Anthropogenic Climate
Changes
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Human production of freons (CFCs) è Ozone Hole
Depletion |
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Human production of CO2 and CH4 è Global
Warming |
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Human change of land use è
Deforestation |
Standard Atmosphere
Measurements of Ozone
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(1)
Number Density |
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Number of molecules per cubic centimeter (molecules/cm3). |
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The number density is typically about 5x1012 molecules/cm3
near 20 to 25 km altitude, near the peak of ozone concentration in the
stratosphere. |
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(2) Layer Thickness |
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The thickness of pure ozone would have at 1 atm pressure. |
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One atmosphere-centimeter (1 atm-cm) is equal to 2.687x1019
molecules/cm2. |
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One Dobson Unit (DU) is equivalent to a layer of pure ozone 0.001 cm
thick at 1 atm pressure. |
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A typical midlatitude ozone column depth is about 300 DU (0.3 atm-cm). |
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Why is Ozone Important?
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The stratospheric ozone layer reduces
the amount of UV-B radiation from the sun reaching Earth’s surface. |
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UV-B exposure can damage human’s immune
system, increase risk of skin cancer, and damage terrestrial plant life,
single-cell organisms, and aquatic ecosystem. |
How Is Ozone Formed?
Ozone Distribution
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The greatest production of ozone occurs
in the tropics, where the solar UV flux is the highest. |
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However, the general circulation in the
stratosphere transport ozone-rich air from the tropical upper stratosphere to
mid-to-high latitudes. |
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Ozone column depths are highest during
springtime at mid-to-high latitudes. |
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Ozone column depths are the lowest over
the equator. |
Polar Vortex
Ozone Production and
Destruction
Radiation and Ozone
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Ozone Production |
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UV photons are required for ozone production: |
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O2 + UV photon à O + O |
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Below about 20 km, UV photons are absorbed by ozone. Therefore, O2 can
be photolyzed only above 20 km. This is why the ozone layer is located in the
stratosphere and not near Earth’s surface. |
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Ozone Destruction |
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O3 can be split by radiation in the visible-light range: |
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O3 + photon à O2 + O |
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Because many more visible photons than UV photons are available, O3 is
photolyzed (ozone destruction) much faster than O2 (ozone
production). |
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Also, O3 can be photolyzed all the way sown to Earth’s surface. |
Other Ozone Destruction
Processes
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Other atmospheric trace constituents,
such as nitrous oxide (N2O), water vapor, and freons, can also be photolyzed.
They produce highly reactive radicals that keep ozone abundances lower than
they would otherwise be. |
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These radicals include: nitric oxide
(NO), atomic chlorine (Cl), bromine (Br) radicals, and hydroxyl (OH)
radicals. |
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These radicals can destroy
stratospheric ozone through “catalytic cycles”. |
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A catalytic cycle is a set of chemical
reactions facilitated by the presence of a catalyst. |
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A catalyst is a substance that
increases the rate of a chemical reaction but is itself unchanged by the
reaction. |
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The Chlorine Catalytic
Cycle
The Chlorine Catalytic
Cycle
The Nitrogen Catalytic
Cycle
Where do those
ozone-depleting catalysts come from?
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(1) Cl – related to natural and
man-made substances |
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(2) Br – related to natural and
man-made substances |
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(3) NO2 reacts with Cl and Br to
produce reservoirs for Cl and Br. |
The Odd Nitrogen Cycle
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N2O: from Earth’s surface, where it is
produced by microbial activity in soil and in the ocean. |
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è In the stratosphere, some of
N2O reacts to form NO, the rest is photolyzed back to N2 and O2. |
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The NO so produced participates in the
ozone-destroying nitrogen catalytic cycle. |
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Once for a while, the resulting NO2
molecule interacts with a hydroxyl radical (OH), producing nitric acid
(HNO3). |
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Nitric Acid (HNO3) then diffuses down
into the troposphere and dissolves into rain. |
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Other Sources of NO
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N2O is currently the largest source of
stratospheric odd nitrogen (NO and NO2). |
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How human activity affect NO? |
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high-flying, supersonic transport airplanes |
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è Jet plane-produced high temperatures from combustion. |
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è combine N2 and O2 to form NO. |
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è injected NO goes to the stratosphere and destroy ozone |
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è This is why jet plans can affect stratospheric ozone layer |
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(even though jet plans fly in the upper
troposphere). |
The Chlorine Cycle
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Natural sources of chlorine: |
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(1) Methyl chloride (CH3Cl): produced by marine plankton. (2) Hydrogen chloride (HCl): produced by
volcanic eruption and by evaporation
of sea spray. |
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These natural-generated chlorine are most removed by precipitation
before they reach the stratosphere. |
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The largest sources of stratospheric
chlorine today are freons (CFCs), which are thropogenic compounds. |
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CFCs do not react in either the
troposphere or lower stratosphere |
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CFCs go all the way to the upper
stratosphere and are photolyzed by UV radiation. |
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è The Cl so produced proceeds
to destroy ozone. |
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è Eventually, Cl react with CH4
and is diffused to the troposphere, where it is precipitated out. |
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Chlorine Sources
Man-Made Sources for CFCs
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There are two kinds of CFCs: freon-11
(CCl3F) and freon-12 (CCl2F2). |
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Freon-11 has been used: |
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(1) as a propellant in spray cans |
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(2) as a blowing agent for producing foams |
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(3) to clean semiconductor
chips. |
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Freon-12 has been used as |
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(1) a refrigerant |
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(2) working fluid in most car air conditioners. |
The Bromine Cycle
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Br is also a ozone-depleting catalyst |
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The bromine cycle is similar to the
chlorine cycle. |
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The natural source of bromine is Methyl
Bromine (Ch3Br), which is a byproduct of biological activity in the ocean.
