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Summer 1999

INDOEX finds surprisingly dirty air

by Carol Rasmussen

Investigators in the Indian Ocean Experiment found the atmosphere over the Indian Ocean both dirtier and more complex than they expected. Although the INDOEX data, collected around the Maldive Islands in February and March, have not been released yet, some of the UCAR participants in the multinational experiment described their initial findings for the UCAR Quarterly.

A thick layer of very polluted air, extending more than 1,000 kilometers (600 miles) offshore from the Indian subcontinent, covered the ocean almost constantly during the six-week observing period. The NSF/NCAR C-130 aircraft recorded aerosol optical depths, a measure of murkiness, as high as 0.7--equivalent to a bad day in downtown Los Angeles. Visibility was often under ten kilometers. Aerosols of soot, sulfates, nitrates, organic particles, fly ash, and mineral dust made up the haze, which was accompanied by the gases carbon dioxide and sulfur dioxide. The presence of these compounds is "conclusive evidence that the haze layer is caused by pollution," according to project director Bruce Gandrud (NCAR Atmospheric Technology Division and Scripps Institution for Oceanography).

Hulule Island in the Maldives was the C-130's base during INDOEX. (Photo by Gene Martin, JOSS.)

Craig Motell (University of Hawaii), one of about 80 INDOEX participants supported by JOSS, installing a satellite antenna to receive data from NASA's Sea-viewing Wide Field-of-View Sensor (SeaWiFS) instrument. (Photo by Gene Martin, JOSS.)

"Clearly, this is global change at its worst," said V. Ramanathan (Scripps), the co-chief scientist and U.S. coordinator for INDOEX. "We can only hope that INDOEX data will have a beneficial policy impact."

Away from the haze over the Southern Hemisphere, the C-130 sampled almost completely clean air, with an aerosol optical depth of 0.1. James Moore (UCAR Joint Office of Science Support, or JOSS), the aircraft coordinator for INDOEX, noted that some researchers "got very interested in studying the interface between the much cleaner air from the Southern Hemisphere and the very polluted air coming from the north--how rapidly the chemistry changed and what a distinct line there was between the two. We were treated very well by Mother Nature in that she brought the clean air close enough to us that the scientists could study that interface a bit more than they thought they would be able to do."

Although the extent of the polluted air was a surprise, the circulation pattern that carried it out to sea is fairly well understood, according to INDOEX participant Joachim Kuettner. Kuettner holds the UCAR Distinguished Chair for Atmospheric Science and International Research, funded by NSF. He explains that in the Hadley Cell atmospheric circulation, air rises over the Southern Hemisphere ocean, flows northeast, descends over the Indian subcontinent, and moves toward the southwest. As the air descends and warms, it dries out; and with no precipitation to clean the air, it becomes dirtier and dirtier. Polluted air flowing southwest from Burma and Indochina compounds the problem. This heavily polluted low-level air is carried all the way to the Southern Hemisphere, where it meets the clouds of the intertropical convergence zone and finally dissipates.

Cloud complexities

Since one focus of INDOEX was the indirect effects of aerosols--how the particles interact with clouds--the scientists were looking forward to sampling in and around clouds. But that turned out to be more complicated than they had anticipated. Ramanathan cited "two major complexities which will have to be sorted out in the coming year or two." The first is that clouds in the region were often embedded in the band of haze, which was up to about four kilometers deep. The radiative interaction between the clouds and haze is not clear yet, according to Ramanathan; "The direct effect of aerosols in such cloudy and hazy regions has not been looked at before and needs very detailed study." The second difficulty was that cirrus clouds often overlay the low clouds, a situation that is also not well understood.

Principal investigator Bill Collins (NCAR Climate and Global Dynamics Division, or CGD) mentioned another surprise. "We had been examining cloud cover in that region using satellite observations for some time, and the satellites were always telling us the total amount of cloud was very small. That's because the clouds were smaller than the finest satellite resolution. There are a lot of tiny little cumulus clouds."

During the experiment, Collins, Philip Rasch, and Brian Eaton (both from CGD) produced forecasts of aerosols over the region using a combination of a chemical transport model and a system for assimilating satellite aerosol observations. With the data from INDOEX, they will create a three-dimensional aerosol analysis for the experiment period. "This allows you to create a large-scale picture of the aerosols that is consistent with the observations," Collins said. "We can use that to extrapolate the in situ observations to a very large region, and then we can begin computing the direct rate of forcing of aerosols over the Indian Ocean."

INDOEX principal investigator Richard Shetter (NCAR Atmospheric Chemistry Division) supplied the experiment with up- and down-looking spectroradiometers that flew on the C-130. The sum of the up- and down-looking data gives the actinic flux--all the radiation that reaches a point in the atmosphere from any direction. Teresa Campos, also of ACD, operated the instrument because Shetter and the rest of ACD's Atmospheric Radiation Investigations and Measurement Group were busy running spectroradiometers in PEM-Tropics B, a concurrent NASA experiment in the South Pacific. During INDOEX, "We saw similar things to what we'd seen in other missions in terms of enhancements of radiation around clouds," said Shetter.

"Our instrument performed well, and we got lots of data," Shetter summarized. Having the same kind of data from both tropical experiments is likely to create some synergy. "Combining the INDOEX data with the NASA data set, we're going to start establishing a climatology in what we'll see for photolysis frequencies."

A unique data set

Not only Shetter but all of the principal investigators were satisfied with their data, according to Ramanathan. Previous observations in this part of the world have been few; the last major field project in the area was the Monsoon Experiment, 20 years ago. Thus the INDOEX data will be useful for answering other questions besides the role of aerosols in climate. For example, the data may offer an increased understanding of the monsoon cycle and its connection with other global cycles such as the El Niño/Southern Oscillation. Any improvement in predicting the intensity of the monsoon would be a socioeconomic boon for the millions of inhabitants of India and Southeast Asia, as well as allowing improved global climate models and the potential for better simulation of future climate change.

JOSS is responsible for INDOEX data management, and all the data will eventually end up there. It will be accessible at a link from the JOSS Web site for field-project data. The INDOEX catalog, giving daily operations summaries, is currently on line.


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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 15:10:57 MDT 2000