Spewing into the Stratosphere

The plane reached cruising altitude smoothly, and I settled back into cattle-car class comfort with reading material in hand: a recent issue of the sprightly British journal New Scientist. Big mistake. It contained an article that turned my family visit Outside into one more guilt trip.

Airline passengers, it seems, are doing more than their share to disrupt the earth's protective ozone layer. We Alaskans do a great deal of passengering on the commercial jets now catching some blame for attacking ozone, and we live at the brink of high latitudes where ozone loss has been most notable. The problem seems one to which we should pay attention.

Thanks to a steady barrage of media coverage, most people now know that the three-atom molecule of oxygen known as ozone blocks much of the hazardous ultraviolet light emitted by the sun. There's now less ozone in the stratosphere available to carry out this protective service because people have mucked up the natural chemistry of the atmosphere. In all innocence, we devised chemicals that functioned superbly as fire extinguishers and air-conditioner fillers (the halogen compounds, especially the ones known as CFCs) but that also broke up ozone molecules in the high portion of the atmosphere called the stratosphere.

The struggles to replace the halogen family of chemicals with ones as useful but more atmospherically friendly have been well reported. Less well reported is the fact that other gases also affect ozone. Among them are the nitrogen oxides produced by combustion. Jet engines bring combustion products right to the threshold of the stratosphere, and even into it.

That happens because the lower edge of the stratosphere (demarked by the tropopause, the cutoff point of the breathable stuff in the troposphere, which skims the solid earth) falls at different altitudes depending on latitude, season, and even shifts in the jet stream. Near the equator, the stratosphere starts about 18 kilometers above the ground. At the poles, it's between 6 and 8 kilometers high.

In terms your captain and flight crew would use, it's easy for a polar-route airliner to enter the stratosphere: 9 kilometers is about 31,000 feet. At the equator, even the high-flying supersonic Concorde should stay within the troposphere. Its cruising altitude of 55,000 feet is near 16.5 kilometers.

The most important contribution commercial flights make to ozone depletion may be as much physical as chemical. Water vapor and nitrogen oxides in the exhaust gases of high-flying aircraft apparently help form polar stratospheric clouds. The particles making up these clouds provide surfaces on which the ozone-destroying chemical reactions can take place.

You might think, as at first I did, that a simple fix would be to force airplanes to fly lower. That burns more fuel, but it would release ozone-destroying combustion products below the tropopause. Unfortunately, measurements made of jets flying in the troposphere show that at those altitudes they're producing photochemical smog that sullies the airlanes just as automobile-produced smog chokes the air over highways. Furthermore, at the lower altitudes their emissions enhance the greenhouse effect that is leading to global warming.

Well, drat. Whether letting in cancer-causing UV radiation or cranking up the global thermostat, it looks as if our beloved jet planes are dirty birds indeed. The only good news is that some mighty big brains are seeking ways to clean up the airplanes' emissions and keep all of us travel addicts flying. The bad news is that some equally prodigious thinkers are advocating massive tax increases for jet fuel to make flying so expensive that few of us would indulge in that convenient form of travel.