We wish to learn:
TROPOSPHERE (TOO MUCH Ozone!!):
The troposphere is the lowest layer of the atmosphere, extending from the ground to roughly 10 – 17 km altitude. The vertical extent of the troposphere varies with latitude and season. With the most intense heating (and subsequent convection) occurring at the Earth’s equator year-round, it is not unusual to find the troposphere extending to ~ 17 km. Near the winter pole, the air is cold and dense and the troposphere may only extend to 9 or 10 km. The temperature in the troposphere decreases with height. That is, as you go higher in altitude, the temperature decreases. If you consider an air parcel that rises in the troposphere, it encounter air parcels having temperatures colder that itself. Since hot air (less dense air) rises, it will continue to rise. The troposphere is a zone of rapid vertical mixing in addition to horizontal winds.
The tropopause separates the troposphere and the stratosphere.
The stratosphere is the layer above the troposphere. It extends from the tropopause to ~ 45 – 55 km altitude. Unlike the troposphere, temperature increases with height in the stratosphere. This means that a parcel of air that gets displaced upward will encounter airparcels that are warmer. The colder, denser air parcel will sink back to its initial position and prevents vertical mixing. The stratosphere is thus a region of high horizontal winds but no vertical mixing, so it is horizontally “stratified” or layered.
Chemical compounds that are emitted naturally or anthropogenically at the Earth’s surface can remain in the atmosphere for long periods of time if they are not reactive (chemically or photolytically), water soluble, or sticky (prone to dry deposition). Most greenhouse gases are long-lived, which means that they react slowly (e.g., CH4) or only partially water soluble (e.g., CO2), or are only vulnerable to breakup by solar radiation in the stratosphere (e.g., N2O and CFCs). (Of course, a gas must also be a powerful absorber of infrared radiation to be a greenhouse gas, so even though N2 is long-lived, it is not a greenhouse gas because it is not an efficient absorber.)
However, many gases that are emitted
from the Earth’s surface are short-lived in the atmosphere because they
are highly reactive, highly water-soluble, or very sticky. For example,
there are numerous compounds that are quickly oxidized by reaction with
OH or other oxidants or that are readily broken up by sunlight (i.e., photodissociated)
(e.g, NO2). And there are many other species that are very water-soluble
(such as acidic compounds like nitric and sulfuric acid) or very sticky
and are removed from the atmosphere by contact with surfaces or vegetation
(like OH and nitric acid).
Important issues associated with
tropospheric ozone and global change:
Ozone – primarily in the stratosphere
- also plays a very important role in protecting organisms at the Earth’s
surface from UV-B radiation. Changes in stratospheric ozone
levels can thus affect human and ecosystem health as well as the chemistry
of the troposphere.
From the above discussion, we can see that ozone protects us from UV light and it is a greenhouse gas in its own right. We next focus on the chemistry of ozone - how is it produced and how is it destroyed?
Stratospheric Ozone Abundance
Ozone occurs in a layer, centered at around 30 km altitude, reaching a peak abundance of ~10 parts per million. Even at the peak of the ozone layer, however, it is still very much a trace constituent - two orders of magnitude down from CO2 and 5 or 6 orders down from O2 and N2. If we were to take all the ozone in a column overhead and bring it down to sea level (room temperature and pressure) it would occupy a layer of only 3 mm in thickness!
It is interesting to notice how different the ozone distribution is from most of the other gases in the stratosphere. Ozone occurs in a layer, while the other gases have simple exponential drop-offs with altitude.
Why does stratospheric ozone exist is a layer? To answer this question, we need to understand the production mechanism for ozone.
Ozone is a deep blue, explosive, and poisonous gas. It is made in the atmosphere by the action of sunlight on molecular oxygen. In the stratosphere, UV light is available that can split up ordinary molecular oxygen into two atomic oxygen atoms.
O2 + UV photon --> O + O
Now, atomic oxygen is a very reactive species - so much so that it is very hard to make in the laboratory - it immediately combines with something else. In the stratosphere, atomic oxygen can quickly combine with molecular oxygen (in the presence of a third body) to yield the almost equally reactive other allotrope of oxygen: ozone or O3.
O + O2 + third body --> O3 + third body
The combination of these two reactions,
mediated by sunlight, converts molecular oxygen into ozone. Thus ozone
is continually being created in the stratosphere by the combination of
molecular oxygen and sunlight.
Ozone is lost through the following pair of reactions:
O3 + UV photon --> O2 + O
O + O3 --> 2O2
The first of these two reactions
serves to regenerate atomic oxygen for the second reaction which converts
the ozone back to molecular oxygen. This second reaction is very slow.
It can be enormously accelerated, however, by catalytic reactions (see
below). In the absence of such catalytic reactions, ozone can survive for
1-10 years in the stratosphere.
Catalytic Destruction of Ozone by Chorine from CFC's
Catalysis refers to the acceleration of a particular chemical reaction by a catalyst, a substance that is not destroyed in the reaction, enabling it to continue having the same accelerating effect time and time again.
Rapid catalytic destruction of ozone is best explained by reference to the famous example of CFC's (also known as freons) in the stratosphere.
Chlorofluorocarbons (CFC's) were developed to be colorless, odorless, non-staining, chemically inert, non-toxic, non-flammable, and to have certain other properties that make them excellent refridgerants, solvents, propellants for aerosol cans, and foam-blowing agents. These same properties make them essentially inert in the troposphere.
In the stratosphere, however, the
CFC's can be broken apart into more reactive fragments under the action
of UV light. When this splitting occurs, free chlorine is liberated which
can catalytically destroy ozone. The process occurs in two steps:
Notice that the net effect of this pair of fast reactions is to turn two ozone molecules into three normal molecules of oxygen. The (catalyst) atomic chlorine is recovered in the second reaction, making it available to start over. In fact, each chlorine atom can destroy hundreds of thousands of ozone molecules!
