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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 |
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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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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. |
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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. |
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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. |
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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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(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. |
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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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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). |
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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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Eventually, Cl react with CH4 and is diffused to the troposphere,
where it is precipitated out. |
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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. |
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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. |
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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). |
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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. |
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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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Chemical Reactions – polar stratospheric clouds
(PSCs) |
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Atmospheric Circulation – stratospheric polar
vortex |
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Sunlight – spring season |
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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. |
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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. |
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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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