Environmental Risk of Supersonic Jets Probed

By William J. Cromie

Gazette Staff

When a high-flying spy plane dove through the exhaust of a

Concorde supersonic jet flying tourists around the world, it produced a scientific surprise. The exhaust contained an unexpectedly high number of particles, a fact that bears on the protective ozone umbrella over our heads and on global warming.

The startling discovery looms as a possible monkey wrench in the plans of the U.S. and other nations to build supersonic commercial airliners that will cut hours off a trip across the Pacific Ocean.

"Many people are trying to predict the effects of exhaust gases and particles on the stratospheric ozone that protects us from harmful ultraviolet radiation from the sun," says Steven Wofsy, Gordon McKay Professor of Atmospheric and Environmental Sciences. "Computer models have been developed in laboratories and many measurements made from research aircraft, but this was the first time we obtained information about real airplanes flying in the stratosphere."

"The results have made a lot of people nervous," continues Paul Wennberg, a research associate in chemistry. "The surprising number of particles are very tiny -- millionths of an inch in size. We have no idea what they are made of, and they could alter our predictions of how the new commercial aircraft will affect ozone levels and, perhaps, global warming."

Four Hours to Tokyo

One thing Wofsy, Wennberg, and their Harvard colleagues know for certain: there have been a lot of surprises in the stratosphere and there will probably be many more. This cold, calm region of our atmosphere begins between 5 and 11 miles high, and extends up to roughly 30 miles. This transition area between the turbulent, cloudy lower atmosphere and near-space contains the ozone layer that shields us from the radiation that is implicated in skin cancer, cataracts, and crop damage and other ecological calamities.

The U.S. runs a $500 million research program to investigate the feasibility of the so-called High Speed Civil Transports (HSCT), which would pass through the lower part of the stratosphere on their way to Asia, Europe, and Australia. In one scenario, you could board a HSCT in Los Angeles in the morning and arrive in Tokyo four hours later.

Before investing billions of dollars on these swift aircraft, however, the airlines industry wants to know how the gases and particles they belch out will affect the environment.

To find out, researchers from various universities and federal agencies put instruments aboard National Aeronautics and Space Administration aircraft that fly as high as 14 miles. These aircraft, formerly called U-2s and now ER-2s, were designed for military spy missions. During the past four years Wofsy's Harvard team has flown instruments on more than 100 flights, which covered the stratosphere from the Arctic to the Antarctic at altitudes ranging from 5 to 13 miles.

One goal of these high-in-the-sky missions is to determine how long gases that rise from the turbulent lower atmosphere stay in the more stable stratosphere. The team uses measurements of carbon dioxide, which comes from smokestacks and tailpipes, to track air movements into and within the stratosphere

Their results were unexpected.

"We determined that such gases move upward in tropical areas and spread rapidly into both the Northern and Southern Hemispheres," notes Kristie Boering, a research associate in atmospheric chemistry. "They stay in the stratosphere an average of five years before descending into the middle latitudes and polar regions. The longer gases stay in the stratosphere, the more pollutants from stratoliners may build up. We estimate that pollutant concentrations from proposed aircraft flying in the stratosphere will be 25 to 100 percent greater than previously expected," Boering wrote in a report published recently in the journal Science.

That sounds like bad news.

"Not necessarily," Boering counters. "It helps us to test whether computer models that simulate the atmosphere are accurate. The models are used to predict the consequences of increased levels of gas and particles, natural and artificial, on ozone depletion and global warming. Our results have already improved these computer simulations. If models and measurements continue to agree, that will increase confidence in our predictions of the impact of HSCT emissions."

Ozone Killers

Carbon dioxide is a major cause of global warming. High-flying stratoliners probably won't spew out enough of the gas to make a big difference in the lives of those living below, but nitrogen oxides, water vapor, and particles of soot and sulfur in their exhausts might.

Another group, led by James Anderson, Weld Professor of Atmospheric Chemistry, flies instruments on the same ER-2 flights to study chemical reactions that can destroy ozone. Three years ago, they made measurements showing that the chemistry of stratospheric ozone was radically different from what scientists had believed since the early 1970s.

They had thought that nitrogen oxides, produced naturally and in airplane exhaust, would be the principal killer of ozone. (These same noxious chemicals, ejected from the tailpipes of cars and trucks, produce smog in Los Angeles and other urban centers.) But measurements made by instruments aboard the spy planes showed just the opposite. Nitrogen oxides were found to play only a minor role in destroying ozone in the lower stratosphere. In fact, these oxides take up other, more dangerous compounds, such as the highly reactive chemicals that eat huge holes in the ozone layer over Antarctica.

"We found that small increases in nitrogen oxides slow ozone destruction at altitudes where we expect the heaviest HSCT traffic," Wennberg says. "However, at higher altitudes, above 13 miles, they will probably lead to ozone loss because of differences in chemistry at these heights."

Flying below 12-13 miles won't solve the problem because, as Boering's and Wofsy's research shows, gases emitted in the lower stratosphere make their way to higher altitudes where they can chew up ozone.

Airlines don't want to order new aircraft if environmental regulations might prevent them from flying. Before the first factory starts to build the first stratoliners, the industry needs accurate computer models to predict the impact of their exhausts. "The name of the game is prediction," says Wofsy.

"Fifteen years ago, such models were developed mainly on the basis of laboratory studies," Wennberg comments. "After years of atmospheric measurements and more recent lab work, they're becoming more precise. But we still can't fully explain what's going on."

Frustrating gaps in knowledge remain. Most HSCT flights will be over the Northern Hemisphere. Will the gases and particles from their exhaust be concentrated in that hemisphere, or will they spread more or less evenly over the entire globe? To answer that, Wofsy's team is preparing to launch huge instrument-carrying helium balloons from Brazil into the high stratosphere. ER-2 aircraft cannot fly higher than 14 miles; the balloons will extend observations to about 18 miles.

Also, there are many questions about air movements and chemistry at the bottom of the stratosphere, where many commercial airliners now fly. Anderson's group is readying for the first measurements in this region, where, it is suspected, highly reactive chemicals could effect global warming. Finally, there's the nagging problem of those particles found in the wake of the Concorde.

"It's increasingly clear that we know a lot more than we used to know," says Wennberg, "but what we know isn't enough yet."