The AOH is an ephemeral (every Oct-Nov) thinning of stratospheric ozone at an altitude of 20-25 km, roughly covering the Antarctic continent; unanticipated, it was discovered serendipitously in 1985 but is now tracked with satellite-borne ozone meters. Its discovery created much panic about an epidemic of skin cancers that led directly to passage of the 1987 Montreal Protocol, an international treaty stopping the manufacture and release into the atmosphere of ozone-depleting chemicals, including CFCs used in refrigeration and bromine-containing fire suppressants.
Recently, there have been many voices, suggesting that the AOH is shrinking, presumably as a result of the Protocol. I am somewhat skeptical of the evidence, but also for theoretical reasons. I am inclined to blame wishful thinking –- a desire to justify post facto the 1987 Montreal Protocol and the economic losses it has produced around the world since then. By implication also, this tends to support the concept of a (largely unrelated) global climate treaty that would severely reduce the release of the greenhouse gas CO2.
For evidence, I refer to a well-written semi-popular story in Eos of 15 August 2016, which relies mainly on a paper in Science magazine [of 30 June, 2016] by MIT chemist Susan Solomon et al. The credibility of the paper derives from the fact that its lead author had identified the correct mechanism for creating the AOH at a time when there was much dispute about its cause; it turned out to be ‘heterogeneous’ reactions of chlorine compounds on the surfaces of polar stratospheric clouds (PSCs), made up of ice particles created from stratospheric moisture by the extremely cold local temperatures. [Heterogeneous reactions involve both gas molecules and solid particles, while ‘homogeneous’ reactions involve only gas molecules.]
These reactions eventually release free chlorine atoms (able to destroy ozone catalytically) from the existing stratospheric chlorine reservoir, gaseous HCl –hydrogen chloride. The relevant chemical reactions commence when solar radiation reaches the Antarctic stratosphere in the beginning of Spring, i.e., in October, after a winter darkness lasting up to six months.
But the same Eos story also quotes NASA atmospheric scientist Susan Strahan, who points to the difficulty of identifying a trend in the presence of “noise,” the year-to-year variation in geographic extent of the AOH. Worse still, the AOH can also be characterized by other varying parameters, like depth of depletion and by its duration. Nevertheless, Solomon extrapolates the somewhat uncertain geographic trend and boldly estimates that the AOH will seal up and disappear by mid-century.
In the American Geophysical Union journal Earth Future, atmospheric chemist Guy Brasseur and colleagues suggest a faster way to “heal” the AOH – by actively releasing ice particles in the stratosphere to deplete HCl, the main reservoir of stratospheric chlorine. But they do not consider the continued existence of natural sources of chlorine compounds: frequent volcanic injections and possibly also oceanic salt spray carried into the stratosphere by convection. Worse still, they ignore the risks of their proposed geo-engineering scheme – the strong greenhouse effects of their ice particles, which would absorb and then re-emit (albeit at a much lower temperature) most of the outgoing long-wave radiation from earth into space, covering even the normally open atmospheric infrared “window” region (of 8 – 12 microns).