CHEM 31132 Environmental Chemistry I
Atmospheric Chemistry (10L)
Textbooks
Finlayson-Pitts
Chemistry of the Upper and Lower Atmosphere
(Academic Press)
Jacob
Introduction to Atmospheric Chemistry (Princeton)
Nigel Bunce
Environmental Chemistry
S. E. Manahan
Fundamentals of Environmental Chemistry
What is the course about?
• This course is about environmental issues
and the chemistry behind them.
• It aims to apply knowledge of chemistry to
understand environmental issues.
• The goal is to provide you the knowledge of
how to do a chemist’s share in improving
environmental quality.
What is environmental chemistry?
Environmental chemistry is the study of the sources, reactions, transport, effects, and fates of chemical species in water, soil, and air environments.”
Stanley E. Manahan. 1991. Environmental Chemistry, Fifth edition.
Climate Change(…1990s…)
Regional Air Pollution(…1950s…)
Acid rain(1970s…)
Stratospheric Ozone depletion
(1985…)
Atmospheric
CHEMISTRY
The Nobel Prize in Chemistry 1995
“for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone”
Paul J Cruzen
Mario J Molina
F. Sherwood Rowland
Martin Schultz, MPI-Met, Hamburg -- Potsdam Summerschool on Scientific Supercomputing in Climate Research, 2002
Key research questions:
• How has the atmospheric composition changed over time ?
• What is the human contribution to this change?
• How will the atmospheric composition change in the future?
• How will atmospheric composition change affect ecosystems, economies, and the quality of life?
Composition of the atmosphere
Gas % by volume
N2 78.08
O2 20.95
Ar 0.93
CO2 0.03
All other gases (Ne, He, Kr, H, etc)
0.01
Water Variable
Dry air
Water vapor in the air
• The % volume of Water vapor is variable, depending on temperature, precipitation, rate of evaporation and other factors at a particular location.
• The percentage of water vapor ranges from 0.1-5%. Generally it is 1-3% (the 3rd most abundant constituents in the air).
Expressing the amount of substances in the atmosphere
• Concentration
– the amount (mass, moles, molecules, etc) of a substance in a given volume of air divided by that volume.
– The example concentration units are μg/m3, mg/m3, mol/m3, molecules/cc.
• Mixing ratio
– the ratio of the molar amount of the substance in a given volume to the total molar amount of all constituents in that volume.
– It is essentially “mole fraction”.
Mixing Ratio (Cx)The number of moles of a substance x per mole of air;
equivalent to the mole fraction.
nx is the molar concentration of x and ntotal is the total molar
concentration of air.
parts per million (ppm)10-6 mmol mol-1
parts per billion (ppb) 10-9 nmol mol-1
parts per trillion (ppt) 10-12 pmol mol-1
total
xx
n
nC
total
xx
n
nppmC
x10)( 6
Conversion between ppm and mg/m3
Example:
The EU Air Quality Objective for ozone is 240 mg/m3. The U.S. National Ambient Air Quality Standard for ozone is 120 ppb. Which standard is stricter at the same temperature (25oC) and the pressure (1atm)?
ppbppmmgppminratiomixing 122122.0/240481001325.1
298314.8 3
5
m
Conversion between ppm and mg/m3
RT
P
V
Nntotal
x
x
xM
mn 10
6
nx: mol/m3
mx: mg/m3
Mx: g/mol
total
x
n
nppminratiomixing 610
x
x
x
x6
6 mpM
RT
RT
p
M
m10
10
3/ mginionConcentratpM
RTppminratiomixing
x
m
Pressure unit and R Constant:
P= 1.01325x105 pascal
R= 8.314 J/k.mol for P in Pa and volume in m3
Typical mixing ratios for some compounds of
environmental importance
Carbon dioxide 355 ppm Carbon monoxide 100 ppb to 20 ppm Ozone 1 to 100 ppb Methane 1.72 ppm Nonmethane hydrocarbon 1 ppt to < 1 ppb Nitric oxide (NO) 5 ppt to 1 ppb Nitrogen dioxide (NO2) 1 to 150 ppb Nitrous oxide (N2O) 310 ppb Sulfur dioxide 1 to 100 ppb CFCl3 (Freon 11) 200 ppt CF2Cl2 (Freon 12) 350 ppt
Martin Schultz, MPI-Met, Hamburg -- Potsdam Summerschool on Scientific Supercomputing in Climate Research, 2002
The system atmospheric chemistry
Sources
Reactions
Reservoir Sinks
Transport Transport
catalytic
cycles
Residence time
reservoir from, outflowor to,inflow of rate
reservoir"in the" substance ofamount timeResidence
The average length of time a given pollutant remains in
the atmosphere
Source: Origin of a particular substance in a reservoir
Sink: It’s destination
Structure of the atmosphere
• The atmosphere is a thin blanket of gas that envelops the earth.
