Harvard Researchers Take Flight

Those who think the air is rarefied in the ivory towers where most academics work should consider the research of a group of Harvard atmospheric scientists.

Paul O. Wennberg '88 and Weld Professor of Atmospheric Chemistry James G. Anderson are part of a team that performed chemical research in a converted spy plane 70,000 feet above the earth's surface, seeking new answers to the problem of ozone depletion.

With the aid of colleagues from across the United States, the Harvard group recently published a research article in Science reporting their work on the impact of supersonic aircraft emissions on the ozone layer.

Their article takes the story that scientists traditionally tell to explain how chemical emissions affect the earth's atmosphere, and stands it on its head.

"The fundamental tenet of atmospheric chemistry is that ozone depletion is controlled by nitrogen oxide radicals and the hydrogen oxide and halogen radicals play a much smaller part," Anderson says. "We have experimentally shown that in the lower ozone layer, the hydrogen oxide radicals are on top and the halogen radicals are dominant. The nitrogen oxide radicals play the smallest role at less than 20 percent."

The group's work was supported by the National Aeronautics and Space Administration [NASA] High Speed Research Program. Howard L. Wesoky, manager of atmospheric effects of aviation at NASA Headquarters, says that the research is a key part of a comprehensive study of the ozone layer.

"The research may have helped us to understand the photo chemistry [of ozone depletion], but we don't know the dynamics," Wesoky says.

"NASA has two programs that sponsor upper atmosphere research," Wesoky says. "We are funding studies to understand the effects of chlorofluoro carbons [CFCs] on the Antarctic ozone layer as well as studies to understand the impact of a fleet of supersonic aircraft on ozone depletion."

"NASA is funding a 150-million-dollar, 12-year study to understand the atmospheric effects of subsonic and supersonic aircraft," Wesoky adds. "Anderson's group is looking at only one element of six areas, but it is the most expensive and complex one."

Jet engine exhaust deposits nitrogen oxides into the atmosphere, where they can chemically react with and deplete ozone.

Supersonic aircraft, which currently include the Concorde and some military planes, produce these nitrogen radicals in their engines. "Estimates show that a 50 percent increase in nitrogen radicals [in the lower ozone layer] is expected from a fleet of supersonic aircraft," Wennberg says.

Supersonic Aircraft

In a speech given in December 1992, NASA Administrator Daniel S. Goldin called high speed civil transport (HSCT) research NASA's first priority for the aeronautics industry. The proposed HSCT will carry 300 people over a range of 6000 miles at almost two and a half times the speed of sound (Mach 2.4).

"This HSCT would go twice as far [as the current Concorde can] with three times as many people at one-10th the ticket price and not damage the ozone layer," Goldin said. "And with its speed, it could do twice the work of a subsonic aircraft."

The Concorde is the only civil supersonic aircraft that currently exists. Boeing is considering whether to build its own supersonic aircraft after extensive further research.

"Boeing is actively studying HSCTs," says Randy Harrison, spokesperson for the Boeing HSCT program. "We first proposed supersonic transports in the 1970s, and we had to stop our program when federal funding was canceled, but we kept a few people to continue to work on the issues."

The firm is currently in the second stage of a three-part research feasibility study. The first phase, HSR 1, was started in 1989 to investigate the engines and emissions, and was headed by Pratt & Whitney and GE.

"Boeing is currently heading a U.S. industrial team that has 440 million dollars from High Speed Research (HSR) II to investigate the feasibility of an airframe and structure for the HSCT program," Harrison adds.

"The goal of HSR is to investigate enabling technologies to answer eventually the ability to decide whether to proceed with the design of HSCT," Harrison says. "It must be environmentally benign and economically viable. Currently, the technological barriers are very significant."

Boeing's projected aircraft would have a baseline design carrying more than 300 passengers, as well as a range of 5800 statute miles, flying at Mach 2.4 [1775 mph] at 60,000 feet [18 kilometers] with four engines each with 50,000 pounds of thrust, according to Harrison. The Concorde currently carries 100 passengers, and has a range of 4000 statute miles flying at Mach 2.

"We currently have no cost projections for the project, but we know that a HSCT will not fly before 2005 and we are shooting for a price in agreement with subsonic aircraft of the day," Harrison says. The most expensive subsonic aircraft flying today, Boeing's 747, costs around 150 million dollars, he said.

"Our requirements for feasibility are that we have to be able to build the aircraft, which will require a great deal of new composite materials that are very different from metal alloys, [and] the price must be such that the customer can afford it," Harrison says. "[A further requirement is that] we can afford to build it so passengers in three classes can afford to pay a slight surcharge, 10 to 15 percent, above the existing subsonic fare of the day for a flight taking 50 percent of the time."

Flyers willing to pay the higher price, Harrison says, would recoup their investment in time saved. He offers the example of a flight from Los Angeles to Tokyo that takes 10.3 hours today, but would take only 4.3 hours in an HSCT.

