Transcript Atmospheric Chemistry - AP Environmental Science

Atmospheric Chemistry
Stratospheric Ozone
1) Absorbs solar radiation (200-315nm)
2) Three types of UV
UV-A (320-400 )
UV-B (280-320nm)
UV-C (200-280 nm)
3) The greatest concentration of O3 is from
15-30 km above Earth
4) The amount of Ozone varies
5) Quantified by the Dobson Unit (DU)
100 DU = 1 mm of Ozone
The Earth averages about 300 DU
Antarctic = 150 DU
Ozone Formation:
Oxygen molecules are hit with UV light
and split into energized atoms that
combine with an oxygen molecule to
form ozone (O3).
1. O2 + UV  2 O•
2. O• + O2 + M  O3
- this second reaction requires a third
party molecule (M) to help absorb the
extra energy – usually another
atmospheric gas
Processes of Ozone destruction
Free Radicals: atoms or molecules with a
free (non-bonded) electron
- highly unstable and reactive
- Most responsible for non-oxygen
destruction of ozone
These include:
HOx•, NOx•, ClOx• gases
Basic Process:
 molecules float to the stratosphere and
are bombarded by UV light
 molecules are broken apart forming free
radicals
 free radicals attack ozone molecules
causing them to break down and form
more free radicals
 process continues until the free radicals
bind to a sequestering molecule to
become stable
Generalized Reaction
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X2 + UV  X• + X•
X• + O3  XO + O2
XO + O•  O2 + X•
Overall Process:
X• is not consumed by the reaction
but remains to continue the
breakdown of O3.
O3 + O  2 O2
Hydroxyl Radicals and the HOX Cycle
Methods of Hydroxyl Generation:
1. O• (energized oxygen) + H2O  2
•OH
2. H2O + UV  •H + •OH
3. H • + O3  •OH + O2
Decomposition of ozone by hydroxyl
radical:
1. •OH + O3  HOO• + O2
2. HOO• + O•  •OH + O2
NOX Cycle
NO2 + UV  NO• + O•
This photodissociation makes possible:
NO• + O3 NO2 + O2
NO2 + O•  NO• + O2 - note the free
“O” is there because it is in the
stratosphere where atomic density is
low
The ClOx cycle
Cl• are mostly anthropogenic (man
made) in source
CH3Cl + UV  •CH3 + •Cl
MEMORIZE THESE!
The main reaction with ozone
CFCl3 + UV (<290 nm)  •CFCl2 + Cl•
Cl• +O3  ClO• + O2
ClO• + O•  Cl• + O2
Net Reaction = O3 + O  O2 + O2
Anthropogenic sources of chlorine
1. Mostly chlorinated fluorocarbons
2. Refrigerants, solvents, propellants
3. Chemically and biologically inert
4. CFCs do not react in the troposphere
but do in stratosphere
Properties of CFCs
1. Can last 60 – 522 years in
troposphere
2. The ODP (ozone depletion potential)
is correlated with the # of Cl
The Ozone Hole - Antarctica
- Results from climatic factors and
accumulation of pollutants
Process:
1) Sun sets for the season – dark and
very cold
2) Air currents form a circumpolar
vortex that channels pollutants to form
stratospheric clouds
Clouds form ice crystals of nitric acid,
chlorine compounds and water
4) The sunlight returns to photolyse and
create free radicals of chlorine and
nitrogen compounds
5) Rapid degradation of ozone
VIDEO Video #2
Video#3
3)
End Result:
1. Ozone thickness decreases to 150 DU
2. Free radicals spread north to South
America, New Zealand and Australia
3. Increases UV damage by allowing more
UV-B radiation into troposphere
Health Effects:
- damage to plants
- increased damage to skin and DNA
(thymine dymers)  skin cancer –
just like going to the Death Boxes known
as tanning bed.
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Montreal Protocol: International
agreement to limit/ban CFC use.
Tropospheric Chemistry
To form photochemical smog, three main
ingredients are needed:
1.
2.
3.
nitrogen oxides (NOx)
hydrocarbons (VOCs)
energy from the sun in the form of
ultraviolet light (UV).
Process:
1. Generation of NO, hydrocarbons
(VOCs) and carbon monoxide (CO)
from combustion in cars
N2 + O22 NO
2. In the air, nitrogen monoxide (nitric
oxide) combines with molecular
oxygen to form nitrogen dioxide
within a few hours.
2NO + O22 NO2
3. Nitrogen dioxide absorbs light energy
and splits to form nitrogen monoxide
and atomic oxygen:
NO2NO + O•
4. Then, in sunlight, the atomic oxygen
combines with oxygen gas to form
ozone
O• + O2O3
- If no other factors are involved, ozone and
nitric oxide then react to form nitrogen
dioxide and oxygen gas.
O3 + NO  NO2 + O2
- This last reaction is reversible and moves in
one direction based on the temperature
and the amount of sunlight.
- If there is a lot of sunlight, the equation
moves to the left, and more ozone is
produced. If nothing else gets in the way, an
equilibrium is reached, and the ozone level
stabilizes.
5. VOCs react with nitric oxide to
produce PAN (peroxyacetyl nitrate).
NO + hydrocarbonsPAN
Two Results:
1. Volatile, reactive organic compounds are
generated directly.
2. Nitric oxide (NO) reacts with
hydrocarbons instead of ozone increasing
the amount of ozone in the troposphere
to damaging levels.
End Product:
- an accumulation of ozone and volatile
organic compounds such as PAN, which are
secondary pollutants
- the sun moves the reactions along forming
photochemical smog
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Health Effects of Photochemical Smog
eye irritation and poor visibility
Strong oxidants such as ozone can damage the
lungs.
Damage to the lungs may stress the heart.
loss of immune system function, increased
susceptibility to infections, and fatigue.
kill plant cells, causing leaves to develop
brown spots or drop off the plant, reduce plant
growth, and make plants more susceptible to
damage from other causes.
corrode and destroy many materials such
as rubber, nylon, fabric, and paint.
Greenhouse Effect, Enhanced Greenhouse Effect,
Global Warming & Climate Change
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Greenhouse: glass of greenhouse allows
light into the plants and soil. Light is
converted to heat (infrared radiation
(IR)). The glass traps the heat making
the greenhouse warm.
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Greenhouse Effect on Earth: Atmospheric
gases act as the glass. Allow light in but
absorb and redirect the IR back to the
Earth.
Not all gases do this. Some gases do this
more than others. The ability to absorb IR and
release it is called the Global Warming
Potential. Carbon dioxide has a GWP value of 1
(base measurement) while CFC’s can be
hundreds or thousands of times greater than
CO2.
THIS IS GOOD otherwise the Earth
would be about -18 C.
Enhanced Greenhouse Effect (aka Global
Warming) is the idea that increased
amounts of Greenhouse gases will
cause the global temperature to
increase.
Effects:
 Change of ocean temperature.
 changes ocean currents = change in
climate
 changes weather patterns = change in
climate = loss of biodiversity. Also a
change in agriculture.
 change in upwelling of nutrients =
loss of biodiversity
 decreases oxygen content = loss of
biodiversity
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Increased CO2 in water
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Loss of Biodiversity
increases acidity of water
decreases productivity
kills coral reefs
Loss of
Biodiversity
Competing Theories