These natural source reacts in the troposphere. |
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The man-made source of bromine is two
chemical compounds: Halon-1211 (CF2ClBr) and Halon-1301 (CF3Br). |
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These two halons are used in certain types of fire extinguishers. |
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Halons diffuse up to the stratosphere,
where they are photolyzed into bromine atmos. They eventually rain out. |
Bromine Sources
Coupling Between Odd
Nitrogen and Chlorine Cycles
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Throughout most of the lower
stratosphere, the nitrogen and chlorine cycles are coupled by the above
chemical reaction. |
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The chlorine nitrate (ClONO2) formed in
this reaction does not react directly with either ozone or atomic oxygen.
Therefore, this coupling reaction keeps chlorine from being in its reactive
forms (Cl and ClO) (which can destroy ozone). |
Polar Stratospheric
Clouds (PSCs)
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In winter the polar stratosphere is so
cold (-80°C or below) that certain trace atmospheric constituents can
condense. |
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These clouds are called “polar
stratospheric clouds” (PSCs). |
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The particles that form typically
consist of a mixture of water and nitric acid (HNO3). |
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The PSCs alter the chemistry of the
lower stratosphere in two ways: |
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(1) by slow down the coupling between the odd nitrogen and chlorine
cycles |
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(2) by providing surfaces on which heterogeneous reactions can occur. |
How PSCs Affect Chlorine?
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In most of the seasons, there are
always abundant NO2 in the stratosphere to tie up a significant fraction of
the available chlorine in the form of chlorine nitrate. |
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In the wintertime Antarctic
stratosphere, NO2 concentrations are low, because most of the odd nitrogen
has been converted into HNO3 and become droplets in PSCs (polar stratospheric
clouds). |
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Therefore, the formation of PSCs allow
reactive chlorine concentration to increases. |
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The PSCs particles also help convert
unreactive forms of chlorine into reactive chlorine by providing surfaces on
which heterogeneous reaction can occur. |
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Three Factors for the
Ozone Hole
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Chemical Reactions – polar
stratospheric clouds (PSCs) |
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Atmospheric Circulation – stratospheric
polar vortex |
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Sunlight – spring season |
The Polar Vortex
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The wintertime circulation over the
South Pole is characterized by a gigantic whirlpool of cold and dense air,
called the polar vortex. |
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The cold and dense cold air in the
middle of the vortex is subsiding. |
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The sinking air carries cloud particles
along with it. |
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Remove odd nitrogen from the
stratosphere. |
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Very little ozone and odd nitrogen can
be brought into the south pole. |
Antarctic Ozone Hole
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The decrease in ozone near the South
Pole is most striking near the spring time (October). |
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During the rest of the year, ozone
levels have remained close to normal in the region. |
Satellite View of the
Ozone Hole
The 1997 Ozone Hole
Ozone Hole Depletion
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Long Antarctic winter (May through
September) |
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The stratosphere is cold enough to form
PSCs |
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PSCs deplete odd nitrogen (NO) |
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Help convert unreactive forms of
chlorine (ClONO2 and HCl) into more reactive forms (such as Cl2). |
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The reactive chlorine remains bound to
the surface of clouds particles. |
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Sunlight returns in springtime
(September) |
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The sunlight releases reactive chlorine
from the particle surface. |
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The chlorine destroy ozone in October. |
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Ozone hole appears. |
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At the end of winter, the polar vortex
breaks down. |
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Allow fresh ozone and odd nitrogen to
be brought in from low latitudes. |
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The ozone hole recovers (disappears)
until next October. |
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Global Total Ozone Change
Why No Ozone Hole in
Artic?