CFCs and ozone
The Industrial Revolution: The Age of Greenhouse Gases
Since the early Holocene and the advent of agriculture,
humankind has altered its environment. Deforestation, artificial lakes,
and asphalt parks have a significant, but mainly regional, climatic
effect. Through the emission of atmospheric pollutants, anthropogenic
climate change is now a global phenomenon. Carbon dioxide is the
most infamous of the atmospheric pollutants that have increased dramatically
since the start of the Industrial Revolution- but not the only pollutant
with the ability to alter the global climate. Below is a summary of
the important greenhouse gases: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons (CFCs), and ozone.
Below is an illustration of the short-term carbon cycle.
Of course, anthropogenic contributions are missing from this cartoon. Once
a CO2 particle is emitted, what happens to it? CO2 molecules
are not destroyed...they are redistributed between carbon reservoirs. The
rate of carbon exchange between the different reservoirs depends on the
turnover time for each reservoir, which ranges from 1 year to decades (the
biosphere and ocean mixed layer) to millenia (the deep ocean).
Sources of anthropogenic CO2:
- Fossil fuel burning
- Cement production
Ocean (the mixed-layer on short time scales)
Presently, about one third (approximately 2 gigatons of carbon per year) of anthropogenic emissions of CO2 are believed to be entering the ocean. The solubility pump is the primary mechanism driving this flux, with the consequence that anthropogenic CO2 is reaching the ocean interior via high latitude sites of deep water formation (particularly the North Atlantic). Ultimately, most of the CO2 emitted by human activities will dissolve in the ocean, however the rate at which the ocean will take it up in the future is less certain.
Carbon dioxide, like other gases, is soluble in water. However, unlike many other gases (oxygen for instance), it reacts with water and forms a balance of several ionic and non-ionic species (collectively known as dissolved inorganic carbon, or DIC). These are dissolved free carbon dioxide (CO2 (aq)), carbonic acid (H2CO3), bicarbonate (HCO3-) and carbonate (CO32-), and they interact with water as follows :
CO2 (aq) + H2O H2CO3 HCO3- + H+ CO32- + 2 H+
The balance of these carbonate species (which ultimately affects the solubility of carbon dioxide), is dependent on factors such as pH. In seawater this is regulated by the charge balance of a number of positive (e.g. Na+, K+, Mg2+, Ca2+) and negative (e.g. CO32- itself, Cl-, SO42-, Br-) ions. Normally, the balance of these species leaves a net positive charge. With respect to the carbonate system, this excess positive charge shifts the balance of carbonate species towards negative ions to compensate. The result of which is a reduced concentration of the free carbon dioxide and carbonic acid species, which in turn leads to an oceanic uptake of carbon dioxide from the atmosphere to restore balance. Thus, the greater the positive charge imbalance, the greater the solubility of carbon dioxide. In carbonate chemistry terms, this imbalance is referred to as alkalinity.
In terms of measurement, four basic parameters are of key importance: Total inorganic carbon (TIC, TCO2 or CT) , Total alkalinity (TALK or AT), pH, and pCO2. Measuring any two of these parameters allows for the determination of a wide range of pH-dependent species (including the above mentioned species). This balance can be changed by a number of processes. For example, the air-sea flux of CO2, the dissolution/precipitation of CaCO3, or biological activity such as photosynthesis/respiration. Each of these has different effects on each of the four basic parameters, and together they exert strong influences on global cycles. The net and local charge of the oceans remains neutral during any chemical process.
In a study of carbon cycle up to the end of the 21st century, Cox et al. (2000) predicted that the rate of CO2 uptake will begin to saturate (reach the maximum rate) at 5 gigatons of carbon per year by 2100. This was partially due to non-linearities in the seawater carbonate system, but also due to climate change. Ocean warming decreases the solubility of CO2 in seawater, slowing the ocean's response to emissions. Warming also acts to increase ocean stratification, isolating the surface ocean from deeper waters. Additionally, changes in the ocean's thermohaline circulation (specifically slowing) may act to decrease transport of dissolved CO2 into the deep ocean. However, the magnitude of these processes is still uncertain, preventing good long-term estimates of the fate of the solubility pump.
While ocean absorption of anthropogenic CO2 from the atmosphere acts to decrease climate change, it causes ocean acidification which is believed will have negative consequences for marine ecosystems.
