|Growth of HCFC-142b in the stratosphere over the past decade as measured from the NASA ER-2 aircraft by NCAR's Whole Air Sampler. The plot uses nitrous oxide (N2O) as a reference for quantifying the penetration and accumulation of this HCFC into the stratosphere. For this HCFC, growth rates in the lower stratosphere are approximately 5090% of the tropospheric growth rates. Currently about one-eighth of the total mass of HCFC-142b in the atmosphere resides in the stratosphere. (Illustration courtesy of Elliot Atlas.)|
The HCFCs "get destroyed quite a bit faster in the lower atmosphere than the CFCs do, so, ultimately, less gets into the stratosphere to cause damage," says Elliot Atlas, head of NCAR's Stratospheric/Tropospheric Measurements (S/TM) Group. "But it has not been well appreciated that some of these compounds do reach the stratosphere. The concentrations of the HCFCs are also increasing there, though at a somewhat slower rate than in the troposphere. Also, the stratospheric lifetimes of the HCFCs are longer than those of the major CFCs, so to the extent that they do accumulate, that will drag out the impact of chlorine on ozone."
To learn the extent of that accumulation, Atlas, NCAR coinvestigator Susan Schauffler, and members of the S/TM group have compiled the most complete studies to date on the amounts of HCFCs and related compounds that are now in the stratosphere. Their research indicates that, although these chemicals are releasing some chlorine atoms into the stratosphere as they decompose, the ozone-depleting atoms are accumulating at a much slower rate than before the Montreal Protocol was implemented. The scientists have presented their work at meetings of the American and European Geophysical Unions and are preparing a paper.
NCAR is a global leader in monitoring HCFCs, CFCs, and other compounds in the stratosphere. Studies at NCAR originated in the 1970s with the work of scientists Leroy Heidt, Walter Pollock, and Richard Lueb. They began by collecting stratospheric air samples from high-altitude balloons and rockets and later collected samples by aircraft. The S/TM group has continued and extended the aircraft studies. In the early days, "They shipped the whole lab to Norway or Chile, and analysis would be done right there," explains Schauffler. Atlas adds, "But now we have modern shipping methodsFedEx and a few more canisters to work with," so field teams ship the samples back to NCAR for analysis. Atlas's group has sent canisters on high-altitude flights during many field programs over the last decades. The most recent high- altitude flights were during the SOLVE (Stratospheric Aerosol and Gas Experiment III Ozone Loss and Validation Experiment) campaign in early 2000.
Determining how fast the compounds are accumulating in the stratosphere is a bit like calculating gas mileage. Schauffler and Atlas use calculations of the age of the air sample (that is, how long it had been in the stratosphere), check surface data to determine how much of each compound the sample contained when it got to the stratosphere, and analyze how much of the compounds the stratospheric air parcel still contains. By subtracting the final amount from the original amount, they can determine how many ozone-depleting atoms were released to the stratosphere.
The HCFCs that Atlas and Schauffler studied were growing at rates from 1 to 5 parts per trillion by volume (pptv) per year. They still have much lower stratospheric concentrations than the CFCs. For example, the foam- blowing agent HCFC-142b (C2H3ClF2) has a concentration of about 20 pptv, whereas CFC-12 (CCl2F2)a refrigerantis at about 550 pptv. Also, of course, each molecule of HCFC-142b has only half as much chlorine as a CFC-12 molecule to begin with.
Most of the HCFC, hydrofluorocarbon, and perfluorocarbon concentrations in the stratosphere agree well with the emission rates reported by industry. That means, Atlas says, "We pretty well understand where [they're] coming from." The exception is HCFC-142b. "There appears to be more of that floating around in the atmosphere than was expected," Atlas notes.
Schauffler comments on the success of the Montreal Protocol. "All of these chlorinated compounds were produced by a very small number of chemical companies. It was a whole lot easier to get them to say, look, this problem is very reallet's get together and figure out some kinds of replacements [for the CFCs], than it [will be] with the whole globe in terms of carbon dioxide."
Edited by Carol Rasmussen,
Prepared for the Web by Jacque Marshall
Last revised: Thu Jun 21 18:56:13 MDT 2001