But Harrison cites problems that foreseeable technology will not be able to overcome. "The aircraft will not be able to fly over populated land masses," Harrison says, "and it must be able to operate subsonic at takeoff, too [because of noise regulations]."

If the proposed airplane design meets environmental regulations, however, that will be an innovation in itself. Currently, the Concorde fails to meet the landing requirements at some US airports, restricting its flying routes. In the future, Harrison claims, supersonic aircraft will be able to comply with all environmental laws.

"The noise and pollution [emissions] will meet the regulations of the day," he says.

If Boeing decides to produce supersonic aircraft, it will be on a large scale.

"The minimum production would have to be 500 aircraft," Harrison says. "We do not believe there is a market for two airplanes."

But even Boeing's resources may not be enough to develop a project of HSCT's size. "We do not believe it will be possible for any one company to do this alone," Harrison says.

"We do not foresee federal support and we do not seek it," Harrison adds. "NASA's role has traditionally focused on technological development, and that goes back to the National Advisory Committee for Aeronautics of the 1930s."

Goldin says that embracing new supersonic technology could bring America's market share in aviation to nearly 80 percent, and would create jobs similar to those that are currently being lost through defense cutbacks.

According to NASA literature, the program will be the cornerstone of NASA aeronautics for the 1990s.

Experimentation

Wennberg and Andersons' research addresses the fundamental question of whether Boeing, or any firm, can build such vast fleets of supersonic aircraft without doing irreparable damage to the lower ozone layer.

To complete their study, Wennberg and Anderson flew instruments in a converted U-2 spy plane 20 kilometers (70,000 feet) above the earth's surface. Taking part in the Stratospheric Photo chemistry, Aerosols and Dynamics Expedition [SPADE], the researchers were able to make many measurements simultaneously. This procedure is "unprecedented" according to Ross J. Salawitch, a research associate in Atmospheric Sciences and a co-author of the paper.

"Most of the nitrogen and hydrogen radicals are produced by natural activities," says Salawitch. "But the halogen radicals, primarily bromine and chlorine, are primarily due to industrial activity."

The U-2 jet was designed to reach extremely high altitudes for reconnaissance during the Cold War, and was converted to become the NASA ER-2, a major part of the SPADE program.

But Anderson acknowledges a weakness in their research. "We only have comprehensive measurements from 13 to 20 kilometers," Anderson says. "The region above is unexplored in hard detail. We know there is a point above 20 kilometers where nitrogen radicals control the depletion of ozone."

The scientists will be unable to determine exactly where this reversal occurs using NASA's ER-2 jet.

"The ER-2 is limited to a ceiling altitude of 71,000-72,000 feet [22 kilometers] unloaded," says James R. Podolske, an atmospheric scientist at the NASA Ames Research Center. "To go higher, you either have to go faster or have a lighter plane."

But some researchers, including Wennberg and Anderson, have been able to take partial measurements at greater altitudes using balloons. "Balloons have been used for years to go to 120,000 feet," says Podolske, "but there are three limitations. You have no control over where the balloon will go, you often have to wait for the right winds and you only get a vertical profile [because the balloon goes up and down]."

"Balloons have short flights with no inherent repeatability," Wennberg says. "Aircraft eliminate these limitations."

The SR-71 Blackbird, originally designed in the 1960s for US Air Force strategic reconnaissance, set the record for flying at Mach 3 in 1976.

"The SR-71 can go to at least 85,000 feet [26 kilometers] and can fly at as fast as Mach 3," says Podolske. "But, as the air goes through the shockwave, it heats up," and .

Another option, flying slower, has its own problems.

"The air is so thin above 21 kilometers, you can't use conventional gas engines," says Podolske. "Either you have to 'supercharge' the air, or carry your own liquid oxygen to support combustion."

John S. Langford, president of Aurora Flight Sciences of Manassis, Va., thinks he has the solution to this problem. "Atmospheric scientists have long had to settle for the hand-me-down platforms from the military," Langford says. "[Aurora is] developing the Perseus and Theseus aircraft, taking advantage of a convergence in technologies, to fly as high as 25 kilometers, or more."

Perseus and Theseus are both "flying robots," Anderson says, unmanned planes that are guided by remote control.

Langford hopes to keep the Perseus and Theseus aircraft inexpensive enough that researchers can afford to use it. "In numbers, the Perseus will cost about $500,000 and each flight will cost $10,000 to $20,000," Langford says.

But even the Perseus has its limitations. "Flights will have to be pretty short -- about one hour at 80,000 feet [24 kilometers]," Langford says. "Perseus will only be able to carry a single instrument, while Thesarus will be able to carry up to 700 pounds of payload."

Perseus is now in the testing and flight stage. Aurora is currently working with a five million dollar grant from NASA.

