WASHINGTON - In a polluted city, it is hard to miss the murky skies created by the tiny particles called aerosols (mostly sulfate, carbon, dust, salt, and nitrate). But, scientists have been hard-pressed to track the global behavior of aerosols, which influence climate, along with visibility and human health.
Satellites and research aircraft provide important detail, but these data are incomplete, and the aerosol pathways are difficult to discern. Now, scientists Philip Rasch, William Collins, Brian Eaton, and colleagues at the National Center for Atmospheric Research in Boulder, Colorado, have found a novel way to bring aerosol data into computer-model projections. The new technique is described in two papers appearing in the April 16 issue of the Journal of Geophysical Research, published by the American Geophysical Union.
"Regional and global exchange of aerosols is a key area of upcoming climate research," says Collins. "This technique is providing perhaps the best current estimate of aerosols available in the world."
The aerosol modeling technique has already produced surprising results from the 1999 Indian Ocean Experiment (INDOEX). Model results suggest that aerosols remain in India's dry winter atmosphere several days longer than previously thought. Since aerosols are thought to have an overall cooling effect, this finding could have global climate implications, if it proves valid in other dry regions.
These results are a consequence of the detail afforded by the new technique. Most models of global chemistry simulate the behavior of aerosols in general terms, instead of tracking their actual motions within day-to-day weather. Collins and Rasch devised a method of incorporating aerosol data into an atmospheric transport and chemistry model created at NCAR, the Scripps Institution of Oceanography and the Max-Planck Institute of Meteorology in Hamburg, Germany.
As the model's predicted weather moves forward in time, satellite data are used to adjust aerosol behavior as needed. This allows aerosol motion to be predicted up to 48 hours in advance. During the Indian Ocean Experiment, the NCAR team combined daily simulations into a three-month-long picture of aerosol transport across the region. The model is now being used to guide this spring's Aerosol Characterization Experiments--Asia, an international effort based in Japan.
Looking to data available in the future, Rasch and Collins have already expanded the technique to incorporate aerosol readings from lidar (radar-like laser) into the model, in addition to the satellite and aircraft data. Lidar can observe the aerosol prevalence at various heights, looking either upward from the ground or downward from space. Two aerosol-observing lidars will be deployed on National Aeronautic and Space Administration satellites, one later this year.
"In the middle and upper atmosphere, aerosols can travel long distances before washing or falling out, so vertical detail provided by the technique is especially important for understanding aerosol transport around the globe and its lifetime in the atmosphere," explains Rasch.
NCAR's primary sponsor is the National Science Foundation. This research was also supported by NASA.
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