Biosphere (including forest regrowth and CO2 fertilization)
Where is the anthropogenic emissions of CO2 stored? 55% is stored between the oceans and the land biota. The remaining
45% is stored in the atmosphere.
from Global Warming, The Complete Briefing by John Houghton.
(A) The rise in atmospheric CO2 concentrations
measured from Antarctic ice cores (1700s-1957) and from the Mauna Loa Observatory
in Hawaii. (B) More detailed measurements of atmospheric CO2 from the Mauna Loa Observatory (thicker line) and the South Pole (thinner
line). What accounts for the differences between the Mauna Loa and South
Pole CO2 records?
from Global Warming, The Complete Briefing by John Houghton.
Cumulative national contributions of CO2 by
fossil fuel combustion and cement manufacturing as of 1992.
CO2 contributions from third world and non
third world countries.
If anthropogenic CO2 emissions were to suddenly
stop, what would be the effect? It would take several hundreds of years
to obtain pre-industrial atmospheric CO2 concentrations. Are
anthropogenic CO2 emissions abating?
The concentration of methane in the
atmosphere is much lower than that of CO2. The overall budget
of atmospheric methane is fairly well established, but the strength
of individual sources remains uncertain. Best estimates of sources and
sinks from various reports are summarized in Table 1 (below).
sources (340 Tg/yr) predominate over natural
sources (160 Tg/yr), and 80% of the total methane
emission is of modern biogenic origin. Only 20% is due to fossil carbon
sources (Wahlen et al. 1989).However, the greenhouse effect of methane
is 7.5 times that of CO2!
Natural sources (millions of
tons per year)
- Coal mining, natural gas, petroleum
paddies (60) By 2020, the world will need to produce
350 million tons more rice per year to feed an anticipated 3 billion
more people than in 1992. Riice field methane emissions are a major
source of atmospheric methane.Flooding a field cuts off the oxygen
supply from the atmosphere, resulting in anaerobic fermentation of
soil organic matter. Methane, a major end product of anaerobic fermentation,
is released from submerged soils to the atmosphere through the roots
and stems of rice plants. Estimates of global methane emission rates
from rice fields range from 20 to 100 Tg per year (1 Tg=1 million
tons), which corresponds to 6 to 29 % of
total annual anthropogenic methane emission.
- Enteric fermentation (belching) from cattle and other stock (85) Ruminant animals, such
as cattle, sheep, buffalo, and goats, are unique. Because of their
special digestive systems, they can convert otherwise unusable plant
materials into nutritious food and fiber. This same helpful digestive
system, however, produces methane, a potent greenhouse gas that can
contribute to global climate change. Livestock production systems
can also emit other greenhouse gases such as nitrous oxide and carbon
How much methane is produced by livestock?
Globally, ruminant livestock produce about 80 million tons of methane
annually, accounting for about 22% of global
methane emissions from human-related activities. An adult
cow may be a very small source by itself, emitting only 80-120 kgs
of methane, but with about 100 million cattle in the U.S. and 1.2
billion large ruminants in the world, ruminants are one of the largest
methane sources. In the U.S., cattle emit about 6 million metric
tons of methane per year into the atmosphere, which is equivalent
to about 36 million metric tons of carbon
- Animal wastes (25) The domestic
animal population has increased by 0.5 to 2.0 % per day during the
last century, according to the Environmental Protection Agency (EPA)
report Policy Options for Stabilizing Global Climate. Figure 4-14
of the report shows the upward trend in domestic animal populations.
One result of this population increase is that emissions from livestock
have become a significant source of atmospheric methane. In fact,
domestic animals currently account for about
15 percent of the annual anthropogenic methane emissions.
Much of the world's
- Domestic sewage treatment (25)
- Landfills (40) Americans generate trash at an astonishing rate of four pounds
per day per person, which translates to 600,000 tons per day or 210
million tons per year! This is almost twice as much trash per person
as most other major countries. What happens to this trash? Some gets
recycled or recovered and some is burned, but the majority is buried
- LFG is created when organic waste in a municipal solid waste landfill
decomposes. This gas consists of about 50
percent methane (CH4), the primary component of natural gas, and about
50 percent carbon dioxide (CO2) and a small amount of non-methane
- Biomass burning (40)
- Atmospheric removal (530) - destroyed upon the reaction with
hydroxyl (OH) radicals
- Removal by soils (30)
- Atmospheric increase (37)
Average residence time for methane: 12
years (much shorter than CO2!)
