental boundary layer. Ion- induced nucleation of binary, ternary, or organic vapors may be found in the upper troposphere and lower stratosphere. Finally, homogeneous nucleation involving iodide species may occur in coastal environments. All of these mechanisms have been observed in laboratory experiments. However, concentrations used in such studies are typically far above those in the atmosphere.
References (abridged):
1, R. J. Weber et al., J. Atmos. Sci. 52, 2242 (1995)
2. F. Yu, R. P. Turco, J. Geophys. Res. 106, 4797 (2001)
3. S.-H. Lee et al., Science 301, [1886] (2003)
4. M. Kulmala, L. Pirjola, J. Mäkelä, Nature 404, 66 (2000)
5. M. Kulmala et al., J. Aeorosol Sci., in press
Science http://www.sciencemag.org
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CLIMATE EFFECTS OF BLACK CARBON AEROSOLS IN CHINA AND INDIA
The following points are made by S. Menon et al (Science 2002 297:2250):
1) China has been experiencing an increased severity of dust storms, commonly attributed to overfarming, overgrazing, and destruction of forests (1). Plumes of dust from north China, with adhered toxic contaminants, are cause for public health concern in China, Japan, and Korea, and some of the aerosols even reach the United States (2). Recent dust events have prompted Chinese officials to consider spending several hundred billion yuan (~$12 billion) in the next decade to increase forests and green belts to combat the dust storms (3).
2) Such measures may be beneficial in any case. However, the authors suggest that the observed trend toward increased summer floods in south China and drought in north China (4), thought to be the largest change in precipitation trends since 950 A.D.(4), may have an alternative explanation: human-made absorbing aerosols in remote populous industrial regions that alter the regional atmospheric circulation and contribute to regional climate change. The authors suggest that if interpretation is correct, reducing the amount of anthropogenic black carbon aerosols, in addition to having human health benefits, may help diminish the intensity of floods in the south and droughts and dust storms in the north.
3) Similar considerations may apply to India and neighboring regions such as Afghanistan, which have experienced recent droughts. Atmospheric aerosols, which are fine particles suspended in the air, comprise a mixture of mainly sulfates, nitrates, carbonaceous (organic and black carbon) particles, sea salt, and mineral dust. Black (elemental) carbon (BC) is of special interest because it absorbs sunlight, heats the air, and contributes to global warming (5), unlike most aerosols, which reflect sunlight to space and have a global cooling effect. BC emissions, a product of incomplete combustion from coal, diesel engines, biofuels, and outdoor biomass burning, are particularly large in China and India because of low-temperature household burning of biofuels and coal (9).
4) In summary: In recent decades, there has been a tendency toward increased summer floods in south China, increased drought in north China, and moderate cooling in China and India while most of the world has been warming. The authors used a global climate model to investigate possible aerosol contributions to these trends. The authors found precipitation and temperature changes in the model that were comparable to those observed if the aerosols included a large proportion of absorbing black carbon ("soot"), similar to observed amounts. Absorbing aerosols heat the air, alter regional atmospheric stability and vertical motions, and affect the large-scale circulation and hydrologic cycle with significant regional climate effects.
References (abridged):
1. H. W. French, New York Times, 14 April 2002, p. 3
2. R. B. Husar, et al., J. Geophys. Res. 106, 18317 (2001)
3. Reuters, http://in.news.yahoo.com/020514/64/1o02h.html 14 May 2002.
4. Q. Xu, Atmos. Environ. 35, 5029 (2001)
5. J. Hansen, M. Sato, R. Ruedy, A. Lacis, V. Oinas, Proc. Natl. Acad. Sci. U.S.A. 97, 9875 (2000)
Science http://www.sciencemag.org
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AEROSOLS, CLIMATE, AND THE HYDROLOGICAL CYCLE
The following points are made by V. Ramanathan et al (Science 2001 294:2119):
1) One of the most visible impacts of human activities is the brownish haze that pervades many industrial regions as well as the rural areas of the tropics and the subtropics that are subjected to heavy biomass burning. Long-range atmospheric transport transforms this haze into a regional-scale aerosol layer. Well-known examples are the Arctic haze, the Indo-Asian haze, the east Asian dust and haze traveling across the Pacific, and the biomass burning and dust plumes from North Africa (Sahara and Sahel regions) that spread over most of the subtropical Atlantic. Unlike the long-lived greenhouse gases, which are distributed uniformly over the globe, aerosol lifetimes are only a week or less, resulting in substantial spatial and temporal variations with peak concentrations near the source.
