Environmental Transport and Fate
Chapter 10 – Ozone Holes
Benoit Cushman-Roisin Thayer School of Engineering Dartmouth College
Recall the vertical structure of the atmosphere
We are now concerned about this lithlayer in the middle of the stratosphere
1 Temperature increases in this layer of the atmosphere because of absorption of solar radiation by oxygen and ozone.
Photochemical reactions are:
O2 + (UV radiation < 242 nm) → O● + O●
O● + O2 + M → O3 + M● (M stands for any molecule nearby)
O3 + (UV-C or UV-B radiation) → O2 + O●
O2 absorption < 242 nm
O3 absorption 200 nm < < 320 nm
2 Cartoon from US EPAUS Cartoon from
3 Example of what is happening on the seasonal scale
of ozone
Where was the stratospheric ozone by the end of the winter?
And, it did get worse for a number of years…
1 DU = 1 Dobson Unit
= thickness of O3 brought to 1 atm @ 0oC (in meters) x 105
Historical springtime vertically integrated ozone over Halley Bay, Antarctica (76oS) (Source: UNEP, 1994)
4 Historical data showing no minimum in spring sphere/winter_bulletins/sh_07/ o Pronounced minimum in springs from mid- 1980s to present. www.cpc.noaa.gov/products/strat / http:/
Hole still occurring in 2010 and 2011
The Antarctic Ozone Hole
The Antarctic Ozone Hole was discovered by the British Antarctic Survey from data obtained with a ground-based instrument at a measuring station located in Halley Bay, Antarctica, in the 1981-1983 period. A first report of October ozone loss was issued in 1985. Satellite measurements then confirm edtattesped that the sprin gteooeossasacotgtime ozone loss was a contin etent-wide f eatur e.
Research conducted during the National Ozone Expeditions to the U.S. McMurdo Station in 1986 and 1987, and NASA stratospheric aircraft flights into the Antarctic region from Chile in 1987 showed conclusively that the ozone loss was related to chlorine-catalyzed chemical destruction which takes place following spring sunrise in the Antarctic polar region.
The chlorine is derived from man-made chloro-fluoro-carbons (CFCs) which have migrated to the stratosphere and have been broken down by solar ultraviolet light, freeing chlorine atoms.
(Source: http://www.ozonelayer.noaa.gov/science/ozhole.htm)
5 Mario J. Molina and F. Sherwood Rowland shared the 1995 Nobel Prize in chemistry for “explaining how certain man-made chemicals can rise into the atmosphere and harm the ozone layer that shields us from the ultraviolet radiation of the sun.”
The chemical culprits
These artificial chemicals were thought to be completely inert. And they indeed are but only under ground-level temperatures, pressures, and solar radiation.
6 Chemical reactions contributing to ozone loss in the stratosphere
1) Chlorine release from the CFCs
ex. CFC-12 CF2Cl2 + UV light → CF2Cl + Cl CF2Cl + UV light → CF2 + Cl
2) Catalytic ozone destruction
Cl + O3 → ClO + O2 One ozone molecule destroyed ClO + O → Cl + O2 as well as one of its precursors, while the chlorine atom is regenerated
3) Eventual removal of chlorine and of chlorine oxide
Cl + CH4 → HCl + CH3 HCl is water soluble → precipitation ClO + NO2 → ClONO2
It has been estimated that a chlorine atom can destroy 1000 ozone molecules before being destroyed in its turn.
Over the poles, there is an extended seasonal period of darkness, during which no ultraviolet radiation is shed.
The chlorine is then converted into other products, which regenerate the chlorine when the sun returns in the spring.
4) In the darkness of the polar nigh (reactions occurring on the surface of icy stratospheric clouds)
ClO + NO2 → ClONO2 ClONO2 + H2O → HOCl + HNO3 HOCl + HCl → Cl2 + H2O
5) Once the sun rises in the spring
Cl2 + sunlight → 2 Cl HOCl + sunlight → OH + Cl
Cl + O3 → ClO + O2
7 These chemical reactions take place in a trapped air zone called the Polar Vortex. The Polar Vortex is a permanent, though variable, cyclone sitting over the pole.
