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Plumes across the Pacific

PACDEX offers new range of insight into Asian aerosols

by Carol Rasmussen

This spring, scientists mounted a far-reaching field project to study plumes of airborne dust and pollutants that originate in Asia and journey to North America. The plumes are among the largest such events on Earth and they affect clouds and precipitation across the Pacific Basin. The Pacific Dust Experiment (PACDEX), led by scientists at NCAR and Scripps Institution of Oceanography, ran from 29 April to 24 May. During the project, scientists observed plumes at various points across the Pacific Ocean between Japan and the West Coast of the United States.

The experiment focused on how the plumes affect the development of clouds and precipitation, the intensity of storms moving across the Pacific, the amount of sunlight that reaches Earth, and the role that these plumes might play in affecting our weather and climate.

"PACDEX comes at a crucial time in our efforts to understand the regional impacts of global warming," says V. Ramanathan, a PACDEX principal investigator based at Scripps Institution of Oceanography. "The data will shed light on one of the major environmental issues of this decade."


This NASA satellite image, taken on 30 April 2005, shows a plume of dust flowing from China to the north of the Korean Peninsula and over the Sea of Japan. The dust almost completely obscures the island of Honshu from satellite view. Such plumes can cross the Pacific and scatter dust across the western United States. (Image courtesy the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE.)

Impacts on climate change and weather

The plumes, which originate as dust storms in the great deserts of Central Asia, pick up emissions from biomass burning, cooking fires, fossil fuel combustion, and industrial activity as they move east over more populated and industrialized regions. Some of the aerosols in the plumes, such as sulfates, cool Earth by blocking solar radiation. Such particles may currently mask up to half the global warming impact of greenhouse gases. Other particles, however, can have a warming effect. For example, black carbon absorbs sunlight both at ground level, if the particles are deposited on snow cover, and in the air, where the sunlight otherwise would have been reflected back into space.

The plumes can also alter global temperatures by interacting with large-scale midlatitude cloud systems over the Pacific, which reflect enormous amounts of sunlight and thus help regulate global climate. The plumes may affect


Jeff Stith. (Photo by Carlye Calvin.)

regional precipitation patterns by providing extra cloud condensation nuclei that increase amounts of rain or snow. In addition, the dust and pollutants reduce the amount of light reaching Earth, contributing to the phenomenon known as global dimming.

Tokyo to Boulder

The PACDEX team used the NSF/NCAR High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER), flying between Tokyo; Anchorage Alaska; and its base near Boulder. The Gulfstream-V aircraft provided an important capability for studying Asian dust: enough range to follow a plume clear across the Pacific Ocean, observing its evolution along the way. The aircraft's high-altitude capability also allowed participants to observe how the aerosols changed from the lower to the upper troposphere. Ground stations in Inner Mongolia (China), Korea, Japan, and California supplemented the aircraft data.

"We were able to find a wide variety of plumes from Asia at various locations across the ocean including a number of encounters with the plumes in large-scale storm clouds," said Jeffrey Stith, a co-investigator for the project. "The long-range capabilities of the aircraft allowed us to gain an ‘ocean-wide' perspective on how these plumes evolve on their journey between Asia and the United States and how they interact with weather systems. This was exactly what we wanted accomplish when we planned the project."

In addition to NCAR and Scripps, the international research team comprised scientists from North American and Asian institutions, including NASA; NOAA; the Naval Research Laboratory; the universities of Alaska, Colorado, and Iowa; Arizona State, Colorado State, and Oregon State universities; the National Autonomous University of Mexico; the Japanese National Institute for Environmental Studies; China's Lanzhou and Peking universities; and South Korea's Seoul National University.


V. Ramanathan. (Photo by Carlye Calvin.)

Early results

The researchers discovered that the plumes may be quite stratified. "We found pollution layers in very thin vertical regions, which may be one of the reasons why it is challenging to model these effects. Some layers are only a few hundred feet thick," Stith says. "Another interesting thing we observed was layers overlying other layers that had significantly different characteristics, such as the humidity threshold where they transform into cloud droplets."

Ramanathan notes that the team found black carbon at very high altitudes. "That worries me greatly, because the higher the aerosols are, the longer their lifetimes in the atmosphere and the greater their impacts on climate," he says, adding that a black carbon aerosol 10 kilometers high in the atmosphere may have a climate warming factor two to three times greater than a black carbon aerosol at 1 km. The team also saw dust and black carbon occurring simultaneously at the higher altitudes. "Theory suggests that this combination will enhance the warming effects of the black carbon even more," Ramanathan says.



Satellite observations of a particularly large dust plume moving from Asia to North America in 2001. The image depicts aerosol optical depth, which shows how much light in a column of the atmosphere is blocked by airborne particles. The purple and blue areas represent no or little dust in the atmosphere; the yellow and orange areas represent moderate to high amounts of dust. The observations were taken by the Moderate Resolution Imaging Spectroradiometer (MODIS), flown aboard NASA's Terra satellite. (Image courtesy NASA.)


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