The historical rise of methane is closely correlated with
the rise in human population.
Nitrous oxide, laughing gas, is also a greenhouse gas.
Its concentration in the atmosphere, 0.3 ppmv, is rising at about 0.25%
per year and is about 13% greater than it was during pre-industrial times.
The residence time of NO2 is 120 years.
The source of nitrous oxide is not well known, though
the chemical industry (nylon production), deforestation, and agricultural
practices play a role.
CFCs and ozone
(Portions of this discussion are from
the Centre for Atmospheric Science, University of Cambridge http://www.atm.ch.cam.ac.uk/tour/part3.html)
CFCs are man-made chemicals used in refrigerators,
the manufacture of insulation, and aerosol cans. The CFCs have 2 environmental
impacts: 1) they lead to ozone destruction and 2) they are a greenhouse
gases. Although found in very small concentrations (1 ppbv), they are
a serious environmental threat and have a long residence time- 100 to
200 years. The greenhouse effect of CFCs is 5000-1000 times greater
Ozone forms a layer in the stratosphere, thinnest in the tropics (around
the equator) and denser towards the poles. The amount of ozone above
a point on the earth's surface is measured in Dobson units (DU) - typically
~260 DU near the tropics and higher elsewhere, though there are large
seasonal fluctuations. It is created when ultraviolet radiation (sunlight)
strikes the stratosphere, dissociating (or "splitting") oxygen
molecules (O2) to atomic oxygen (O). The atomic oxygen quickly combines
with further oxygen molecules to form ozone:
O2 + hv -> O + O (1)
O + O2 -> O3 (2)
(1/v = wavelength < ~ 240 nm)
It's ironic that at ground level, ozone is a health hazard - it is a
major constituent of photochemical smog. However, in the stratosphere
we could not survive without it. Up in the stratosphere it absorbs some
of the potentially harmful ultra-violet (UV) radiation from the sun
(at wavelengths between 240 and 320 nm) which can cause skin cancer
and damage vegetation, among other things.
Although the UV radiation splits the ozone molecule, ozone can reform
through the following reactions resulting in no net loss of ozone:
O3 + hv -> O2 + O (3)
O + O2 -> O3 (2) as above
Ozone is also destroyed by the following reaction:
O + O3 -> O2 + O2 (4)
The Chapman Reactions
The reactions above, labelled (1)-(4) are known as the "Chapman
reactions". Reaction (2) becomes slower with increasing altitude
while reaction (3) becomes faster. The concentration of ozone is a balance
between these competing reactions. In the upper atmosphere, atomic oxygen
dominates where UV levels are high. Moving down through the stratosphere,
the air gets denser, UV absorption increases and ozone levels peak at
roughly 20km. As we move closer to the ground, UV levels decrease and
ozone levels decrease. The layer of ozone formed in the stratosphere
by these reactions is sometimes called the 'Chapman layer'.
The Missing Reactions..
But there was a problem with the Chapman theory. In the 1960s it was
realized that the loss of ozone given by reaction (4) was too slow.
It could not remove enough ozone to give the values seen in the real
atmosphere. There had to be other reactions, faster reactions that were
controlling the ozone concentations in the stratosphere.
Evidence that human activities affect the ozone layer have been building
up over the last 20 years, ever since scientists first suggested that
the release of chlorofluorocarbons (CFCs) into the atmosphere could reduce the amount of ozone over our
The breakdown products (chlorine compounds) of these gases were detected
in the stratosphere. When the ozone hole was detected, it was soon linked
to this increase in these chlorine compounds. The loss of ozone was
not restricted to the Antarctic - at around the same time the first
firm evidence was produced that there had been an ozone decrease over
the heavily populated northern mid-latitudes (30-60N). However, unlike
the sudden and near total loss of ozone over Antarctica at certain altitudes,
the loss of ozone in mid-latitudes is much less and much slower - only
a few percentage per year. However, it is a very worrying trend and
one which is the subject of intense scientific research at present.
What Is The Ozone Hole?
