CFC Police Win the Day
A detective story to savour ...
A welcome update to a caveat to the Ozone Success story in Saving our Skins.
A couple of papers out in Nature last week brought good news for our atmosphere. They showed that the rate of decline in the concentrations of CFC-11 in the atmosphere accelerated significantly over the last year, indicating that we’re back on track for a complete ozone recovery in the second half of the century.
First, a short background tutorial. A digression for the uninitiated …
CFC-11 - otherwise known as trichlorofluoromethane (CFCl3) - was widely manufactured since the mid 20th century for use in refrigeration, as a spray-can propellant and in foam-blowing agents. As the name suggests, it contains three atoms of chlorine, which, when released from that molecular form can efficiently destroy ozone in the atmosphere. It’s one of the two major ozone depleting chemicals (the other being CFC-12). Under the terms of the Montreal Protocol on Protection of the Ozone Layer, its phase-out began in the early 1990s, and all production has been banned since 2010. But its lifetime in the atmosphere is long – with a half-life of more than fifty years.
CFC-11’s great stability in the lower atmosphere arises from the strong chemical bonds that hold it together. They’re strong enough to withstand the bombardment from any UV radiation that’s present (solar radiation at the Earth surface is limited to wavelengths greater than 290 nm, but the greater photon energies from wavelengths less than 230 nm are needed to break it down). Only after it rises high into the stratosphere is it exposed to the short wavelength UV radiation needed to break it down and release those ozone-damaging chlorine atoms.
It takes a long time for gases to get up to those altitudes because the high concentrations of ozone present in the stratosphere - above 15 km or so - lead to an inversion layer, which acts as a ‘lid’ to vertical motions. In the lower atmosphere (i.e., the troposphere), where temperatures decrease with altitude, air is rapidly mixed by turbulence. But that changes in the stratosphere above, where the air is warmed through ozone’s absorption of sunlight (and outgoing radiation from the earth). As a result, temperatures no longer decrease with altitude. That’s just the condition needed for an inversion layer where vertical movement is curtailed (just as for the more familiar inversion layers of fog and smog that we see on a cold winter morning when the air temperature above the ground is higher than at the surface). That means there’s little turbulent mixing, and the air is stratified into layers (which is why the region is called the stratosphere). Because of that ‘lid’ effect, only a tiny fraction of the air in the lower atmosphere even makes it into the stratosphere each year, and that slow rate of exchange is a limiting factor in the lifetime of CFCs. Consequently, its concentration in the atmosphere declines over multi-decadal time scales.
But enough lecturing on the topic. Back to the main issue ….
About three years ago, researchers responsible for measuring atmospheric CFC-11 noticed that the rate of decline in its concentrations had become even slower than it should have been. Somebody was up to mischief. Persons unknown must have been involved in the clandestine production of the prohibited gas.
With an ingenious piece of sleuthing they used data from several monitoring sites in a global network, in conjunction with calculations of back trajectories of air movements to show that the illegal production was from Eastern China. Once the secret was out, the Chinese Government vowed to remedy the problem, but with strained relations between China and the rest of the world, who knew what would really happen?
Thankfully, as Nature reports, data from the just-published papers showed that they were as good as their word. The rate of reduction in atmospheric CFC-11 last year was 1 percent per year, the fastest rate recorded.
And back-trajectory analyses clearly show the improvement is due to reduced emissions from Eastern China caused by the closure of the rogue manufacturing. All credit to China for rooting out the problem. And well done the atmospheric science detective team for alerting the world of the problem in the first place, and then verifying the fix.
But it’s not all beer and skittles. The studies also showed that China wasn’t the only culprit. While changes in China accounted for most of the observed improvement, other countries - still to be identified - also apparently buckled to the international pressure. But will they continue to do so?
Continued internationally-coordinated monitoring is needed in the decades ahead to ensure that all nations of the world continue to meet their emissions obligations for both ozone-depleting chemicals and greenhouse gases.
Although we do measure CFCs at Lauder, none of our data were used in these new studies. But I was pleased to see that both papers at least cited one that I worked on a few years ago with colleagues in the UK. 😊
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Hi Richard,
Good news to see reduction of CFC concentration in the atmosphere. Please can you show historical concentrations in the atmosphere of both ozone and CFC's on a single graph.