Air-Profiling and UV

Contributing to our harsher intensities down under ...

As promised in Brisbane ….

The media are always harping on the ozone layer. It’s not really a ‘layer’, which to me implies something thin with sharp edges (or a hen). In reality it’s more a like a broad region of ‘high’ ozone concentrations (but really only a few parts per million of air!) centred around 25 km, with decreasing concentrations above and below. That shape - or ‘profile’ as we like to call it - arises because you need two things to make ozone:
(1) oxygen molecules, and
(2) UV radiation with enough energy to break them apart to form oxygen atoms.
It’s the recombination of those oxygen atoms with normal molecules of oxygen that produces ozone (O + O₂ → O₃). High in the atmosphere, there isn’t enough oxygen in the thinner air (its density reduces by about a factor of ten for each 15-kilometre increase in altitude), and low in the atmosphere there isn’t enough UV from sunlight - because it’s been blocked out by the ozone generated above.

A couple of examples are shown in the left panel below where I’ve compared the mean summer profiles of ozone measured by balloon-borne instruments over mid-latitude sites in the northern hemisphere (Hohenpeissenberg, Germany) and the southern hemisphere (Lauder, New Zealand). The plots are beautified versions of figures from a paper I wrote two decades ago when I was trying to better understand why the UV was so much more intense in the south.

The first thing to notice is that there’s much less ozone at the southern site (the red line). The overall difference here is about 15 percent. But the latitudes aren’t quite congruent. The German site is 3 degrees of latitude closer to the pole, and summer ozone concentrations tend to be larger at higher mid-latitudes. When you compare latitude-for-latitude, the summer differences in total column ozone are a bit smaller - less than 10 percent.

The other thing to notice is that there’s much less ozone in the lowest part of the atmosphere at the southern site. That’s nothing to do with the Antarctic ozone hole. It’s because of our cleaner air. Higher concentrations of ozone occur in polluted air. In Europe, its concentration in the air around us is about 40 parts per billion, about twice that at the pristine Lauder site, where the concentration is more like what would have been present in Europe prior to the industrial revolution. These differences in the lower atmosphere are significant for UV radiation, as we’ll soon see.

The right-hand panel shows corresponding profiles of temperature measured during the same balloon flights. Unlike the ozone profiles, they’re remarkably similar at the two sites, especially in the lower atmosphere where the temperatures decrease rapidly at both - it gets about 6 degrees colder for each kilometre increase in altitude (you might have already noticed that it’s much colder in mountainous regions than at sea level). At the cruising altitude of jet aircraft - around 10 km altitude - it’s a freezing 50 degrees below zero Celsius!

At higher altitudes the temperature begins to increase again because of the higher concentrations of ozone present, which absorb energy from incoming sunlight. Interestingly, a significant party of the heating is due the absorption at longer wavelengths in the visible and infra-red regions (including outgoing infrared radiation from below). At UV wavelengths, where ozone absorption is strongest, the amount of energy available in sunlight is much smaller.

The turnaround altitude, where the temperature stops decreasing and starts to increase, is called the tropopause. The region below is called the troposphere (the ‘weather’ sphere), and the region above is called the stratosphere (sometimes called the ozone layer).

The reason I included these temperature plots is because of their effect on UV radiation. For most gases, the amount of absorption increases with temperature; and ozone is no exception. Its warmer temperature in the troposphere means that each molecule is more effective at blocking UV. But this increased ozone at lower altitudes in the north is doubly important because by the time UV from sunlight has penetrated to those depths low in the atmosphere, it has been largely scattered by air molecules and pollutants. Consequently, even when the sun is high in the sky, the radiation comes into that lower region at a range of different angles instead of through the short direct path straight from the sun.

For these two reasons, on a per-molecule basis, ozone in the troposphere is more effective than stratospheric ozone at blocking UV. For the profiles shown, the combined effect is a further 2 percent increase in UV at the southern site due to differences in profile shapes alone (after re-normalising both profiles to a common ozone amount). That’s over and above the much larger difference mentioned earlier due to the lower total amounts of ozone in the south.

It’s just another example of our clean air unfairly working against us innocent antipodeans, and fortuitously (😊) protecting our northern neighbours against the risk of UV damage. Last time it was about the direct effect of those aerosols in blocking UV. Now it’s the indirect effect of pollution causing an increase in the amounts of UV-blocking ozone in the air we breathe.