Cold and Polar Vortex
Since there is no sunlight during the polar winter, the
air within the "polar vortex" can get very cold. So cold that
special clouds can form once the air temperature gets to below about
-80C. These clouds are called Polar Stratospheric Clouds (or PSCs for
short) but they are not the clouds that you are used to seeing in the
sky which are composed of water droplets. PSCs first form as nitric
acid trihydrate. As the temperature gets colder however, larger droplets
of water-ice with nitric acid dissolved in them can form. However, their
exact composition is still the subject of intense scientific scrutiny.
These PSCs are crucial for ozone loss to occur.
So, we have the first few ingredients for our 'ozone loss
recipe'. We must have:
-- Polar winter leading to the formation of the polar vortex which
isolates the air within it.
-- Cold temperatures; cold enough for the formation of Polar Stratospheric
Clouds. As the vortex air is isolated, the cold temperatures persist
Chlorine and the Ozone Hole
It is now accepted that chlorine and bromine compounds in the atmosphere
cause the ozone depletion observed in the `ozone hole' over Antarctica
and over the North Pole. However, the relative importance of chlorine
and bromine for ozone destruction in different regions of the atmosphere
has not yet been clearly explained. Nearly all of the chlorine, and
half of the bromine in the stratosphere, where most of the depletion
has been observed, comes from human activities.
The main long-lived inorganic carriers (reservoirs) of chlorine
are hydrochloric acid (HCl) and chlorine nitrate (ClONO2). These
form from the breakdown products of the CFCs. Dinitrogen pentoxide (N2O5)
is a reservoir of oxides of nitrogen and also plays an important role
in the chemistry. Nitric acid (HNO3) is significant in that it sustains
high levels of active chlorine .
Production of Chlorine Radicals
One of the most important points to realise about the chemistry of the
ozone hole is that the key chemical reactions are unusual. They cannot
take place in the atmosphere unless certain conditions are present:
our first two ingredients in our recipe for ozone loss.
The central feature of this unusual chemistry is that the chlorine
reservoir species HCl and ClONO2 (and their bromine counterparts) are
converted into more active forms of chlorine on the surface of the polar
stratospheric clouds. The most important reactions in the destruction
of ozone are:
HCl + ClONO2 -> HNO3 + Cl2 (1)
ClONO2 + H2O -> HNO3 + HOCl (2)
HCl + HOCl -> H2O + Cl2 (3)
N2O5 + HCl -> HNO3 + ClONO (4)
N2O5 + H2O -> 2 HNO3 (5)
It's important to appreciate that these reactions can only take place
on the surface of polar stratospheric clouds, and they are very fast.
This is why the ozone hole was such as surprise. Heterogeneous reactions
(those that occur on surfaces) were neglected in atmospheric chemistry
(at least in the stratosphere) before the ozone hole was discovered.
Another ingredient then, is these heterogeneous reactions which allow
reservoir species of chlorine and bromine to be rapidly converted to
more active forms.
The nitric acid (HNO3) formed in these reactions remains in the PSC
particles, so that the gas phase concentrations of oxides of nitrogen
are reduced. This reduction, 'denoxification' is very important as it
slows down the rate of removal of ClO that would otherwise occur by
ClO + NO2 + M -> ClONO2 + M (6)
(where M is any air molecule)
... and so helps to maintain high levels of active chlorine. Here is
some more information on Polar Stratospheric Clouds.
Lastly note that we have still only formed molecular chlorine (Cl2)
from reactions (1)-(5). To destroy ozone requires atomic chlorine.
Molecular chlorine is easily photodissociated (split by sunlight):
Cl2 + hv -> Cl + Cl
This is the key to the timing of the ozone hole. During the polar winter,
the cold temperatures that form in the 'vortex' lead to the formation
of polar stratospheric clouds. Heterogeneous reactions convert the reservoir
forms of the ozone destroying species, chlorine and bromine, to their
molecular forms. When the sunlight returns to the polar region in the southern hemisphere spring (northern
hemisphere autumn) the Cl2 is rapidly split into
chlorine atoms which lead to the sudden loss of ozone. This sequence
of events has been confirmed by measurements before, during and after
the ozone hole.
There is still one more ingredient for our recipe of ozone destruction.
