While the SST supersonic jet has been the focus of concern about emissions in the stratosphere, subsonic jets at lower altitudes make many more trips across the Atlantic -- some 400 to 700 times a day, depending on the season. Along with soot, water vapor, and sulfur oxides being studied by other NASA missions, oxides of nitrogen are a major cause of concern. Ozone is another concern addressed by SONEX: in addition to greenhouse effects caused by too little ozone in the upper atmosphere, too much in the lower atmosphere could promote global warming.
"Until recently most emissions tests were done by measuring what came out of airplane engines on the ground, then extrapolating," Littlejohn says. "We wanted to find out what was actually being emitted aloft. In some cases we were flying in the path of planes that had just passed by."
The OPTIMA experiment was designed to monitor gas-phase nitric acid and nitrogen dioxide in undisturbed air, rather than collecting these compounds through narrow tubes or other probes. "Chemical changes can result just from the air going four or five hundred miles an hour, hitting the collecting tube, and heating up," Littlejohn explains, so OPTIMA detection takes place in the "free stream region" over the wing.
A laser beam is directed out a window and bounced back and forth between a mirror on top of the left inboard engine pylon and another mirror on the fuselage. Ideally the beam makes 64 trips before absorption measurements are made. (Nitric acid and nitrogen dioxide have easily-identifiable absorption features.)
Because the pylon-mounted mirror moves constantly in relation to the fuselage, the system is actively adjusted by means of guide lasers, position sensors, and servo-actuators controlled by computer. These serve to make sure the beam length stays constant. The set-up is monitored by a computer in the cabin, while another computer stores the fast-accumulating data.
"Typically we'd be in the air one day, spend a day going over the data we'd collected, and then spend another day prepping the next flight," Littlejohn says. The main bases for the mission were Bangor, Maine, on this side of the Atlantic, Shannon, Ireland on the other, and the Azores in midocean, with side trips that took the researchers south to the Canary Islands and north to Norway.
Although the work was interesting, there was little time off, says Littlejohn -- except when the aircraft was on the ground six days after sucking a seagull into one of its four engines. "The pilot didn't abort the flight, but when we were back on the ground he thought it would be good to make sure there was no damage."
Except for some erosion of the mirror surfaces, OPTIMA performed well. "We weren't sure it would work at all," says Littlejohn. "Not only did it work, we got lots of good data."
Since the flights ended, the teams have been crunching numbers. The results of most of the SONEX experiments will be announced in March and will be available on the SONEX website at http://telsci.arc.nasa.gov/~sonex/.
Jim Podolske and Dave Littlejohn, as co-investigators, have proposed a second series of tests for the NASA airborne mission planned for 1999 in the Central and South Pacific. Instead of Bangor, Maine, the plane will stop at places like Tahiti, Hawaii, and Costa Rica. Not bad places to be stuck on the ground, should an unfortunate encounter with a seagull occur.