A Stratospheric "Clock" to Measure Upper Atmosphere Circulation
Aircraft exhaust from midlatitude flight paths moves at an unknown rate into the tropics, is lofted within the tropics and moved at an unknown rate to higher latitudes where it could significantly deplete the protective ozone layer in the upper stratosphere. The air motions within the stratosphere are poorly constrained, since direct observation at these altitudes is difficult. Therefore, we rely upon observations of "tracers" — chemical species with known sources and sinks that are carried along with the stratospheric circulation — to reveal this circulation.
Two potentially useful stratospheric tracers are isotopes of beryllium — 10Be and 7Be — which are generated by the collision of high-energy particles from space with nitrogen atoms in the atmosphere. Most tracer production occurs between about 30° 70° latitude in both hemispheres of the lower stratosphere, as indicated by the circled regions on the figure. These tracers, which are borne on aerosol particles, are removed from the stratosphere by radioactive decay. While beryllium-7 decays relatively quickly, with a half-life of 53 days, 10Be's decay rate is negligible. The only sink for 10Be occurs after it enters the troposphere, where the radionuclides are efficiently removed by precipitation. Therefore, if we look at the ratio of 10Be/7Be as air moves from the midlatitude production region to other parts of the stratosphere, the ratio will generally increase, as 7Be decays. Thus, the 10Be/7Be acts as a "clock" of airmass age.
The figure shows the 10Be/7Be ratio calculated in the GISS general circulation model (GCM) during January and March. In the tropical stratosphere, air rises from the troposphere and continues to ascend, but exchange with higher latitudes is inhibited. The 10Be/7Be ratio is very high (white region) since slow penetration of air from the mid-latitude production region allows much of the 7Be to decay. During the early northern hemisphere spring, air from the lower tropical stratosphere moves to higher latitudes relatively quickly. The result is the green blob of relatively high 10Be/7Be air at around 10 km overlying the North Pole as shown in the March figure. Some observations of the isotopes indicate that the ratio should be even higher, for example, as high as 5. Thus, perhaps the transport from the tropics to the pole should be even stronger in the model. It is also possible that too much air leaks into the troposphere in the model, depleting the ratio in the lower stratosphere.
Unfortunately, very few observations of 10Be exist, and none exist in the tropics. A better understanding of the rate of exchange between the tropics and higher latitudes in the stratosphere is crucial. Additional observations of 10Be and 7Be, combined with modeling studies would enable us to determine how long it takes air to move between the tropics and higher latitudes of the stratosphere. This information would help elucidate the mechanisms of upper atmospheric circulation and the development of the ozone hole.
Koch, D., and D. Rind 1998. Beryllium 10/beryllium 7 as a tracer of stratospheric transport. J. Geophys. Res. 103, 3907-3917.
Please address all inquiries about this research to Dr. David Rind.