We have most of it but we have still not explained the chemical reactions
that the atomic chlorine actually takes part in to destroy the ozone.
We'll discuss this next.
Catalytic Destruction of Ozone
Measurements taken of the chemical species above the pole show the high
levels of active forms of chlorine that we have explained above. However,
we still have many more atoms of ozone than we do of the active chlorine
so how it is possible to destroy nearly all of the ozone?
The answer to this question lies in what are known as 'catalytic cycles'.
A catalytic cycle is one in which a molecule significantly changes or
enables a reaction cycle without being altered by the cycle itself.
The production of active chlorine requires sunlight, and sunlight drives
the following catalytic cycles thought to be the main cycles involving
chlorine and bromine, responsible for destroying the ozone:
(I) ClO + ClO + M -> Cl2O2 + M
Cl2O2 + hv -> Cl + ClO2
ClO2 + M -> Cl + O2 + M
then: 2 x (Cl + O3) -> 2 x (ClO + O2)
net: 2 O3 -> 3 O2
(II) ClO + BrO -> Br + Cl + O2
Cl + O3 -> ClO + O2
Br + O3 -> BrO + O2
net: 2 O3 -> 3 O2
The dimer (Cl2O2) of the chlorine monoxide radical involved in Cycle
(I) is thermally unstable, and the cycle is most effective at low temperatures.
Hence, again low temperatures in the polar vortex during winter are
important. It is thought to be responsible for most (70%) of the ozone
loss in Antarctica. In the warmer Arctic a large proportion of the loss
may be driven by Cycle (II).
The Recipe For Ozone Loss
To summarise then, we have looked at the 'ingredients' or conditions
necessary for the destruction of ozone that we see in Antarctica. The
same applies more or less to the loss of ozone in the Arctic stratosphere
during winter. Although in this case the loss is not nearly so severe.
To recap then, the requirements for ozone loss are:
The polar winter leads to the formation of the polar vortex which isolates
the air within it.
Cold temperatures form inside the vortex; cold enough for the formation
of Polar Stratospheric Clouds (PSCs). As the vortex air is isolated,
the cold temperatures and the PSCs persist.
Once the PSCs form, heterogeneous reactions take place and convert
the inactive chlorine and bromine reservoirs to more active forms of
chlorine and bromine.
No ozone loss occurs until sunlight returns to the air inside the polar
vortex and allows the production of active chlorine and initiates the
catalytic ozone destruction cycles. Ozone loss is rapid. The ozone hole
currently covers a geographic region a little bigger than Antarctica
and extends nearly 10km in altitude in the lower stratosphere.
A single molecule of chlorine can devour thousands of
molecules of ozone- chlorine catalyzes ozone destruction.
the seasonal change in ozone over the South Pole.
The "Ozone hole" over Antarctica was discovered
in 1985 by Joe Farman, Brian Gardiner, and Jonathan Shanklin at the
British Antarctic Survey.
Realizing the immediate and serious consequences of CFCs,
the international community drafted and signed the Montreal Protocol (1987),
requiring CFCs to be phased out completely in industrialized countries
by 1996 and in developing countries by 2006.
ozone is important to life because it shields the Earth from harmful UV
radiation, which encourages cell mutations leading to skin cancer. CFCs
hasten the natural destruction of of ozone by adding extra chlorine to
Ozone and CFCs are greenhouse gases. In the mid-latitudes,
the destruction of ozone and the addition of CFCs counterbalance each other-
so there is no net greenhouse effect due to these gases. At high latitudes,
the CFCs more than compensates for the ozone destruction- so there is a
net greenhouse effect due to CFCs.
- blown from land surfaces (particularly desert regions)
- forest fires
- sea spray
- biomass burning (particularly sulfates)
- fossil fuel burning
Aerosols have a very short residence time in the atmosphere-
about 5 days on average. Over limited regions, aerosols have a negative
radiative effect countering that of anthropogenic greenhouse gases.
Sulfate aerosols (sulfur dioxide) contributes to acid
pollution. Acid rain degrades forests and fish stocks in lakes. As a result,
many countries are trying to curb emissions of these gases.
Radiative forcing due to anthropogenic emission of gases
(and compared to solar variability).
Future global warming potentials- comparison of the "strength"
of various greenhouse gases.
Predicted future greenhouse forcing.