Ozone: change is the norm!

In the discussions about climate change, one often gets the impression that the media and the political people aim for a stable climate (actually they mean “weather”), something like a paradise-like smooth state without any nasty, big and extreme variations and perturbations. The chaotic nature of our atmosphere should tell them that this can never be the case, and that battling to stop climate change is and will remain futile. Here I will write some words on the variability of the total ozone column (TOC). The atmosphere has a variable concentration of ozone (O3), that delicate gas that comes and goes according to solar, temperature and presence of precursor gas conditions.

1. Ground ozone.

We find ozone everywhere, but two regions are the most important: the ground layer where we live and breathe can have O3 concentrations that change up 200 ug/m3 (about 100 ppb) during good (pre-) summer days. Many factors impact this concentration, the most important being (besides temperature) the availability of UVB radiation and precursor gases. Here the most important of these are the natural isoprenes emitted by trees and plants, and the NO2 which mostly comes from traffic related emissions (there are other gases like industrial VOC’s, which have a smaller impact). As vehicles also emit NO, which destroys ozone, we have very different nightly profile in clean air rural and high traffic urban places. Look at the following picture, which gives the ground ozone levels measured at Bonnevoie ( = Luxembourg-City) and Diekirch (=semi-rural) during the week ending the 26th October 2015.

The blue rectangle shows the situation in the night of the start of the 22th October 2015: in a same interval of about 6 hours, the ozone concentration in the city location (top) diminishes by 27 ug/m3 (about 13.5 ppb), whereas at Diekirch without not much nightly traffic the fall is only 8 ug/m3 (4 ppb). The rapid decrease in Luxembourg-City is due to the emitted NO, which destroys the existing ground ozone, a removal that is much slower in Diekirch where there is not much nocturnal emitted NO !

So looking at a daily ozone pattern tells you immediately if the location was urban or rural.

Every May/June, when ground ozone levels are on the rise, the Luxembourg environmental agency issues warnings, as they take wrongly and stubbornly as a reference the O3 levels at the Mont Saint Nicolas in Vianden, a very rural location without only a minimum traffic, but a very rich tree cover. The natural isoprenes, together with clear and non-polluted air (i.e. rich UVB irradiance) makes the ozone levels at this location the highest for Luxembourg. This has nothing to do with noxious human activities, but is an absolute natural phenomenon.

As I will speak in this blog on variability, just look at the extreme swings in the ground ozone concentration: the O3 levels never are constant, but vary from close to zero in the morning to their late afternoon peak.

2. Total ozone column and UVB  irradiance.

The major part of the ozone is located in the stratosphere, between 15 and 50 km with a maximum around 25 km; the concentration there is about 6 times higher than at ground level. This “good ozone” layer absorbs the short-wave and dangerous UV-C radiation (with wavelength below 280 nm) completely, and also part of the UVB radiation (280 to 320 nm). The total ozone column is measured in Dobson Units (DU): if all the ozone contained in a vertical column would be compressed to normal atmospheric pressure, the height of that column would be about 3 mm or 300 DU. Usual numbers in our region vary from 250 to over 400 DU. About 10 to 20% corresponds to the ozone at the ground layer (the “bad ozone”), the major part is stratospheric ozone (the “good ozone”).

The influence of the thickness of the ozone layer on the UVB irradiance can be shown from our measurements at meteoLCD (see this paper). The next figure from the cited paper documents that a thinning ozone layer will increase ground UVB irradiance.

The 22 April 2013 the TOC (total ozone column) was 381.8 DU, and the effective UVB irradiance about 1.5 MED  (MED = minimal erythemal dose); the next day with the same meteorological conditions, the TOC fell down to 265.8 DU and the effective UVB irradiance increased by 0.68 MED, about 2 UVI (UV index). A dip of 100 DU would correspond to an increase of 1.7 UVI.

3. The extremely variable total ozone column

Many factors influence the thickness of the total ozone column, which varies often in  a spectacular manner. Look at the next figure which shows the TOC measured at Uccle (near Brussels, Belgium) for this year 2015:

The read line represents the DU readings for this year 2015 up to the 25th October, the grey part of the plot are the readings from last year. Uccle has one of the longest DU series in Europe, starting 1979. The sine-wave represents the average of all measurements from 1979 to today. Clearly the TOC is highest in spring and lowest in autumn. The next 3D diagram shows this in a more beautiful way for the global region between 45° and 50° latitude North.

So we have an average smooth sine-curve over the year, but the actual measurements present a totally different pattern. Look at the extreme variations in the Uccle plot, where the ozone column can plunge from a 500 DU peak to a 300 low in a few days; often the peak and troughs follow in very short time, a day or even less.

These are the measurements at meteoLCD for 2015, up to the 26th October: Look at what happened around the 10th April: in two days the thickness of the ozone column increased from 300 to 450 DU, and fell back to 363 DU the next day.

What all these measurements show is that our atmosphere is a very dynamic beast; change is the norm, and no change the exception! I remember that in the past when the TOC fell rapidly, the media were fast with alarmist articles about a vanishing ozone layer (evidently caused by human activity!) and our eradication by skin cancer. Had the authors waited a couple of days and had they not been ignorami of natural variations, these silly articles would not have been written.

There is no cause for alarm, as the total ozone layer has not been thinning since many years. The last figure shows the trend from our meteoLCD measurements (meteoLCD is still the only station measuring the TOC in Luxembourg).

The general trend from 1998 to 2014 is positive, and that of the last 14 years practically flat. So no cause for alarm here!

4. Conclusion

The measurements of the ground ozone and the total ozone thickness document an extremely variable situation.  There is no even spread out of the ozone concentration, no well mixed situation, but a breathtaking variability. The atmosphere is a turbulent beast, not a smooth pudding!

PS: Do not forget to look form time to time at our ozone data by clicking on the “DOBSON (total O3)” link at http://meteo.lcd.lu