The Future

With the ability to go to even greater altitudes than the ER-2 does, Wennberg and Anderson are anxious to gain a more complete understanding of the chemistry of ozone depletion.

"There are essentially two issues [in determining ozone depletion]: the rapid change of aerosol loading, dictating the amount of surface area that is exposed, and the change in ultraviolet radiation," Anderson says. Regional and seasonal variations can lead to vast differences in ozone levels, he adds. "The polar regions are almost different planets."

"There is a three-week period over the Antarctic with massive destruction due to high chlorine, some time and a lot of sunlight," Anderson says. "The preparation is the same over the Arctic, but there isn't enough sun to generate the ozone hole like that over the Antarctic."

Recent NASA measurements from Russian satellites indicate that the Antarctic ozone hole is maintaining a size roughly the same as the North American land mass.

Salawitch cautions that even with the current data, there are still a lot of uncertainties.

"The results are uncertain because lot of key reaction rates have not been measured at low temperature in the lower ozone layer," Salawitch says. "We also really don't have an accurate understanding of how engine exhaust is transported to higher altitudes."

And Wennberg notes that the mechanism by which aircraft exhaust is diffused has yet to be explained. "If 10 percent of the exhaust is transported upwards in the atmosphere and 90 percent down, the net ozone loss would be zero," Wennberg says.

Anderson says that this area is very active in the research community. "The indication from tracers is that the atmosphere mixed horizontally and vertically," he notes.

And Anderson's group is already moving to understand more of the related chemistry.

Recently, the ER-2 flew through the exhaust of the Concorde off Christchurch, New Zealand at an altitude of 53,000 feet. Anderson's group is currently analyzing the data.

"Right now, we only have models to predict what we will find above 22 kilometers," Wennberg says. "In situ data will be much more precise and unparalleled."

"With the Perseus-like technology, we will be able to look in good detail" to understand the chemistry of the ozone layer, Wennberg says. "We are measuring levels that are 1 part in a trillion, and then changes that are 0.01 of 0.001 parts per trillion."

It is unlikely such measurements will be possible from remote payloads such as those on satellites or the Space Shuttle, Wennberg says.

Despite the technological difficulties Wennberg and Anderson have encountered, atmospheric researchers are continuing their efforts to understand all areas of the ozone layer. And scientists involved with their research are optimistic about future efforts.

"The paper is another step along the road [to understanding the chemistry of the ozone layer]," says David W. Fahey, a research physicist at the National Oceanic and Atmospheric Administration Aeronomy Laboratory, and co-author of the paper. "The stage has been nicely set."Photo Courtesy the US Air ForceThe scientists flew their instruments in a converted version of this U-2 jet, which NASA calls the ER-2.

Supersonic Aircraft

In a speech given in December 1992, NASA Administrator Daniel S. Goldin called high speed civil transport (HSCT) research NASA's first priority for the aeronautics industry. The proposed HSCT will carry 300 people over a range of 6000 miles at almost two and a half times the speed of sound (Mach 2.4).

"This HSCT would go twice as far [as the current Concorde can] with three times as many people at one-10th the ticket price and not damage the ozone layer," Goldin said. "And with its speed, it could do twice the work of a subsonic aircraft."

The Concorde is the only civil supersonic aircraft that currently exists. Boeing is considering whether to build its own supersonic aircraft after extensive further research.

"Boeing is actively studying HSCTs," says Randy Harrison, spokesperson for the Boeing HSCT program. "We first proposed supersonic transports in the 1970s, and we had to stop our program when federal funding was canceled, but we kept a few people to continue to work on the issues."

The firm is currently in the second stage of a three-part research feasibility study. The first phase, HSR 1, was started in 1989 to investigate the engines and emissions, and was headed by Pratt & Whitney and GE.

"Boeing is currently heading a U.S. industrial team that has 440 million dollars from High Speed Research (HSR) II to investigate the feasibility of an airframe and structure for the HSCT program," Harrison adds.

"The goal of HSR is to investigate enabling technologies to answer eventually the ability to decide whether to proceed with the design of HSCT," Harrison says. "It must be environmentally benign and economically viable. Currently, the technological barriers are very significant."

Boeing's projected aircraft would have a baseline design carrying more than 300 passengers, as well as a range of 5800 statute miles, flying at Mach 2.4 [1775 mph] at 60,000 feet [18 kilometers] with four engines each with 50,000 pounds of thrust, according to Harrison. The Concorde currently carries 100 passengers, and has a range of 4000 statute miles flying at Mach 2.

"We currently have no cost projections for the project, but we know that a HSCT will not fly before 2005 and we are shooting for a price in agreement with subsonic aircraft of the day," Harrison says. The most expensive subsonic aircraft flying today, Boeing's 747, costs around 150 million dollars, he said.

