Research Highlight: Clouds of Mystery

Like smoke signals issued from a distant fire, huge puffs of dust and particulate pollution have spent the month of May traveling across the Pacific Ocean from Asia to North America in bursts every three or four days.

For the first time, scientists from Scripps Institution of Oceanography at UC San Diego and elsewhere have flown out to meet these plumes to take an unprecedented look at how they travel across the Pacific Ocean into North America and influence climate in the Pacific Ocean region. In a field campaign called the Pacific Dust Experiment, or PACDEX, the researchers are using a highly specialized aircraft capable of long-range flights that can span altitudes ranging from near sea level to the lower stratosphere.

“This dust plume is part of a massive spring migration that’s been happening before Genghis Khan’s time, but what is more recent is that this dust plume is mixed with plumes of man-made particles such as soot, sulfates, and ash among other pollutants,” said V. Ramanathan, a Scripps climate and atmospheric sciences professor who originally proposed the experiment and serves as science director and co-principal investigator of PACDEX. “The interaction of the dust-soot layers with the Pacific cloud systems has potentially large consequences to global and North American climate change.”

The aircraft, a modified Gulfstream-V with a range of more than 6,000 miles, carrries an array of instruments that enable scientists to both measure clouds and bring dust, pollutants, and cloud particles into the aircraft for study. For example, the researchers have captured ice particles from clouds for analysis. They evaporate them, study the residue and then try to recreate the particle in a special moistened chamber to mimic the temperature and moisture conditions that enabled the original ice particle to form.

Flight transects, using airports in Japan, Alaska and Colorado as hubs, were scheduled to take place through May 25. They featured a May 5 flyover from Alaska toward the eastern Pacific Ocean in which Ramanathan, as the mission director, monitored dust and soot layers from east Asia.

The dust plumes can be hundreds of miles across and are among the largest atmospheric movements of solid material in the world. In an online exchange with reporters on May 15, Ramanathan said he had been able to observe how yellow dust clouds rising from the deserts of Mongolia turned brown as the dust moved eastward and mingled with smoke generated by biomass burning and particulate pollution from Asian cities.

The Asian plumes are one part of a worldwide transport of particles that continually take place, but most prominently in spring in the Northern Hemisphere. Dust and particulate pollution from American sources might fall on other parts of North America or remain aloft long enough to reach Europe.

Atmospheric and ocean dynamics in the Pacific Ocean region influence climate around the world. Because of these global impacts and because of the plumes’ large size, the phenomenon is especially important to understand. At levels of 15 kilometers (50,000 feet) or greater, particles in the plumes can remain aloft for more than a week. In that time, they can influence weather phenomena over the Pacific Ocean such as storm tracks and cloud formation. The plumes, containing aerosols ranging from sulfates to the black carbon found in soot, can have dimming effects at the ocean surface and both cooling and warming effects at stratospheric altitudes.

“By focusing on these plumes, PACDEX will shed light on one of the major environmental issues of this decade,” said Ramanathan.

Scripps is working on PACDEX in conjunction with the National Center for Atmospheric Research (NCAR) and more than a dozen other institutions from the United States, China, Japan, and Korea. NCAR's main sponsor, the National Science Foundation (NSF) owns the Gulfstream-V and is providing most of the funding for PACDEX.

— Robert Monroe