Professor Koehler explores jet fuel-climate link

Finding Professor Birgit Koehler’s laboratory on the second floor of Thompson Chemistry isn’t difficult: just let your ears lead you to the gentle roar of a steam locomotive. Inside the door of the laboratory, instead of a 19th century choo-choo, several vacuum pumps chug out a steady rhythm behind a metal chamber adorned with tubes and nozzles. This chamber is actually nothing like the boilers of old fashioned trains. Instead of holding in high pressure steam, this device simulates the extremely cold, low pressure conditions of the tropopause, the region of the troposphere six miles up that has become the domain of jet aircraft. Dr. Koehler is one of many atmospheric scientists currently studying the behavior of anthropogenic pollutants at altitude in the hope of eventually understanding how exhaust from high-flying jet engines is affecting our climate.

It is often easy to spot several airplanes trailing long thin white lines overhead while walking across campus on a clear day. These condensation trails – or contrails – form when the hot, moist exhaust from a jet engine meets the frigid air of the upper troposphere. Water vapor in the exhaust and in the atmosphere freezes into billions of tiny ice crystals, forming cloud particles similar to the mist that appears when you exhale on a cold winter day.

The criss-crossing contrails seem harmless enough at 30,000 feet, but recent meteorological studies suggest that they are actually tweaking the climate on the ground. In effect, the world’s jumbo jets have become a fleet of artificial cloudmakers. As turbulent winds spread contrails out into thin sheets, the ice crystals become identical to naturally occurring cirrus clouds, which are “warming clouds.” Like the glass panes of a greenhouse, cirrus clouds allow short-wavelength ultraviolet radiation from the sun to pass through the atmosphere, but trap longer wavelength infrared radiation (ie. heat energy) near the Earth. The buzzword is “greenhouse effect,” and climatologists fear that airplane induced cirrus clouds are accelerating global warming.

Jet exhaust also stimulates cloud formation indirectly, by spewing tiny aerosol particles of sulfur dioxide (SO2) and soot, the black carbon byproduct of incomplete combustion of fossil fuels. At some point between leaving the jet engine and becoming the contrail, SO2 is believed to stick to the soot particles, which leads to a chemical reaction that produces sulfuric acid (H2SO4) droplets. The sulfuric acid droplets then grab nearby water molecules, essentially becoming cloud “seeds” by providing surfaces upon which water molecules can condense or freeze.

As an initial step towards understanding contrail formation, Professor Koehler’s research has focused on how SO2 binds to soot at various temperatures and pressures. A tour through the lab with Professor Koehler’s thesis student, Annabel Muenter ’99, reveals that this is not as easy as it sounds. Muenter explains that a wafer of germanium – a semiconductor about the size and shape of a quarter – is first coated with a thin layer of soot from burned hexane. The wafer is then positioned horizontally in the center of the chamber where a beam of infrared light passes directly through it and the soot coating. When almost all of the air is evacuated from the chamber, SO2 is pumped in and interacts with the soot. Sensors on the wafer monitor the temperature of the soot, while a Fourier Transform Infrared Spectrometer (FTIR) monitors how much of the infrared light passing through the wafer is being absorbed. Essentially, as more SO2 binds to the soot surface, less infrared light is able to pass through. This setup allows them to quantitatively measure the amount of SO2 attached to soot at various temperatures and pressures.

NASA has taken a different approach to studying the chemistry of jet exhaust. In a 1996 field project titled SUCCESS (SUbsonic aircraft: Contrail & Cloud Effects Special Study), a team of scientists sampled fresh exhaust plumes by trailing sensors directly behind cruising jetliners. In the experiment, a DC-8 aircraft, loaded with a suite of gaseous, particulate, radiative, and meteorological instruments, tracked the chemical “scent” of a 757 from distances as great as 10 miles. A smaller Saberliner jet was also used for closer approaches to the 757, at times hanging only 150 meters behind the engines to collect fresh samples. According to the scientists, the sampling was not always smooth sailing. The 757’s backwash – a tight horizontal tornado spiraling at over 100 mph – occasionally flipped over the light Saberliner completely, causing scientists and instruments to go temporarily weightless before the pilots righted the upside-down plane. Fans of the fighter jet movie Top Gun will recall a similar incident where Maverick (Tom Cruise) loses control of an F-14 after flying through the jet wash of his wingman. Along with using all of their available air-sickness bags, the SUCCESS scientists discovered that sulfuric acid emissions at altitude are actually 10% higher than those measured from jet engines on the ground, indicating that aircraft are better cloud seeders than previously thought.

The results of Professor Koehler’s SO2 research might explain why the SUCCESS scientists found more sulfuric acid than they expected. At the high altitude conditions inside Koehler’s vacuum chamber, significant amounts of SO2 accumulated on the sooty wafer of germanium, which could possibly indicate the first stages of sulfuric acid formation. In order to learn more about the way soot interacts with other trace gases, Professor Koehler is now studying the behavior of ammonia (a byproduct of agricultural waste) at low temperatures and pressures.

Contrail research tends to be a very mediagenic aspect of atmospheric science, because jet travel and cirrus clouds are eye-catchy and easily recognizable. The connections between SO2, soot, and zooming airplanes is exciting, but it is important to remember that Professor Koehler’s research does not actually revolve around contrails and jet technology. “Our research is aimed at understanding the basics of atmospheric chemistry,” she explains. “It’s a nice bonus that our work, in turn, facilitates practical applications in climatology and in environmental considerations of jet aircraft.” Professor Koehler encourages anyone interested in learning more about atmospheric science, contrails, or noisy vacuum chambers to attend her Faculty Lecture Series presentation at Brooks-Rogers on March 11, where she will discuss her research in greater depth.