"Our requirements for feasibility are that we have to be able to build the aircraft, which will require a great deal of new composite materials that are very different from metal alloys, [and] the price must be such that the customer can afford it," Harrison says. "[A further requirement is that] we can afford to build it so passengers in three classes can afford to pay a slight surcharge, 10 to 15 percent, above the existing subsonic fare of the day for a flight taking 50 percent of the time."

Flyers willing to pay the higher price, Harrison says, would recoup their investment in time saved. He offers the example of a flight from Los Angeles to Tokyo that takes 10.3 hours today, but would take only 4.3 hours in an HSCT.

But Harrison cites problems that foreseeable technology will not be able to overcome. "The aircraft will not be able to fly over populated land masses," Harrison says, "and it must be able to operate subsonic at takeoff, too [because of noise regulations]."

If the proposed airplane design meets environmental regulations, however, that will be an innovation in itself. Currently, the Concorde fails to meet the landing requirements at some US airports, restricting its flying routes. In the future, Harrison claims, supersonic aircraft will be able to comply with all environmental laws.

"The noise and pollution [emissions] will meet the regulations of the day," he says.

If Boeing decides to produce supersonic aircraft, it will be on a large scale.

"The minimum production would have to be 500 aircraft," Harrison says. "We do not believe there is a market for two airplanes."

But even Boeing's resources may not be enough to develop a project of HSCT's size. "We do not believe it will be possible for any one company to do this alone," Harrison says.

"We do not foresee federal support and we do not seek it," Harrison adds. "NASA's role has traditionally focused on technological development, and that goes back to the National Advisory Committee for Aeronautics of the 1930s."

Goldin says that embracing new supersonic technology could bring America's market share in aviation to nearly 80 percent, and would create jobs similar to those that are currently being lost through defense cutbacks.

According to NASA literature, the program will be the cornerstone of NASA aeronautics for the 1990s.

Experimentation

Wennberg and Andersons' research addresses the fundamental question of whether Boeing, or any firm, can build such vast fleets of supersonic aircraft without doing irreparable damage to the lower ozone layer.

To complete their study, Wennberg and Anderson flew instruments in a converted U-2 spy plane 20 kilometers (70,000 feet) above the earth's surface. Taking part in the Stratospheric Photo chemistry, Aerosols and Dynamics Expedition [SPADE], the researchers were able to make many measurements simultaneously. This procedure is "unprecedented" according to Ross J. Salawitch, a research associate in Atmospheric Sciences and a co-author of the paper.

"Most of the nitrogen and hydrogen radicals are produced by natural activities," says Salawitch. "But the halogen radicals, primarily bromine and chlorine, are primarily due to industrial activity."

The U-2 jet was designed to reach extremely high altitudes for reconnaissance during the Cold War, and was converted to become the NASA ER-2, a major part of the SPADE program.

But Anderson acknowledges a weakness in their research. "We only have comprehensive measurements from 13 to 20 kilometers," Anderson says. "The region above is unexplored in hard detail. We know there is a point above 20 kilometers where nitrogen radicals control the depletion of ozone."

The scientists will be unable to determine exactly where this reversal occurs using NASA's ER-2 jet.

"The ER-2 is limited to a ceiling altitude of 71,000-72,000 feet [22 kilometers] unloaded," says James R. Podolske, an atmospheric scientist at the NASA Ames Research Center. "To go higher, you either have to go faster or have a lighter plane."

But some researchers, including Wennberg and Anderson, have been able to take partial measurements at greater altitudes using balloons. "Balloons have been used for years to go to 120,000 feet," says Podolske, "but there are three limitations. You have no control over where the balloon will go, you often have to wait for the right winds and you only get a vertical profile [because the balloon goes up and down]."

"Balloons have short flights with no inherent repeatability," Wennberg says. "Aircraft eliminate these limitations."

The SR-71 Blackbird, originally designed in the 1960s for US Air Force strategic reconnaissance, set the record for flying at Mach 3 in 1976.

"The SR-71 can go to at least 85,000 feet [26 kilometers] and can fly at as fast as Mach 3," says Podolske. "But, as the air goes through the shockwave, it heats up," and .

Another option, flying slower, has its own problems.

"The air is so thin above 21 kilometers, you can't use conventional gas engines," says Podolske. "Either you have to 'supercharge' the air, or carry your own liquid oxygen to support combustion."

John S. Langford, president of Aurora Flight Sciences of Manassis, Va., thinks he has the solution to this problem. "Atmospheric scientists have long had to settle for the hand-me-down platforms from the military," Langford says. "[Aurora is] developing the Perseus and Theseus aircraft, taking advantage of a convergence in technologies, to fly as high as 25 kilometers, or more."

Perseus and Theseus are both "flying robots," Anderson says, unmanned planes that are guided by remote control.

Langford hopes to keep the Perseus and Theseus aircraft inexpensive enough that researchers can afford to use it. "In numbers, the Perseus will cost about $500,000 and each flight will cost $10,000 to $20,000," Langford says.

