The basics of Ozone depletion

This blog will briefly outline the factors involved ozone depletion. The video in the previous post explains very succinctly the processes, please give the video a watch as it will add great visual demonstration to my basic definitions below.

proteckllc.com

Firstly, Chlorofluorocarbons (CFCs), manufactured in refrigeration, air conditioning, aerosols and solvents, contain chlorine compounds. Chlorine is then released in the stratosphere due to a photolytic reaction (breaking up of particles by photons in light waves) with incoming UV rays. In turn, the release of chlorine into the ozone rich layer of the stratosphere initiates a catalytic (chlorine is not destroyed) reaction:

CI + O3 -> CIO + O2, CIO + O -> CI + O2

This reaction depletes ozone leaving chlorine and oxygen present. Rowland & Molina first suggested this reaction in 1975, but it was not until the ozone “hole” was first detected in 1985 that any measures were put in place to limit the production of CFCs. These measures were directed through the Montreal Protocol, which will be discussed in more detail on a future blog.

jpl.nasa.gov

Polar Stratospheric Clouds (PSCs) form in extremely cold temperatures (about -80 degrees Celsius) during winter from aggregations of water vapour and nitric acid. PSCs were discovered to be the surface upon which Chlorine was converted to its most reactive (and ozone depleting) form, causing the above photolytic and catalytic reactions to be enhanced when sunlight reaches the poles in springtime.

The influence of the Polar Vortex (as discussed in detail by Schoeberl and Hartmann 1991) on ozone depletion is through the indirect effect of providing and sustaining the conditions necessary for PSC formation and existence till springtime. The vortex is a cyclone generated by the steep temperature gradient between high and mid-latitudes present especially during winter, combined with the earths rotation. This steep gradient described as the polar front fluctuates according to many factors including incoming solar radiation and rossby wave strength which determine the scale of the Polar Vortex. When it is especially strong (for example Arctic winter 2011) the PSCs are present much later into spring and therefore more ozone depletion occurs.

It is important to consider that these factors combine to rapidly enhance ozone depletion at the poles. I have only covered the factors briefly, but these processes provide the basis for the creation of the ozone hole and will be refered to in my blog when I introduce case studies of how the past, present and future ozone levels are measured.

The Antarctic Ozone Hole - Video

http://www.youtube.com/watch?v=AU0eNa4GrgU&noredirect=1

Good morning, just discovered this video. Its a fantastic explanation of the science of the ozone hole and I would highly recomend giving it a watch!

The Arctic Ozone Hole



Map of Arctic: showing Ozone column deficit (DU)

adapted from Fig. 5 from Manney et a 2011.

White contour line shows area in which the data was averaged



Research published recently in Nature, carried out by Gloria Manney and a worldwide team of experts on the ozone layer demonstrates the highly variable manner of ozone depletion over the Arctic. This Arctic ozone layer variability has been observed in 2005, 2000 and 1996 but never before to the extent of 2011 which is for the first time described as comparable to the ozone hole over Antarctica.

The team modelled ozone depletion using a comprehensive set of data from a mixture of satellites, lidar, spectrometers and a network of balloon-borne ozone soundings (ozone sondes). 

Ozone deficit during the 26th March 2011 shown on the map to the above is measured in Dobson Units (DU) which is the measure of ozone area density. The region over the Arctic of purple to light blue is the most adversely affected area when compared to an average ozone background within the white contour line. The most shocking statistic to note is that Ozone loss exceeded 80% at altitudes of 18 to 20km.

The conclusions of the research found that an anomalously prolonged cold period over the Arctic, stretching into spring 2011, provided the conditions necessary for extreme stratospheric ozone depletion. In order to understand the processes involved in ozone reduction my next few blogs will introduce the combing factors.

Ouch...the holy Ozone layer

Mike Agliolo/Science Photo Library

In February this year I left the bitterly cold British winter for the sunshine of Australia’s east coast. Having spent several days sightseeing in Melbourne and the Great Ocean Road my mates were keen to hit the beaches further north at Byron Bay and soak up some of the glorious sunshine. One morning the weather was disappointingly overcast and mild so we didn’t bother using sun cream, having used several tonnes the previous few days at the beach. Big mistake, by the afternoon we were all horrifically sunburnt and resembled (to much laughter of the locals) three "Mr Tomato Heads".

I had heard that Australia had significantly high incoming UV rays and the highest skin cancer incidence rate in the world, but I had never imagined that on such an overcast day we would be caught out like that.

Watching BBC news a few weeks ago the headline was broadcast that ozone depletion over the Arctic had hit a record level, creating a hole in stratospheric ozone that reached as far as Scandinavia and could even be compared to the ozone hole over Antarctica. I became worried, would the incidence of UV rays increase dramatically over the UK? Would I be confined to the shade during daylight hours, constantly at risk of sunburn and skin cancer?!

Ozone depletion is a glaring example of human impact upon nature, and the signing of the Montreal Protocol in 1987, which banned the production of many ozone depleting substances, demonstrates man’s role as “Stewards of the Earth System” as Will Steffen et al state in their 2007 review paper The Anthropocene: Are Humans Now Overwhelming the Great Forces of Nature?.

My starting point is to examine the causes of ozone depletion, first looking at the recent Arctic depletion event, the importance of Chlorofluorocarbons (CFCs) and Polar Stratospheric Clouds (PSCs), the chemical processes research from Paul Crutzen that was awarded the Noble Prize in 1995 and also study the interactions of tropospheric ozone which in high volumes causes photochemical smog. I will also compare the influence of ozone on the climate with other aerosols, both natural and man made.