September 9, 2008 Human beings blow vast quantities of aerosols into the air with their cars, power plants and heating systems. Fires set to clear forests also release these floating particles which, in some cases, measure just a few thousandths of a millimeter, or even less. Before humans had any impact, the aerosol load in air over land was only double that in the air over the sea. However, today, the former can amount to one hundred times more than the latter. There is no doubt that natural and human-made aerosols have an impact on our climate. But what effect do they have, exactly? Some say they lead to more clouds and more precipitation. Others say they mean fewer clouds and less precipitation. "Both sides are right," says Meinrat Andreae, Director at the Max Planck Institute for Chemistry in Mainz, "but it depends on the number of particles. This is what determines how the energy needed to evaporate water and transport air is distributed." Background Aerosols are created in natural and human processes. Aerosol particles can consist of sea salt, sand grains, soot particles, sulfates and other materials of organic and inorganic origin. The natural processes include volcanic eruptions, the occasional forest fire, sandstorms, and breaking ocean waves, while traffic, forest clearance by fire, changes in land use and industrial emissions are the major human sources. Not all aerosol particles can act as condensation nuclei; among other things, this depends on whether they are water-soluble and how large they are. Clean air over land contains typically around 2,000 particles per cubic centimeter. Polluted air over land contains around 10,000; air over a city can contain up to 100,000 particles per cubic centimeter. In pure oceanic air, the value is around 500. The cleanest air is over the Antarctic plateau, with readings of only 43 particles per cubic centimeter. By contrast, moderate concentrations of aerosols delay the fall of rain, as initially less water evaporates, and more and therefore lighter drops form. These rise again and reach altitudes where the atmosphere is so cold that they freeze. This releases heat, just as condensing does. The air becomes warmer and can continue to rise. The moisture bound to the aerosols transports energy to where larger clouds form. This stimulates the circulation of the atmosphere and more rain can fall, possibly even as hail. This precipitation can also be transported over quite large distances as clouds do not rain immediately, but first mature. Moderate concentrations of aerosols cause the highest rainfall, as well as extreme events and storms, since the energy for forming clouds and circulation is at a maximum. Where concentrations of aerosols are very high, the sun umbrella effect and the cloud processes weaken the atmospheric circulation. On the one hand, less water evaporates. On the other, there are so many aerosol particles that the small amount of moisture is very thinly spread: only tiny rain drops and powdery ice crystals are created. As the microdrops and the powdery crystals are too light to fall, they evaporate after a while. In the process, they extract the heat from the air that they released when they condensed and froze. This brings the circulation to a standstill as the air masses can no longer further rise. The result: rain fails to materialize - and droughts become more frequent. "These results finally allow us to predict the effects of aerosols in climate models more accurately. Currently, it is the conflicting effects of the aerosols that are preventing us from making more precise predictions about the future of the climate," says Meinrat Andreae with reference to the study’s significance. ## Contact: Kirsten Achenbach Max-Planck-Gesellschaft 49-613-130-5362 achenbach@mpch-mainz.mpg.de This text derived from: http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/2008/pressRelease200809091/ Recommend this Article to a Friend Back to: News |
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