Play movie from
http://www.esrl.noaa.gov/gmd/dv/spo_oz/movies/index.html
8 The polar vortex – always in a meandering state
except that polar high at ground level means polar low above
polar vortex over South Pole, too
As long as the polar vortex retains its integrity, chemicals are trapped inside the vortex, but, when the meanders of the vortex break off, some of the chemicals are discharged to lower latitudes where they are exposed to sunlight within hours.
Example of trajectory originating http://ww from the polar vortex (rotating counterclockwise) and proceeding to lower latitudes. w .esrl.noaa.gov/gmd/dv/traj/plots/b nd.html
BND = Observation station at Bondville, Illinois
9 http://www.cpc.noaa.gov / products/stratosphere/winter_bull e tins/nh_05-06/
We note that not all years have been equally bad. The reason lies in the timing of the breakup of the polar vortex.
The crucial period is very early spring. At that time, sunlight emerges, and the polar vortex becomes very unstable. It very much depends on what happens first.
● If the polar vortex breaks early, then the precursor chemicals are dispersed to lower latitudes before they have a chance to release many chlorine atoms. There is less damage to the ozone layer, and the hole is not very deep.
● On the contrary, should the polar vortex retain its integrity a while longer, the precursor chemicals find time to generate many chlorine atoms, and the damage to the ozone layer can be severe (in terms of decreased O3 concentrations, or in terms of the area over which ozone concentration falls below a certain threshold).
10 Effect at lower latitudes than the polar vortex: Noticeable but not catastrophic http://www.cp c .noaa.gov/products/stratosphere/ w inter_bulletins/nh_05-06/
Largest recorded ozone hole
according to NASA: http://ozonewatch. 24 September 2006 g sfc.nasa.gov/
False-color view of total ozone over the Antarctic pole. Purple and blue colors are where there is the least ozone, whereas yellows and reds are where there is more ozone.
11 The 2010 ozone hole had two remarkable features: It was one of the latest forming ozone holes observed and it was one of the longest lasting ozone holes observed.
Ozone deppypygletion typically begins in late Jul y and earl ygy August with an observed ozone hole size of 10 million square kilometers by mid-August. The “ozone hole” is defined as the area in the polar latitudes where the total column ozone amounts are less than 220 Dobson Units (DU). In 2010 the ozone hole was not observed via satellite measurements until the last week in August. It grew to a maximum size of 20.6 million km2 on September 26, 2010.
From this point on, the South Hemispheric (SH) polar vortex and the ozone hole decreased in size at a much slower rate than previous years. The SH polar circulation was minimally affected by poleward wave propagation during the austral spring time, remaining zonally symmetriThbic. The absence of fb substant ilial poldleward hflheat flux exten ddhded the trans iifition from winter to summer circulation patterns over the South Pole in the lower stratosphere. The 2010 ozone hole and polar vortex remained almost intact well into December.
(Source: NOAA – Climate Prediction Center)
http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_bulletins/sh_10/ http://www.cpc.ncep.noaa.g o v/products/stratosphere/winter_b u lletins/sh_10/
12 http://www.cpc.ncep.noaa .gov/products/stratosphere/winter _ bulletins/sh_10/
13 It appears that the situation is no longer getting worse. Could we have rounded the corner?
On the way back up?
ODS = Ozone Depleting Susbtance
14 Latest predictions
Recovery at mid-latitudes circa 2049
Recovery over Antarctica circa 2078.
Closing concern
Following the phasing out of CFCs per the Montreal Protocol of 1989, chemists have been hard at work to design new refrigerants, solvents, blowing agents and foams for fire extinguishers.
In a first wave, they have developed so-called transitional alternatives called Hydr ochlo ro fl uoroca r bon s ((CCs),sucasHCFCs), such as CHFCl2 (di-chloro-fluoro-methane) C2HFCl4 (tetra-chloro-fluoro-ethane), all of which still contain some chlorine…
These HCFCs are more reactive and thus have shorter atmospheric lifetimes than CFCs and deliver less reactive chlorine to the stratosphere. It is expected that these chemicals will contribute much less to stratospheric ozone depletion than the earlier CFCs. Because they still contain chlorine and have the potential to destroy stratospheric ozone, they are viewed only as temporary substitutes, with phase-out by 2020.
On the longer horizon, plans are to return to well tried but less physically safe
chemicals such as ammonia (NH3) and carbon dioxide (CO2, in supercritical state).
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