The Montreal Protocol, the 1987 UN treaty that restricted the use of the major chlorine- and bromine-containing halocarbons that were damaging the stratospheric ozone layer, has led to a decrease in the main stratospheric chlorine reservoir, hydrogen chloride. This paper reports a recent increase in hydrogen chloride in the Northern Hemisphere. With the help of a model study, the authors attribute the phenomenon to a short-term slowdown in the Northern Hemisphere atmospheric circulation that transports aged air, with a larger relative conversion of chlorine source gases to hydrogen chloride in the lower stratosphere. The increase in hydrogen chloride is not the result of unaccounted emissions from chlorine-containing source gases and the Montreal treaty continues to lead to an overall long-term reduction in stratospheric halogens.
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