Road works - traffic cones have been up for ages, but look, some diggers have actually moved in.

As everyone knows, pressure decreases with increasing altitude. In other words, the higher up in the earths atmosphere you get, the fewer molecules are present. This makes the comparison of absolute concentrations of certain species at different altitudes pretty useless, as {{Do some pV=nRT calcs later, get some proper numbers}}100 molecules cm^-3 would be a tiny fraction of the molecules present in the air at ground level, but all of the molecules present high up in the stratosphere. To overcome this, it is common to express the amount of a species present at a particular altitude as a fraction of the total number of molecules of substance present at that altitude:

The units of volume cancel, leaving the mixing ratio dimensionless. The numbers involved are often very small, so it is common to see the mixing ratio expressed in terms of ppmV, ppbV, and pptV, which stand for parts per million, parts per billion, and parts per trillion at any volume respectively.


Thus a mixing ratio of 4.5 x 10^-8 may also be expressed as 45 ppt.

The ozone layer occurs about 30 km above the surface of the earth, and has a normal mixing ratio of xyz ppb. However, during the arctic spring, the mixing ratio of ozone can get as low as zxy ppb over the south pole. The reasons for this local drop in the mixing ration are given below:

During the constant darkness of winter (june to august) the antarctic becomes a uniquely harsh environment. Temperature differentials cause strong westerly winds which set up a polar vortex around the antarctic. The strong winds around the vortex mean that the warmer air from the southern tropics mixes very poorly with the cold air inside, which becomes trapped in the extremely cold¹ core of the vortex.

This cold core is almost completely isolated fromm the rest of the stratosphere. The unique conditions inside the vortex allow the formation of polar stratoshperic clouds (PSCs) as high as 15km above the surface of the earth. These clouds provide a surface for reactions which convert chlorine resevoir species [--LINK--] into photochemically active species:

ClONO₂ + H₂O _(cloud) ----> HNO₃ + HOCl


ClONO₂ + HCl _(cloud) ----> HNO_{3 (cloud)} + Cl₂

Because of the constant darkness these species remain in the stratosphere, and their concentrations gradually build up over time. The accumulated HOCl and Cl₂ then provide a strong burst of Cl radicals in the spring at polar sunrise:

HOCl + hv ----> Cl + OH

Cl₂ + hv ----> Cl + Cl

The subsequent destruction of ozone [--LINK-CFCs--] is drastic [--LINK-external, graph--], and is driven by a slightly different catalytic cycle from normal due to the lower concentration of O atoms present at the start of the polar spring:

This sudden burst of photochemical activity every arctic spring is what gives rise to the annual ozone hole over the south pole. The ozone hole is a rather misleading term.

What it means for planet? ice. temperature in general. sun burn.