Chlorofluorocarbons (CFCs) are heavier than air, so how do scientists suppose that these chemicals reach the altitude of the ozone layer to adversely affect it?

Jean M. Andino, in the department of environmental engineering sciences at the University of Florida, replies:

"One must consider two issues: the mechanisms for mixing between the troposphere (the bottom layer of the atmosphere) and the overlying stratosphere, and the average time that CFCs remain in the troposphere before chemical processes scrub them from the air. In very general terms, mixing within the atmosphere is caused by differences in temperature and by pressure gradients. These irregularities make some parcels of air buoyant, which results in the transport of pollutants throughout the atmosphere. Given sufficiently large variations in temperature and pressure, air parcels containing contaminants can be transported through the troposphere and into the stratosphere, in much the way that a hot air balloon can be used to loft people high above the ground and transport them from one place to another. Pollutants can reach the stratosphere, however, only if there are no major mechanisms that pull them out of the air while they are still in the troposphere.

"In general, there are two main mechanisms that remove compounds in the atmosphere: deposition and reaction. A common example of deposition is 'rain out': compounds that are soluble in water can be removed from the atmosphere by precipitation. This phenomenon is responsible for acid rain. The most abundant CFCs emitted into the troposphere are CFC 11 and CFC 12. These CFCs are not soluble in water, so deposition does not removed them from the air.

"The only other mechanism that removes compounds from the troposphere is reaction with an abundant oxidizing agent--such as hydroxyl radicals, ozone, or nitrate radicals. Atmospheric researchers have determined the rates at which several CFCs react with hydroxyl radicals; the lifetimes for these CFCs with respect to hydroxyl radicals is approximately 80 years. In other words, if hydroxyl radicals were the only thing reacting with the CFCs, it would take 80 years to completely remove them from the atmosphere. That is a long time! In comparison, methanol, a component of some alternative fuels, has a lifetime with respect to hydroxyl radical reaction of just 17 days. Ozone and nitrate radicals are even less effective at breaking down CFCs.

"Because CFCs are so long-lived in the lower atmosphere, there is ample time and opportunity for them to become well mixed and eventually to reach the stratosphere."

F. Sherwood Rowland of the University of California at Irvine, who won a Nobel Prize for his work on atmospheric chemistry, answers:

"This is indeed a persistent question--so much so that the most recent report of the World Meteorological Organization, entitled 'Scientific Assessment of Ozone Depletion: 1994,' included it among a list of common questions that have been persistently raised and long since answered. Susan Solomon of NOAA Aeronomy Laboratory in Boulder and I are listed in the document as the Coordinators of Common Questions about Ozone. We had as many as 22 of them, but pared them down to the most frequently asked ones.

"The response to this particular question reads as follows."

Although the CFC molecules are indeed several times heavier than air, thousands of measurements have been made from balloons, aircraft and satellites demonstrating that the CFCs are actually present in the stratosphere. The atmosphere is not stagnant. Winds mix the atmosphere to altitudes far above the top of the stratosphere much faster than molecules can settle according to their weight. Gases such as CFCs that are insoluble in water and relatively unreactive in the lower atmosphere (below about 10 kilometers) are quickly mixed and therefore reach the stratosphere regardless of their weight.

Much can be learned about the atmospheric fate of compounds from the measured changes in concentration versus altitude. For example, the two gases carbon tetrafluoride (CF4, produced mainly as a by-product of the manufacture of aluminum) and CFC-11 (CCl3F, used in a variety of human activities) are both much heavier than air. Carbon tetrafluoride is completely unreactive in the lower 99.9 percent of the atmosphere, and measurements show it to be nearly uniformly distributed throughout the atmosphere as shown in the figure. There have also been measurements over the past two decades of several other completely unreactive gases, one lighter than air (neon) and some heavier than air (argon, krypton), which show that they also mix upward uniformly through the stratosphere regardless of their weight, just as observed with carbon tetrafluoride. CFC-11 is unreactive in the lower atmosphere (below about 15 kilometers) and is similarly uniformly mixed there, as shown. The abundance of CFC-11 decreases as the gas reaches higher altitudes, where it is broken down by high energy solar ultraviolet radiation. Chlorine released from this breakdown of CFC-11 and other CFCs remains in the stratosphere for several years, where it destroys many thousands of molecules of ozone.

"The measurements of CFC-11 in the stratosphere were first described in 1975 by two research groups in Boulder, Colorado, and have been similarly observed innumerable times since. The uniform mixing of CF4 versus altitude was reported from balloons around 1980 and many times since, and from an infrared instrument aboard the space shuttle Challenger (which exploded in 1986) in 1985. My own research group has measured CFC-11 in hundreds of air canisters filled while flying in the NASA DC-8. We once did a descent directly over the North Pole and found uniform mixing in the lower atmosphere, and slightly less CFC-11 in the stratosphere.

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