The largest amounts of chlorofluorocarbons (CFC's) have been emitted over the eastern United States and western Europe, as shown in this visualization of the annual mean CFC-11 emissions rate. Red marks the highest emissions.

In the history of invention, good ideas sometimes turn out to be disasters. Our own time has such a story in chlorofluorocarbons (CFCs).

CFCs were thought to be the perfect designer chemicals: they were cheap and did not react with anything. Only after air conditioners and spray cans ejected CFCs for decades did scientists realize their role in depleting the ozone that protects life from the sun's ultraviolet rays.

Photo by Judy Conlon

Richard Turco, UCLA

“The CFCs were designed to be inert and safe, and thus they can survive long enough in the atmosphere to reach the ozone layer,” says Richard Turco, professor of atmospheric sciences at the University of California, Los Angeles (UCLA). Up in the stratosphere, CFCs become reactive and break up ozone molecules, opening a massive ozone hole over Antarctica each year.

Turco has worked on the ozone hole problem since its discovery in the mid-1980s. Using a NASA supercomputer, he and colleagues recently simulated the slow emission of 20 million tons of CFCs, including the reactions that change the chemicals into ozone-eaters.

Making history

For their first global chemistry calculation, the UCLA team wanted to do nothing less than explore the full history of the two most widely used CFCs, substances known as CFC-11 and CFC-12. CFC usage was relatively low from 1931 until 1965, so
the researchers combined emission estimates for that period and used them
as starting conditions for a simulation
that ran through 2000.

Photo by Judy Conlon

C. Roberto Mechoso leads development of UCLA climate models, which include coupled atmospheric chemistry and general circulation models used in a 35-year global simulation of CFC-11 and CFC-12. NASA has funded model development since 1992.

“Nobody had performed a similar 35-year simulation of these species,” says Mohan Gupta, a team member now at the Electric Power Research Institute in Palo Alto, Calif. “This is a landmark effort, doing global chemistry and dynamics at the same time.”

The UCLA team couples two mathematical models to mimic the actual journeys CFCs undergo. The group's Atmospheric Chemistry Model (ACM) does the chemistry, keeping track of CFC emissions and subsequent chemical reactions. Their Atmospheric General Circulation Model (AGCM) does the dynamics, its winds moving the CFCs through the atmosphere.

Simulating all the relevant factors for 35 years would be impossible without a supercomputer. The work is especially challenging because the ACM calculates CFC quantities and reactions every simulated hour. Meanwhile, the AGCM changes atmospheric conditions every four simulated minutes. Such brutal demands consumed 22,225 processor hours on the CRAY T3E system at NASA Goddard Space Flight Center. “We spent many late nights looking after the simulations and analyzing the results,” Gupta says.

CFCs everywhere

The simulations begin by emitting CFCs all over the Earth's surface. Emission quantities come from reports of CFC sales and consumption, with projections for non-reporting countries. The team maps emission locations according to each country's Gross Domestic Product, electricity use and population density.

Image by GSFC, Scientific Visualization Studio

Up in the stratosphere, a chlorine atom released from a CFC attaches iteself to an ozone molecule and steals one of its three oxygen atoms. This proces destroys ozone by converting it into oxygen and contrbiutes to a massive ozone hole over Antartica each year.

After release, the CFCs drift around the globe to altitudes as high as 30 miles, or the top of the stratosphere. Once they pass the ozone layer (around 12 miles), CFCs strongly absorb the sun's ultraviolet rays, which are also treated in the model. The rays break molecular bonds, unloosing the CFCs' chlorine atoms (CFC-11 has three chlorine atoms; CFC-12 has two).

The free chlorine atoms then drift off to destroy ozone, but that process will have to wait for a future simulation. “We will need very large computers to achieve the degree of detail required by the ozone hole problem,” says C. Roberto Mechoso, the atmospheric sciences professor who leads development of the UCLA models, an effort NASA has funded since 1992. “Working in this area is one of our goals,” he adds.

Mechoso is confident they will tackle ozone depletion because their CFC simulation got the first part of the problem right: it produced CFC trends that look very much like reality. “We now have a 3-D picture of how CFCs emitted at the surface move around in the atmosphere to end up in remote places,” he says. The ground truth came from a network of eight observation stations scattered throughout the globe. For
CFC-11 and CFC-12, the simulation typically differed by just a few percentage points from each station's readings.

Image by NASA/Total Ozone Mapping Spectrometer

In September 2000, a NASA satellite observed a record-sized ozone hole roughly three times larger than the United States.

Success in matching observations worsened after 1996, a discrepancy due to the vagaries of human behavior rather than any model defect. A worldwide ban on CFC production took effect in 1996. So, lacking other specific information, the UCLA group assumed zero emissions from that year onwards to test the effectiveness of the phase-out. With observations showing significantly more CFCs than the simulation, it became clear that substantial emissions continued after the ban. “This outcome was not unexpected because many tons of CFCs are still locked in refrigerators and air conditioners manufactured years ago,” Turco says.

Unfortunately, CFCs will continue to leak for decades and contribute to ozone depletion, as well as global warming. Like other greenhouse gases, CFCs trap heat coming off the planet surface and radiate that heat back through the atmosphere. How long the Earth will have to endure these effects depends on the ultimate lifetimes of CFCs in the air.

“People have been reporting different CFC lifetimes, which are very important because of the global warming potential of those molecules,” Gupta says. The new UCLA studies have significantly narrowed the uncertainty in those lifetimes.

Photo by Judy Conlon

Mohan Gupta, EPRI

Gupta was surprised at how much CFC lifetimes decrease when the vertical limit of the simulation is the full stratosphere rather than the stratosphere's lower realms where most of the CFC break-up occurs. The estimated lifetime for CFC-12 drops more than 50 percent, from 220 years to 103 years. CFC-11 longevity has a smaller drop, from 69 years to 54 years.

Whatever their lifetimes, most CFCs are still with us. The simulations suggest that 74 percent of all CFC-11 emissions and 84 percent of all CFC-12 emissions remained in the atmosphere at the end of 1996. Those figures translate into 16 million tons of CFCs floating about, before accounting for more recent emissions.

“In essence, society has locked in the ozone hole and ozone depletion for many, many generations,” Turco says. “It is the legacy of rampantproduction of CFCs in previous years.”

Image by Joseph Spahr, UCLA	Graph by Mohan Gupta, EPRI	Graph by Mohan Gupta, EPRI
This world map shows the concentration of CFC-11 in the lowest model layer (centered at about 0.14 miles) averaged for the northern hemispheric winter in 1990. The range is from 210 parts per million (black) to 350 parts per million (red).	Simulation results for CFC-12 differ by only a few percentage points from surface observations in both hemispheres. Results worsen after 1996, when the simulations stop emissions to mimic an ideal ban on CFC production and release.	Another finding is that estimated CFC lifetimes decrease when the calculations inlcuded the full height of the stratospher,e which extends to 50 kilometers or 30 miles .

Credits for Insights Magazine go to the following people along with the writers and photographers who contribute to each issue and the researchers and specialists whose material is highlighted:

Program manager: Dr. Eugene L. Tu
Deputy program manager: Dr. William R. Van Dalsem
Editor, writer, photographer and designer: Judy Conlon
Web curator: John Hardman

Insights was published by the HPCC Program office and produced by Raytheon contractor staff at NASA Ames Research Center.