Why is the ozone hole a seasonal event?

Answer 1

Ozone in the ozone layer is made by UV-C from our Sun breaking apart oxygen molecules, some of these oxygen atoms form ozone when interacting with whole oxygen molecules. Ozone is naturally unstable (decays with time), and contaminants (H2O, Cl, Br, etc) accelerate the decay of ozone. The ozone hole forms when the pole stops getting UV-C from the Sun, and ozone decays away (for the various reasons above). The ozone that is there, is largely "long lived" remnants, ozone that has diffused in from adjacent regions, and ozone formed by cosmic rays from space. The ozone hole starts healing in the spring, when UV-C from the Sun starts arriving and ozone is made. By summer, ozone has reached its peak equilibrium concentration. UV-C from the Sun is now striking the atmosphere over the pole, now that it is spring there, and the ozone hole heals shut by summer. The ozone hole is a natural occurance, that occurs once a year at each pole. The southern polar hole is larger than the northern polar hole due to Earth's current magnetic alignment. The *size* of the hole is what is of concern... and something that the affairs of Man can play a part in. Ozone decays naturally with time. With the axial tilt that Earth has, once each year (local winter) each pole stops receiving the UV-C that turns some oxygen into ozone. So the ozone starts decaying, and a hole forms. The only ozone the pole gets at this time, diffuses in from areas that are still receiving UV-C. So the ozone would be exceedingly thin at this time. Add contaminants to the mix, and the amount of ozone drastically decreases. Water vapor (natural and Man-sourced), chlorine (most commonly Man-sourced, carried by CFCs), and bromine (most commonly natural, but likely some Man-sourced, carried form example in halon) all have shown abilities in depleting ozone. The ozone hole was discovered by Joseph Farman, Brian Gardiner, and Jonathan Shanklin in 1985. But no reason to believe that it has not existed as long as Earth has had an ozone layer (to have a hole in), an axial tilt (for winter, loss of UV-C which makes ozone), and a magnetic field (to make one hole larger than the other). The concentration of ozone at any point is a balance of incident UV-C from the Sun (both making and destroying ozone), UV-B from the Sun (destroys ozone when absorbed), time, and compounds that can accelerate the decay of ozone. The ozone holes are a natural phenomenon, and some sort of "ozone hole" will form at the pole that is experiencing winter. As spring arrives, UV-C from the Sun arrives and makes ozone... "healing" or filling the hole. The issue with ozone depletion is how little ozone is in the "ozone layer", due to the effect of contaminants. These same contaminants control how big the hole is, how little ozone there is still left in the hole, and how long it takes to heal up. During winter, the atmosphere above the pole receives no UV-C from the Sun to make new ozone. So the ozone decays both naturally, and as a result of contaminants up in the atmosphere with it. So the equilibrium value of ozone above the pole depends on diffusion from adjacent areas that are still making ozone. Spring, summer, and fall, the ozone hole heals up, is essentially non-detect, then slowly starts forming again.

Answer 2

Ozone depletion occurs mostly in winters. This is why ozone hole is a seasonal event.