These two steps turn a very unreactive chemical into a devastatingly effective destoyer of ozone. Whenever free chlorine atoms exist in the stratosphere, ozone is quickly depleted. Other species (such as bromine and fluorine) can also act as ozone-destroying catalysts.
Given this chemistry, it is useful to consider a typical life history of CFC's in the atmosphere:
The Antarctic Ozone Hole
The figure shown below (figure 4) illustrate the satellite view of the same phenomena for the years leading up to the present. There are now several satellite missions that are dedicated to unraveling the chemistry and dynamics of ozone. These include the Total Ozone Mapping Satellite, TOMS and the Upper Atmosphere Research Satellite, UARS.
The hole deepens and becomes enlarged from year to year, as well as deeper although not monotonically.
The Antarctic Ozone Hole is now well
understood. Briefly what happens can be summarized as follows:
Northern Hemisphere Ozone
The Northern hemisphere is not immune from Ozone Holes. In the north, the stratospheric polar vortex is not as well formed as in the south. This is because of the larger contrast between land and water in the northern hemisphere. The existence of land masses tends to break up the symmetry of the polar vortex in the north. However, the same processes operate as in the south and satellite data show the effect occurring in March (Springtime in the northern hemisphere).
Sooner or later, we will see colder than usual northern polar stratospheric temperatures in the early Spring and heavily populated areas will be warned of unusually low ozone levels. Since ozone depleting compounds will be in the atmosphere for many tens of years, we have to live with these effects. Ultimately, chlorine compounds will cleanse themselves from the stratosphere and the Earth's ozone shield will return to normal - for our grandchildren's children.
For a movie showing the latest in
Northern Hemisphere ozone hole formation, click here. For a movie showing
the 1997 hole formation, click
Potential Effects of Depleted Ozone
Of primary concern are the enhanced levels of UV radiation that reach the Earth's surface for a depletion in stratospheric ozone. It is customary to break up the UV spectrum into two parts:
UV-A: 400 - 320 nmThe more energetic UV-B portion of the spectrum is responsible for sunburn, cataracts, potential ecological damage, and skin cancer. It can be absorbed by glass as well as by sunscreens and hats.
Relatively little is known or understood
about the consequences of enhanced UV-B levels. We do know, however, that
a 1% decrease in ozone abundance causes a ~2% increase in UV-B. Increased
UV-B exposure at the Earth’s surface can impact human, agriculture and
forest growth, marine ecosystems, biogeochemical cycles, and materials.
Table 1 summarizes some of the potential effects of UV-B increases.
Effects on Human Health
Figure 6 illustrates the rate
of skin cancer as a function of latitude. While the data has some scatter,
the trend is clear. A decrease of ~110 in latitude results in an increase
of a factor of 2 in skin cancer occurrence. This occurs because the UV-B
exposure increases towards the equator (~ a factor of 50 from pole to equator).
An increase of ozone of 1% gives an increase of ~20,000 cases of skin cancer
per year. This is equivalent to a southward shift in the average
latitude of the U.S. population by only ~12 miles.
Effects on Plants
UVB radiation affects plant physiological and developmental processes and can affect plant growth. Indirect changes, such as in the manner in which nutrients are distributed within the plan, the timing of developmental phases and secondary metabolism and plant form, may be as or more important than the directly damaging effects of UVB.
Effects on Marine Ecosystems
Phytoplankton are the foundation of aquatic food webs, and their productivity is limited to the upper layer of the water column in which there is sufficient sunlight to support net productivity. Exposure to solar UVB radiation affects phytoplankton orientation mechanisms and motility and lowers survival rates for these organisms. UVB radiation has also been found to damage early developmental stages of fish, shrimp, crab, amphibians and other animals.
Effects on Biogeochemical Cycles
Increases in solar UV radiation might affect terrestrial and aquatic biogeochemical cycles, which could affect sources and sinks of greenhouse and a number of other trace gases e.g., carbon dioxide (CO2), carbon monoxide (CO), carbonyl sulfide (COS) and possibly ozone. Such changes would contribute to interactions between the atmosphere and biosphere that attenuate or reinforce the atmospheric buildup of these gases.
Effects on Materials
Although a number of materials are
now somewhat protected from UVB by special additives, synthetic polymers,
naturally occurring biopolymers, and other materials of commercial interest
are adversely affected by solar UV radiation. Increases in solar
UVB levels will therefore accelerate their breakdown and limit their useful
Following the publication of the Farman et al. Findings in 1985, a series of ground-based and airborne measurements campaigns were conducted to develop an understanding of the chemistry and dynamics associated with the Antarctic Ozone Hole. This understanding lead to the Montreal Protocol on Substances that Deplete the Ozone Layer in October 1987. It required a freeze on the annual use of CFCs as early as 1990 with decreases leading to a 50% reduction by the year 2000. In 1990, the Montreal Protocol was amended to take into account the severe losses during the ozone hole events and the downward trends in global ozone. The participating countries substantially strengthened the protocol, calling for accelerated reductions in emissions, and requiring complete phase out of CFCs and other major ozone-depleting substances by 2000. The Montreal Protocol was further amended in 1992, calling for the complete phase out of CFCs, etc, by 1996.
In 1996, Molina, Rowland and Crutzen
received the first Nobel Prize (for Chemistry) to ever be awarded in Atmospheric
Sciences. These scientists were instrumental in predicting the problem
and in developing the needed scientific case for governmental action.