• The gases that make up the atmosphere are held close to the earth by the pull of gravity.
• With increasing distance from the earth’s surface, the temperature, density, and composition of the atmosphere gradually change
• On the basis of air temperature, the atmosphere can be divided vertically into four major layers.
Atmospheric structure
Atmospheric structure
Lower Atmosphere is “Flat”!
Troposphere
• The troposphere is the layer from the earth’s surface to the tropopause, which is at 10-15 km altitude depending on latitude and time of year. (Mt. Everest 8.85km)
• As altitude increases, air temperature decreases at a rate of about 3.5o per 1000 ft. The tropopause has a temperature of about –57oC.
• The lower part of the troposphere interacts directly with the surface of the earth–this part of the troposphere is generally called air.
• The atmosphere in this layer is heated from below by convection and radiation from the earth’s surface.
• Most of our weather occurs in the troposphere.
Stratosphere
• The stratosphere is the layer above the troposphere and extends to about 50 km.
• The temperature rises with increasing altitude, reaching a maximum of about –1oC at the stratopause.
• The ozone layer is in the stratosphere. Ozone absorbs UV, causing the rising temperature with altitude in this layer.
• The temperature structure keeps the air calm in this layer. (That’s why jet aircraft fly in the lower stratosphere!)
Mesosphere
• The mesosphere extends from the top of stratopause to ~80 km.
• In the mesosphere, the temperature decreases with altitude.
Thermosphere
• The layer of air above mesosphere is called thermosphere.
• In the thermosphere, temperature rises with altitude, caused by absorption of UV solar radiation by N2 and O2.
Chemistry
of the
Environment
• The profile makes a
Z-shape from
mesosphere to the
ground.
The lower atmosphere
• The troposphere and the stratosphere together are called the lower atmosphere.
• The lower atmosphere account for 99.9% of total atmospheric mass
• The lower atmosphere is the domain of main interest from an environmental perspective.
– Ozone depletion (stratosphere)
– Air pollution (troposphere)
Ionosphere
• Ionosphere is a region where ions and electrons are most abundant.
• This region is located at altitude above 60 km, therefore lie within the mesosphere and above.
• Ionosphere acts as a conducting layer in the upper atmosphere that would allow a transmitted electromagnetic signal to be reflected back toward the Earth.
atmospheric pressure
The atmospheric pressure is the weight exerted by the overhead atmosphere on a unit area of surface
A B
h
vacuum
Mercury barometer
ghP HgA
Units for pressure
• International System of Units: Pascal (N/m2)
• Hectopascal (hPa)
• mm Hg or Torr
• Millibar (mbar)
• psi (lb/in2)
1 atm = 1.01325 x 105 Pascal (Pa) = 1.01325 x 103hPa
1 atm = 760 mmHg = 760 Torr
1 atm = 1013.25 mbar
1 atm = 14. 7 psi
Pressure profile
0
10
20
30
40
50
60
70
80
0.01 0.1 1 10 100 1000
Pressure, hPa
Alt
itu
de
(k
m)
Astronomer Fred
Hoyle once said,
"Outer space is not far
at all; it's only one
hour away by car if
your car could go
straight up!"
INTERNATIONAL FOOD POLICY RESEARCH INSTITUTE
Last week……
The lower atmosphere is the domain of main interest from an environmental perspective.
