ClO concentrations in the Arctic vortex are as high as those seen in the Antarctic, more than about 1 p.p.b.v. (part per billion by volume), and when such high concentrations are exposed to sunlight, it is well established that ozone loss will take place at a high rate. It is nitric acid (HNO3) that makes the difference: in the Antarctic, temperatures are low for long enough that particles containing nitric acid and water grow large enough to fall out of the stratosphere, leading to denitrification and dehydration. In the Arctic, temperatures are low enough for these particles to form, but not for long enough to lead to large-scale denitrification or dehydration. Because there is a lot more nitric acid in the Arctic atmosphere than in the Antarctic, ozone loss occurs in competition with recovery of ClO to chlorine nitrate (ClONO2) via reaction with NO2released from nitric-acid photolysis. So less ozone loss can occur here than in the Antarctic, even though the Arctic atmosphere is entirely processed by heterogeneous reactions on the surface of ice and water droplets that convert inert chlorine molecules to ClO.
Yet from initial amounts of the order of 450 Dobson units (DU), Arctic ozone concentrations have been falling to about 300 DU in the spring. (Baseline ozone amounts in the Antarctic before the emergence of the ‘ozone hole’ were 300 DU; they now routinely fall to about 100 DU.) So why are we now seeing these large ozone losses in the Arctic? The answer appears to be longer stratospheric winters. Even though there is little denitrification, temperatures do remain cold long enough to tie up nitrogen as HNO3well into the sunlit period in late winter and early spring, and to cause sporadic reprocessing of the air by heterogeneous reactions in polar stratospheric clouds. The temporary lack of nitrogen allows more ozone removal; and, because absorption by ozone warms the stratosphere, these losses feed back and may keep the stratosphere cold later into the spring. In fact, Rex et al.2 saw a region in the Arctic in early 1996 where some denitrification occurred and ozone loss continued unabated for more than two months.
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