But even the Perseus has its limitations. "Flights will have to be pretty short -- about one hour at 80,000 feet [24 kilometers]," Langford says. "Perseus will only be able to carry a single instrument, while Thesarus will be able to carry up to 700 pounds of payload."

Perseus is now in the testing and flight stage. Aurora is currently working with a five million dollar grant from NASA.

The Future

With the ability to go to even greater altitudes than the ER-2 does, Wennberg and Anderson are anxious to gain a more complete understanding of the chemistry of ozone depletion.

"There are essentially two issues [in determining ozone depletion]: the rapid change of aerosol loading, dictating the amount of surface area that is exposed, and the change in ultraviolet radiation," Anderson says. Regional and seasonal variations can lead to vast differences in ozone levels, he adds. "The polar regions are almost different planets."

"There is a three-week period over the Antarctic with massive destruction due to high chlorine, some time and a lot of sunlight," Anderson says. "The preparation is the same over the Arctic, but there isn't enough sun to generate the ozone hole like that over the Antarctic."

Recent NASA measurements from Russian satellites indicate that the Antarctic ozone hole is maintaining a size roughly the same as the North American land mass.

Salawitch cautions that even with the current data, there are still a lot of uncertainties.

"The results are uncertain because lot of key reaction rates have not been measured at low temperature in the lower ozone layer," Salawitch says. "We also really don't have an accurate understanding of how engine exhaust is transported to higher altitudes."

And Wennberg notes that the mechanism by which aircraft exhaust is diffused has yet to be explained. "If 10 percent of the exhaust is transported upwards in the atmosphere and 90 percent down, the net ozone loss would be zero," Wennberg says.

Anderson says that this area is very active in the research community. "The indication from tracers is that the atmosphere mixed horizontally and vertically," he notes.

And Anderson's group is already moving to understand more of the related chemistry.

Recently, the ER-2 flew through the exhaust of the Concorde off Christchurch, New Zealand at an altitude of 53,000 feet. Anderson's group is currently analyzing the data.

"Right now, we only have models to predict what we will find above 22 kilometers," Wennberg says. "In situ data will be much more precise and unparalleled."

"With the Perseus-like technology, we will be able to look in good detail" to understand the chemistry of the ozone layer, Wennberg says. "We are measuring levels that are 1 part in a trillion, and then changes that are 0.01 of 0.001 parts per trillion."

It is unlikely such measurements will be possible from remote payloads such as those on satellites or the Space Shuttle, Wennberg says.

Despite the technological difficulties Wennberg and Anderson have encountered, atmospheric researchers are continuing their efforts to understand all areas of the ozone layer. And scientists involved with their research are optimistic about future efforts.

"The paper is another step along the road [to understanding the chemistry of the ozone layer]," says David W. Fahey, a research physicist at the National Oceanic and Atmospheric Administration Aeronomy Laboratory, and co-author of the paper. "The stage has been nicely set."Photo Courtesy the US Air ForceThe scientists flew their instruments in a converted version of this U-2 jet, which NASA calls the ER-2.

According to NASA literature, the program will be the cornerstone of NASA aeronautics for the 1990s.

Experimentation

Wennberg and Andersons' research addresses the fundamental question of whether Boeing, or any firm, can build such vast fleets of supersonic aircraft without doing irreparable damage to the lower ozone layer.

To complete their study, Wennberg and Anderson flew instruments in a converted U-2 spy plane 20 kilometers (70,000 feet) above the earth's surface. Taking part in the Stratospheric Photo chemistry, Aerosols and Dynamics Expedition [SPADE], the researchers were able to make many measurements simultaneously. This procedure is "unprecedented" according to Ross J. Salawitch, a research associate in Atmospheric Sciences and a co-author of the paper.

"Most of the nitrogen and hydrogen radicals are produced by natural activities," says Salawitch. "But the halogen radicals, primarily bromine and chlorine, are primarily due to industrial activity."

The U-2 jet was designed to reach extremely high altitudes for reconnaissance during the Cold War, and was converted to become the NASA ER-2, a major part of the SPADE program.

But Anderson acknowledges a weakness in their research. "We only have comprehensive measurements from 13 to 20 kilometers," Anderson says. "The region above is unexplored in hard detail. We know there is a point above 20 kilometers where nitrogen radicals control the depletion of ozone."

The scientists will be unable to determine exactly where this reversal occurs using NASA's ER-2 jet.

"The ER-2 is limited to a ceiling altitude of 71,000-72,000 feet [22 kilometers] unloaded," says James R. Podolske, an atmospheric scientist at the NASA Ames Research Center. "To go higher, you either have to go faster or have a lighter plane."