• Ozone depletion (stratosphere)
• Air pollution (troposphere)
Lecture 2
Stratospheric Chemistry
Ozone in the atmosphere Good Ozone and Bad Ozone
The ozone layer
Ozone Distribution in the Atmosphere
0 0.2 0.4 0.6 0.8 1.00
4
8
12
16
20
24
28
32
36
40
Atmospheric Pressure (atm)
Altitu
de
(km
)
Troposphere
Stratosphere
50
Pressure gradient
0 5 10 15 20 25
Ozone partial pressure (mPa)
Ozone concentration curve “Ozone layer”
UV A 315 – 400 nm
UV B 280 – 315 nm
UV C 200 – 280 nm
UV radiation includes wavelengths from 200 to 400 nm
Ozone Absorption in the UV
Band
• UV-C
• Nearly all UV-C is absorbed in the upper atmosphere
• UV-B
• 90% of UV-B is absorbed by the atmosphere, mostly by O3
• UV-A
• Not strongly absorbed by the atmosphere
Amount of UV Radiation That Reaches Earth’s Surface
Ultraviolet protection by ozone
Ozone absorbs UV light in the solar irradiation that is harmful to life
Express ozone abundance
• Dobson Units (DU)
named after G.M.B. Dobson, a scientist who conducted pioneering measurements of the stratosphere in the 1920s and 1930s.
• One DU is the thickness, measured in units of hundredths of a millimeter (0.01 mm), that the ozone column would occupy at standard temperature and pressure (273 K and 1 atm)
What is a Dobson unit?
• 1 Dobson Unit (DU) is
defined to be 0.01 mm
thickness at STP - (00C
and 1 atm pressure).
• A slab 3mm thick
corresponds to 300 DU
Typical ozone column values
• Total ozone column value ranges from 290 to 310 DU over the globe.
• If all the atmosphere's ozone were brought down to the earth's surface at standard pressure and temperature, it would produce a layer of about 3mm thick.
• Ozone depletion: when sum of ozone over height is lower than 2/3 of the normal value, we say "ozone depletion" occurs.
What is ozone?
Ozone is a stable molecule composed of three oxygen atoms.
O
O
O
While stable, it is highly reactive. The Greek word ozein means “to smell” and O3 has a strong pungent odor.
Ozone formation and destruction in the stratosphere
Chapman Mechanism
• The presence of a high-altitude ozone layer in the atmosphere was first determined in the 1920s
• A theory for the origin of this ozone layer was proposed in 1930 by a British scientist, Sydney Chapman
Ozone formation and destruction in the stratosphere
Chapman Cycle
a) O2+ hv 2O - R1
b) O+O2+M O3+M - R2
c) O3 + hv O +O2 - R3
d) O + O3 2O2 - R4
Where M is a random air molecule (O2 or N2)
formation
DestructionNatural ozone
removal
O + O2O3 (ozone)
+
heat
Over 300,000 T of ozone are formed and destroyed NATURALLY in the
stratosphere every day
Net result:• removal of almost all ultraviolet energy with wavelengths less than 240 nm from solar energy that reaches earth•the stratosphere warms up at higher altitudes
Dynamic Equilibrium
creation of ozone
breakdown of ozone
Anthropogenic Ozone Depletion
creation of ozone
breakdown of ozone
INTERNATIONAL FOOD POLICY RESEARCH INSTITUTE
Last week……
The lower atmosphere is the domain of main interest from an environmental perspective.
• Ozone depletion (stratosphere)
• Air pollution (troposphere)
Chapman Mechanism
Chapman theory describes how sunlight converts the various forms of oxygen from one to another
2/1
43
21
2
3 )][
(][
][
kk
Mkk
O
O
Steady-state O3
concentration
Rate coefficients for each reaction have been measured in the lab
Prediction by Chapman theory vs. Observation
Using Chapman theory
Q: Why does Chapman overpredict?
A: Catalytic Ozone loss cycles
Reaction (4) has a significant barrier and so is slow at stratospheric temperatures
There must be other O3 destruction pathways
Catalytic ozone destruction
X + O3 = XO + O2
XO + O = X + O2
O + O3 = 2 O2Net reaction
X is regenerated in the process – act as a catalyst.
The chain reaction continues until X is removed by some side reaction.
The important catalysts for stratospheric O3 destruction
• Hydroxy radical (OH).OH + O3 = HO2
. + O2
HO2. + O = .OH + O2
Net: O + O3 = 2 O2
• Chlorine and bromine (Cl and Br)Cl. + O3 = ClO. + O2
ClO. + O = Cl. + O2
Net: O + O3 = 2 O2
• Nitric oxide (NO)NO + O3 = NO2 + O2
NO2 + O = NO + O2
Net: O + O3 = 2 O2
HOx cycle
ClOx cycle
NOx cycle
Hydroxy radical
• Accounts for nearly one-half of the total ozone destruction in the lower stratosphere (16-20 km).
