Abstract
Abstract. Without the Montreal Protocol, the already extreme Arctic ozone losses in the boreal spring of 2020 would be expected to have produced an Antarctic-like ozone hole, based upon simulations performed using the specified dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM) and using an alternate emission scenario of 3.5 % growth in ozone-depleting substances from 1985 onwards. In particular, we find that the area of total ozone below 220 DU (Dobson units), a standard metric of Antarctic ozone hole size, would have covered about 20 million km2. Record observed local lows of 0.1 ppmv (parts per million by volume) at some altitudes in the lower stratosphere seen by ozonesondes in March 2020 would have reached 0.01, again similar to the Antarctic. Spring ozone depletion would have begun earlier and lasted longer without the Montreal Protocol, and by 2020, the year-round ozone depletion would have begun to dramatically diverge from the observed case. This extreme year also provides an opportunity to test parameterizations of polar stratospheric cloud impacts on denitrification and, thereby, to improve stratospheric models of both the real world and alternate scenarios. In particular, we find that decreasing the parameterized nitric acid trihydrate number density in SD-WACCM, which subsequently increases denitrification, improves the agreement with observations for both nitric acid and ozone. This study reinforces that the historically extreme 2020 Arctic ozone depletion is not cause for concern over the Montreal Protocol's effectiveness but rather demonstrates that the Montreal Protocol indeed merits celebration for avoiding an Arctic ozone hole.
Highlights
In the 1970s, Molina and Rowland (1984) issued a prescient warning to humanity that the chlorofluorocarbons (CFCs) contained in popular refrigerants, building foams, and aerosol cans posed a danger to the stratospheric ozone layer
We have demonstrated that, were it not for the Montreal Protocol, the meteorological conditions seen in 2020 would have produced the first Antarctic-like ozone hole over the Arctic, an area with a substantial human population and vibrant ecosystem
Nitric acid observations and modeling for 2020 help improve our understanding of the role of denitrification in accurately assessing Arctic ozone loss, and further refinements of this will be the subject of future studies
Summary
In the 1970s, Molina and Rowland (1984) issued a prescient warning to humanity that the chlorofluorocarbons (CFCs) contained in popular refrigerants, building foams, and aerosol cans posed a danger to the stratospheric ozone layer. Definitions for what constitutes an ozone hole have been debated in the scientific literature (see Langematz et al, 2018, and references therein), but for purposes of comparison to the discovery of the Antarctic ozone hole and its impact on policy of the era, here we use the historical definition of total ozone area below 220 DU (Dobson units; Stolarksi et al, 1990) Another important metric is extreme locally depleted ozone mixing ratios in the lower stratosphere (Hofmann et al, 1997), providing an important fingerprint for chemical ozone loss driven by chlorine chemistry on PSCs. In response to the increasing ozone depletion, the global community came together to pass the 1987 Montreal Protocol on Substances that Deplete
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