But some researchers, including Wennberg and Anderson, have been able to take partial measurements at greater altitudes using balloons. "Balloons have been used for years to go to 120,000 feet," says Podolske, "but there are three limitations. You have no control over where the balloon will go, you often have to wait for the right winds and you only get a vertical profile [because the balloon goes up and down]."

"Balloons have short flights with no inherent repeatability," Wennberg says. "Aircraft eliminate these limitations."

The SR-71 Blackbird, originally designed in the 1960s for US Air Force strategic reconnaissance, set the record for flying at Mach 3 in 1976.

"The SR-71 can go to at least 85,000 feet [26 kilometers] and can fly at as fast as Mach 3," says Podolske. "But, as the air goes through the shockwave, it heats up," and .

Another option, flying slower, has its own problems.

"The air is so thin above 21 kilometers, you can't use conventional gas engines," says Podolske. "Either you have to 'supercharge' the air, or carry your own liquid oxygen to support combustion."

John S. Langford, president of Aurora Flight Sciences of Manassis, Va., thinks he has the solution to this problem. "Atmospheric scientists have long had to settle for the hand-me-down platforms from the military," Langford says. "[Aurora is] developing the Perseus and Theseus aircraft, taking advantage of a convergence in technologies, to fly as high as 25 kilometers, or more."

Perseus and Theseus are both "flying robots," Anderson says, unmanned planes that are guided by remote control.

Langford hopes to keep the Perseus and Theseus aircraft inexpensive enough that researchers can afford to use it. "In numbers, the Perseus will cost about $500,000 and each flight will cost $10,000 to $20,000," Langford says.

But even the Perseus has its limitations. "Flights will have to be pretty short -- about one hour at 80,000 feet [24 kilometers]," Langford says. "Perseus will only be able to carry a single instrument, while Thesarus will be able to carry up to 700 pounds of payload."

Perseus is now in the testing and flight stage. Aurora is currently working with a five million dollar grant from NASA.

The Future

With the ability to go to even greater altitudes than the ER-2 does, Wennberg and Anderson are anxious to gain a more complete understanding of the chemistry of ozone depletion.

"There are essentially two issues [in determining ozone depletion]: the rapid change of aerosol loading, dictating the amount of surface area that is exposed, and the change in ultraviolet radiation," Anderson says. Regional and seasonal variations can lead to vast differences in ozone levels, he adds. "The polar regions are almost different planets."

"There is a three-week period over the Antarctic with massive destruction due to high chlorine, some time and a lot of sunlight," Anderson says. "The preparation is the same over the Arctic, but there isn't enough sun to generate the ozone hole like that over the Antarctic."

Recent NASA measurements from Russian satellites indicate that the Antarctic ozone hole is maintaining a size roughly the same as the North American land mass.

Salawitch cautions that even with the current data, there are still a lot of uncertainties.

"The results are uncertain because lot of key reaction rates have not been measured at low temperature in the lower ozone layer," Salawitch says. "We also really don't have an accurate understanding of how engine exhaust is transported to higher altitudes."

And Wennberg notes that the mechanism by which aircraft exhaust is diffused has yet to be explained. "If 10 percent of the exhaust is transported upwards in the atmosphere and 90 percent down, the net ozone loss would be zero," Wennberg says.

Anderson says that this area is very active in the research community. "The indication from tracers is that the atmosphere mixed horizontally and vertically," he notes.

And Anderson's group is already moving to understand more of the related chemistry.

Recently, the ER-2 flew through the exhaust of the Concorde off Christchurch, New Zealand at an altitude of 53,000 feet. Anderson's group is currently analyzing the data.

"Right now, we only have models to predict what we will find above 22 kilometers," Wennberg says. "In situ data will be much more precise and unparalleled."

"With the Perseus-like technology, we will be able to look in good detail" to understand the chemistry of the ozone layer, Wennberg says. "We are measuring levels that are 1 part in a trillion, and then changes that are 0.01 of 0.001 parts per trillion."

It is unlikely such measurements will be possible from remote payloads such as those on satellites or the Space Shuttle, Wennberg says.

Despite the technological difficulties Wennberg and Anderson have encountered, atmospheric researchers are continuing their efforts to understand all areas of the ozone layer. And scientists involved with their research are optimistic about future efforts.

"The paper is another step along the road [to understanding the chemistry of the ozone layer]," says David W. Fahey, a research physicist at the National Oceanic and Atmospheric Administration Aeronomy Laboratory, and co-author of the paper. "The stage has been nicely set."Photo Courtesy the US Air ForceThe scientists flew their instruments in a converted version of this U-2 jet, which NASA calls the ER-2.

Experimentation

Wennberg and Andersons' research addresses the fundamental question of whether Boeing, or any firm, can build such vast fleets of supersonic aircraft without doing irreparable damage to the lower ozone layer.

