Will spring 2005 be a bad one for Arctic ozone?

Guest Commentary by Drew Shindell (NASA GISS)

The current winter and early spring have been extremely cold in the Arctic stratosphere, leading to the potential for substantial ozone depletion there. This has been alluded to recently in the press (Sitnews, Seattle Post Intelligencer), but what’s the likely outcome, and why is it happening?

Min Arctic Strat Temp 50N-90N 2004/2005(current conditions)

First, let’s go over some background. The recipe for massive springtime ozone loss in the polar regions, such as the annual ozone hole seen over Antarctica during the past two decades, is fairly simple. The two main ingredients are reactive halogen gases such as chlorine or bromine and sunlight. To prepare, keep the halogens at extremely cold temperatures, typically below –78 C (195 K). Use a strong polar vortex to mix the halogens to help achieve the required temperatures. When the mixture has been properly chilled, add sunlight and you’ll get rapid ozone destruction.

In the real world, both chlorine and bromine are readily available in the stratosphere worldwide, especially chlorine from chlorofluorocarbons which are well mixed in the lower atmosphere (where they are stable), before entering the stratosphere where they are photochemically decomposed. The halogens that are released generally end up in fairly unreactive forms where they have only a small effect on ozone. Under extremely cold conditions, however, ice and supercooled liquid droplets (so-called Polar Stratospheric Clouds – PSCs) can form even at the low densities present in the lower stratosphere (~15-25 km altitude). Chemical reactions on the surfaces of these particles can rapidly convert halogens into very reactive forms. Ozone depletion is therefore extremely sensitive to small changes in temperature when the stratosphere is near this freezing point. The temperatures themselves are greatly influenced by the strength of the polar vortex, a wind that swirls around the pole and when strong, can keep air confined throughout the winter in the polar night, allowing it to cool dramatically. The primary reason ozone depletion has been weaker over the Arctic than over Antarctica is than Arctic temperatures are typically about 10 degrees warmer as the Arctic vortex is generally weaker than its Antarctic counterpart. This is because of the differences in layout of the continents in the two hemispheres affects the dynamics of stratospheric circulation.

The other key factor is that even if chemical conversion into reactive forms occurs during the cold, dark polar winter, the reactive chlorine must stick around until sunlight returns to the polar region for ozone destruction to take place. This is why ozone depletion over the poles is a springtime phenomenon. Even following a very cold winter, if temperatures warm quickly during spring very little ozone loss may take place. Alternatively, a milder winter, provided it was still cold enough to lead to chemical processing of halogens, could be followed by greater springtime ozone losses if temperatures stayed cold longer. Thus temperatures during the period when a lot of sunlight first returns to the polar areas following winter, March in the Arctic and September in the Antarctic, are crucial.

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