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Researchers' work sheds light on atmosphere's multi-tiered structure

Researchers at MIT and two European institutions reported in the March 25 issue of Nature that the atmosphere is more like a Napoleon pastry than a four-layer cake. Consequently, current systems that track weather and pollution may not "see" important fragments of the atmosphere that are less than two kilometers thick.

The researchers from MIT, France and Germany said that new data about water vapor and ozone--collected from instruments stowed in a handful of commercial airplanes--suggest that about 15 percent of the troposphere is stratified, which is more than was previously believed.

The models used for weather forecasting, investigating the time evolution of airborne chemicals and studying climate change may need a resolution of two-tenths of a kilometer--about the length of two football fields--or better, which is much higher than present-day models, they said.

"These results imply that models used for forecasting weather and pollution transport need to improve their vertical resolution so that they can see the thinner layers" of the atmosphere, said Professor Reginald E. Newell of earth, atmospheric and planetary sciences. The other authors from MIT are visiting scientist Valerie Thouret of the Laboratoire d'Aerologie in Toulouse, France; research scientist John Y. N. Cho; and former graduate Patrick Stoller, now at Lawrence Livermore National Laboratory.

Combining data from the commercial aircraft with that of research flights, the researchers found that there are four types of layers that occur in the troposphere (the lowest 10 to 18 kilometers of the atmosphere), depending on the season and the latitude. "We suggest that this universality reflects basic, previously unexplored characteristics of the atmosphere, with potential implications for furthering our understanding of a wide variety of atmospheric processes," the authors wrote.

The atmosphere is divided into four major layers related to differences in the temperature gradient. The thick troposphere, closest to Earth, where the temperature decreases with increasing altitude, is thought to be well-mixed. Weather happens and planes fly in the troposphere.

Above that is the stratosphere, where the ozone layer absorbs harmful solar rays. The mesosphere, between 50 and 80 kilometers altitude, is where meteors burn up. The thermosphere is where the space shuttle orbits.

The layers described in the study are within the troposphere, between a half and 1.3 kilometers thick and at an average altitude between 5.5 and 6.6 kilometers. The researchers identified four different combinations of ozone and water vapor that they said probably came from continental pollution, stratospheric air or pollution (the stratosphere contributes ozone to the troposphere), convection from the boundary layer, and subsiding air originally raised in deep convection over the ocean.

One can classify the layers into types by whether a layer has an excess or deficit of the trace gases, which also then gives clues about the origin of the layers, Professor Newell said. For example, a layer high in ozone and low in water vapor most likely came from the stratosphere, because that combination characterizes the air in the stratosphere.

Tropospheric ozone, created partly by pollution, is dangerous for human health. Although tropospheric ozone is increasing while scientists bemoan the decreasing ozone layer in the stratosphere, there is no way to "refill" the upper reservoir with excess ozone from lower ones.

The four types of layers are remarkably constant throughout the world and at different times of the year. "This means that there is something universal about the way these layers are formed, transported and dissipated globally," Professor Newell said.

The data will help scientists make important predictions about levels of tropospheric ozone, which can come from pollution or drift down from the stratosphere. Partly because of industrialization, ozone in the troposphere has been increasing since it was first measured around the turn of the century.

"Most developing countries are now in the tropics and Asia, areas that have typically had low levels of tropospheric ozone. We have to find out what amount of pollution we can expect from this region and how it can influence the world," Dr. Thouret said.

While scientists have used research aircraft for years to observe atmospheric trace gases like ozone, carbon monoxide, methane and water vapor, five commercial airliners belonging to Air France, Lufthansa, Austrian Air and Sabena recently have been equipped with ozone and water vapor sensors to yield a vast global coverage of these trace gas measurements. The measurements are made by a unit about the size of two large cabinets that is housed underneath and behind the cockpit. Only two sensors protrude from the fuselage.

Unlike research planes, the commercial aircraft continually traverse the planet collecting data on ozone and water vapor, as well as automatically collecting vertical profile information on landing and takeoff. The data they have been gathering since 1994 through the MOZAIC program have provided scientists with a new global coverage and a more precise picture of the makeup of the atmosphere.

By measuring trace gases in the atmosphere, researchers can learn about the structure and motion of the air. Dr. Cho said it's akin to stirring cream into a cup of black coffee.

"When you stir black coffee, it's hard to see how the liquid is moving around, because it's all the same color. When you add cream, the white streamers can tell you how the liquid is moving around inside your cup, because the cream acts as a 'tracer,'" he said.

Despite the intermittent presence of storms and turbulence that would tend to move the atmosphere up and down, trace gases often persist in thin layers, which slide around more or less horizontally. It is as though much of the atmosphere is behaving like a deck of cards that is constantly being shuffled, with certain cards moving through the pack from top to bottom and others moving from bottom to top, Professor Newell said.

The analysis work is funded by NASA's Atmospheric Chemistry Modeling and Analysis Program and NASA's Goddard Space Flight Center. Many of the research flights have also been by funded by the NASA Global Tropospheric Experiment (GTE) program, while the MOZAIC program has been funded by the European Community.

A version of this article appeared in the April 7, 1999 issue of MIT Tech Talk (Volume 43, Number 25).

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