To complete their study, Wennberg and Anderson flew instruments in a converted U-2 spy plane 20 kilometers (70,000 feet) above the earth's surface. Taking part in the Stratospheric Photo chemistry, Aerosols and Dynamics Expedition [SPADE], the researchers were able to make many measurements simultaneously. This procedure is "unprecedented" according to Ross J. Salawitch, a research associate in Atmospheric Sciences and a co-author of the paper.

"Most of the nitrogen and hydrogen radicals are produced by natural activities," says Salawitch. "But the halogen radicals, primarily bromine and chlorine, are primarily due to industrial activity."

The U-2 jet was designed to reach extremely high altitudes for reconnaissance during the Cold War, and was converted to become the NASA ER-2, a major part of the SPADE program.

But Anderson acknowledges a weakness in their research. "We only have comprehensive measurements from 13 to 20 kilometers," Anderson says. "The region above is unexplored in hard detail. We know there is a point above 20 kilometers where nitrogen radicals control the depletion of ozone."

The scientists will be unable to determine exactly where this reversal occurs using NASA's ER-2 jet.

"The ER-2 is limited to a ceiling altitude of 71,000-72,000 feet [22 kilometers] unloaded," says James R. Podolske, an atmospheric scientist at the NASA Ames Research Center. "To go higher, you either have to go faster or have a lighter plane."

But some researchers, including Wennberg and Anderson, have been able to take partial measurements at greater altitudes using balloons. "Balloons have been used for years to go to 120,000 feet," says Podolske, "but there are three limitations. You have no control over where the balloon will go, you often have to wait for the right winds and you only get a vertical profile [because the balloon goes up and down]."

"Balloons have short flights with no inherent repeatability," Wennberg says. "Aircraft eliminate these limitations."

The SR-71 Blackbird, originally designed in the 1960s for US Air Force strategic reconnaissance, set the record for flying at Mach 3 in 1976.

"The SR-71 can go to at least 85,000 feet [26 kilometers] and can fly at as fast as Mach 3," says Podolske. "But, as the air goes through the shockwave, it heats up," and .

Another option, flying slower, has its own problems.

"The air is so thin above 21 kilometers, you can't use conventional gas engines," says Podolske. "Either you have to 'supercharge' the air, or carry your own liquid oxygen to support combustion."

John S. Langford, president of Aurora Flight Sciences of Manassis, Va., thinks he has the solution to this problem. "Atmospheric scientists have long had to settle for the hand-me-down platforms from the military," Langford says. "[Aurora is] developing the Perseus and Theseus aircraft, taking advantage of a convergence in technologies, to fly as high as 25 kilometers, or more."

Perseus and Theseus are both "flying robots," Anderson says, unmanned planes that are guided by remote control.

Langford hopes to keep the Perseus and Theseus aircraft inexpensive enough that researchers can afford to use it. "In numbers, the Perseus will cost about $500,000 and each flight will cost $10,000 to $20,000," Langford says.

But even the Perseus has its limitations. "Flights will have to be pretty short -- about one hour at 80,000 feet [24 kilometers]," Langford says. "Perseus will only be able to carry a single instrument, while Thesarus will be able to carry up to 700 pounds of payload."

Perseus is now in the testing and flight stage. Aurora is currently working with a five million dollar grant from NASA.

The Future

With the ability to go to even greater altitudes than the ER-2 does, Wennberg and Anderson are anxious to gain a more complete understanding of the chemistry of ozone depletion.

"There are essentially two issues [in determining ozone depletion]: the rapid change of aerosol loading, dictating the amount of surface area that is exposed, and the change in ultraviolet radiation," Anderson says. Regional and seasonal variations can lead to vast differences in ozone levels, he adds. "The polar regions are almost different planets."

"There is a three-week period over the Antarctic with massive destruction due to high chlorine, some time and a lot of sunlight," Anderson says. "The preparation is the same over the Arctic, but there isn't enough sun to generate the ozone hole like that over the Antarctic."

Recent NASA measurements from Russian satellites indicate that the Antarctic ozone hole is maintaining a size roughly the same as the North American land mass.

Salawitch cautions that even with the current data, there are still a lot of uncertainties.

"The results are uncertain because lot of key reaction rates have not been measured at low temperature in the lower ozone layer," Salawitch says. "We also really don't have an accurate understanding of how engine exhaust is transported to higher altitudes."

And Wennberg notes that the mechanism by which aircraft exhaust is diffused has yet to be explained. "If 10 percent of the exhaust is transported upwards in the atmosphere and 90 percent down, the net ozone loss would be zero," Wennberg says.

Anderson says that this area is very active in the research community. "The indication from tracers is that the atmosphere mixed horizontally and vertically," he notes.

And Anderson's group is already moving to understand more of the related chemistry.

Recently, the ER-2 flew through the exhaust of the Concorde off Christchurch, New Zealand at an altitude of 53,000 feet. Anderson's group is currently analyzing the data.

"Right now, we only have models to predict what we will find above 22 kilometers," Wennberg says. "In situ data will be much more precise and unparalleled."