2) On account of the large spatial and temporal variability of these aerosols, remote sensing from satellites delivers the most reliable information about global aerosol distributions. The measurable quantity from space is the "aerosol optical depth" (AOD), which is derived from the solar radiation reflected to space. The AOD is the vertical integral of the aerosol concentration weighted with the effective cross-sectional area of the particles intercepting (by scattering and absorption) the solar radiation at the wavelength of interest. The globally and annually averaged value of AOD (at 0.55 microns wavelength) is approximately 0.12 (+- 0.04). Anthropogenic sources contribute almost as much as natural sources to the global AOD. Anthropogenic aerosols are typically in the submicrometer-to micrometer-size range and are composed of numerous inorganic and organic species falling under four broad categories: sulfates, carbonaceous aerosols [black carbon and organic carbon, dust, and sea salt. Global anthropogenic emissions of sulfates, organics, and black carbon even exceed natural sources. Such a large perturbation of the global aerosol loading is a major environmental concern.
3) The authors suggest that in addition to human-made aerosols enhancing scattering and absorption of solar radiation, they also produce brighter clouds that are less efficient at releasing precipitation. These in turn lead to large reductions in the amount of solar irradiance reaching Earth's surface, a corresponding increase in solar heating of the atmosphere, changes in the atmospheric temperature structure, suppression of rainfall, and less efficient removal of pollutants. These aerosol effects can lead to a weaker hydrological cycle, which connects directly to availability and quality of fresh water, a major environmental issue of the 21st century.
References (abridged):
1. J. E. Penner et al., in Climate Change 2001: The Scientific Basis [Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), Cambridge Univ. Press, Cambridge, 2001], pp. 289-348.
2. J. Haywood and O. Boucher, Rev. Geophys. 38, 513 (2000)
3. Y. J. Kaufman, et al., J. Geophys. Res. 102, 17051 (1997)
4. W. D. Collins, et al., J. Geophys. Res. 106, 7313 (2001)
Science http://www.sciencemag.org
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ANTHROPOGENIC ATMOSPHERIC AEROSOLS AND GLOBAL CLIMATE CHANGE
The following points are made by S.E. Schwartz and P.R. Buseck (Science 2000 288:989):
1) Most considerations of global climate change caused by human activities have focused on the warming influence of greenhouse gases. However, aerosols are another important atmospheric constituent that influences climate and that has been affected by human activities. In general, aerosol particles increase scattering and absorption of shortwave (solar) radiation, increase cloud reflectance, enhance cloud lifetimes, and suppress precipitation. These phenomena are all thought to exert a cooling influence on climate. Recent data indicate that anthropogenic aerosols reduce cloud droplet size and suppress precipitation downward of major urban areas and industrial facilities, which is consistent with earlier hypotheses.
2) The influences of aerosols on climate are more complex than those of greenhouse gases. Bulk aerosol composition is highly variable spatially and temporally because of different sources and production mechanisms and short atmospheric residence times (from less than a day to more than a month). Particles sizes range from nanometers to microns, and within the same size class, particles can exhibit widely different compositions and morphologies, with different constituents present within the same particle (e.g., 10 nanometer carbon spherules can be found embedded within much larger sulfate particles). The inhomogeneities in properties and geographical distribution of aerosols make it difficult to characterize their influences on climate and to represent these influences in models.
3) Recent analysis of the consequence of absorption of shortwave radiation by aerosols indicates that the heating of the atmosphere can evaporate clouds. Clouds exert both cooling and warming influences on climate: cooling in the shortwave (because of their reflectance), and warming in the longwave (because of absorption and re-emission of thermal infrared radiation). The shortwave component dominates, so a reduction in cloud coverage would result in a net warming influence.
4) The authors conclude: "Recent studies demonstrate both the importance of aerosol effects on climate and the complexity of aerosol-cloud interactions. Unfortunately for those would like a quick and accurate assessment of anthropogenic climate forcing over the industrial period, the studies also demonstrate that there is much to be learned before such an assessment can confidently be given."
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