"With the Perseus-like technology, we will be able to look in good detail" to understand the chemistry of the ozone layer, Wennberg says. "We are measuring levels that are 1 part in a trillion, and then changes that are 0.01 of 0.001 parts per trillion."

It is unlikely such measurements will be possible from remote payloads such as those on satellites or the Space Shuttle, Wennberg says.

Despite the technological difficulties Wennberg and Anderson have encountered, atmospheric researchers are continuing their efforts to understand all areas of the ozone layer. And scientists involved with their research are optimistic about future efforts.

"The paper is another step along the road [to understanding the chemistry of the ozone layer]," says David W. Fahey, a research physicist at the National Oceanic and Atmospheric Administration Aeronomy Laboratory, and co-author of the paper. "The stage has been nicely set."Photo Courtesy the US Air ForceThe scientists flew their instruments in a converted version of this U-2 jet, which NASA calls the ER-2.

Another option, flying slower, has its own problems.

"The air is so thin above 21 kilometers, you can't use conventional gas engines," says Podolske. "Either you have to 'supercharge' the air, or carry your own liquid oxygen to support combustion."

John S. Langford, president of Aurora Flight Sciences of Manassis, Va., thinks he has the solution to this problem. "Atmospheric scientists have long had to settle for the hand-me-down platforms from the military," Langford says. "[Aurora is] developing the Perseus and Theseus aircraft, taking advantage of a convergence in technologies, to fly as high as 25 kilometers, or more."

Perseus and Theseus are both "flying robots," Anderson says, unmanned planes that are guided by remote control.

Langford hopes to keep the Perseus and Theseus aircraft inexpensive enough that researchers can afford to use it. "In numbers, the Perseus will cost about $500,000 and each flight will cost $10,000 to $20,000," Langford says.

But even the Perseus has its limitations. "Flights will have to be pretty short -- about one hour at 80,000 feet [24 kilometers]," Langford says. "Perseus will only be able to carry a single instrument, while Thesarus will be able to carry up to 700 pounds of payload."

Perseus is now in the testing and flight stage. Aurora is currently working with a five million dollar grant from NASA.

The Future

With the ability to go to even greater altitudes than the ER-2 does, Wennberg and Anderson are anxious to gain a more complete understanding of the chemistry of ozone depletion.

"There are essentially two issues [in determining ozone depletion]: the rapid change of aerosol loading, dictating the amount of surface area that is exposed, and the change in ultraviolet radiation," Anderson says. Regional and seasonal variations can lead to vast differences in ozone levels, he adds. "The polar regions are almost different planets."

"There is a three-week period over the Antarctic with massive destruction due to high chlorine, some time and a lot of sunlight," Anderson says. "The preparation is the same over the Arctic, but there isn't enough sun to generate the ozone hole like that over the Antarctic."

Recent NASA measurements from Russian satellites indicate that the Antarctic ozone hole is maintaining a size roughly the same as the North American land mass.

Salawitch cautions that even with the current data, there are still a lot of uncertainties.

"The results are uncertain because lot of key reaction rates have not been measured at low temperature in the lower ozone layer," Salawitch says. "We also really don't have an accurate understanding of how engine exhaust is transported to higher altitudes."

And Wennberg notes that the mechanism by which aircraft exhaust is diffused has yet to be explained. "If 10 percent of the exhaust is transported upwards in the atmosphere and 90 percent down, the net ozone loss would be zero," Wennberg says.

Anderson says that this area is very active in the research community. "The indication from tracers is that the atmosphere mixed horizontally and vertically," he notes.

And Anderson's group is already moving to understand more of the related chemistry.

Recently, the ER-2 flew through the exhaust of the Concorde off Christchurch, New Zealand at an altitude of 53,000 feet. Anderson's group is currently analyzing the data.

"Right now, we only have models to predict what we will find above 22 kilometers," Wennberg says. "In situ data will be much more precise and unparalleled."

"With the Perseus-like technology, we will be able to look in good detail" to understand the chemistry of the ozone layer, Wennberg says. "We are measuring levels that are 1 part in a trillion, and then changes that are 0.01 of 0.001 parts per trillion."

It is unlikely such measurements will be possible from remote payloads such as those on satellites or the Space Shuttle, Wennberg says.

Despite the technological difficulties Wennberg and Anderson have encountered, atmospheric researchers are continuing their efforts to understand all areas of the ozone layer. And scientists involved with their research are optimistic about future efforts.

"The paper is another step along the road [to understanding the chemistry of the ozone layer]," says David W. Fahey, a research physicist at the National Oceanic and Atmospheric Administration Aeronomy Laboratory, and co-author of the paper. "The stage has been nicely set."Photo Courtesy the US Air ForceThe scientists flew their instruments in a converted version of this U-2 jet, which NASA calls the ER-2.