• Sources
O3 + hv = O2 + O1D (2%)
= O2 + O3P (98%)
O1D + H2O = 2 .OH (major)
Termination reaction.OH + NO2 HNO3
Human activity increases the amount of
naturally occurring methane and nitrogen oxides
in the stratosphere
Anthropogenic Ozone Depletion
The major causes are
photochemical
decomposition of
chlorinated and
brominated
hydrocarbons CFCl3 (CFC-11)
Anthropogenic Ozone Depletion
Ultraviolet light causes photochemical
breakdown, releasing Cl or Br free radicals
Chlorine atomSources: Photolysis of Cl-containing compounds in the stratosphere.
CFCl3 + hv CFCl2.+ Cl
.
CF2Cl2 + hv CF2Cl.+ Cl
.
Subsequent reactions of CFCl2 and CF2Cl more Cl atoms
Some principal Cl-containing species are:CF2Cl2, CFCl3, CCl4, CH3CCl3
Sources for Cl-containing compounds (need to be long-lived in the troposphere)
•Man-made: e.g. CFCs
•Natural: e.g. methyl chloride from biomass burning.
Chlorofluorocarbons (CFCs)
• CFCs is the abbreviated form of ChloroFluoroCarbons, a collective name given to a series of compounds containing chlorine, fluorine and carbon atoms. Examples: CFCl3, CF2Cl2, and CF2ClCFCl2.
• The major natural carrier of chlorine to the stratosphere is CH3Cl
CF2Cl2 2 Cl. + other products
CFCl3 3 Cl. + other products
The Montreal Protocol
Lifetimes of CFC’s One of the primary problems with CFC’s is that
they do not react in the troposphere, so can
diffuse into the stratosphere for a very long time
CFC-11
Trichlorofluoromethane (45 years)
CFC-115
Monochloropentafluoroethane (1700 years)
Adding hydrogen to the molecule dramatically
speeds up its decomposition in the troposphere
HCFC-21
Dichlorofluoromethane (2 years)
CFC substitutes
• The main strategy has been to explore the suitability of hydrochlorofluorocarbons
– The Cl and/or F substituents lend HCFCs some of the desirable properties of CFCs (e.g. low reactivity, fire suppression, good insulating and solvent characteristics, boiling point suitable for use in refrigerator cycles)
– The presence of C-H bond reduces the tropospheric lifetime significantly
• HCFCs are only transitional CFC substitutes
The Future Although no longer allowed, there are
still large amounts of CFC’s in
already produced goods.
It is estimated that the
ozone layer will not return
to its pre-1980 level until at
least 2050.
The Oxides of Nitrogen
• NOx is the ensemble of NO and NO2
• NO is produced abundantly in the troposphere, but all of it is converted into NO2 HNO3 (removed through
precipitation)
• NO in the stratosphere produced from nitrous oxide (N2O), which is much less reactive than NO.
N2O + hv N2 + O (95%)
N2O + O 2 NO (~5%)
X=NO in the catalytic cycle
NO2 + hv NO + O
O + O2 + M O3 + M
NO + O3 NO2 + O2
Net reaction:
“nul cycle”
No net O3 is destroyed
Provide rapid cycling between NO and NO2.
Removal processes:
NO2 + .OH HNO3
ClO. + NO2 ClONO2
Inhibit the HOx
and ClOx cycles
INTERNATIONAL FOOD POLICY RESEARCH INSTITUTE
Last week……
Chapman Mechanism
The two-sided effect of NOx
• NOx provides a catalytic chain mechanism for O3 destruction.
• NOx inhibit the HOx and ClOx cycles for O3
destruction by removing radical species in the two cycles.
• The relative magnitude of the two effects is altitude dependent.– >25 km, the net effect is to destruct O3.
– (NOx accounts for >50% of total ozone destruction in the middle and upper troposphere.)
– In the lower stratosphere, the net effect is to protect O3 from destruction.
The catalytic destruction reactions described so far, together with the Chapman cycle, account for the observed average levels of stratospheric ozone, they are unable to account for the ozone hole over Antarctica.
The ozone depletion in the Antarctica is limited both regionally and seasonally. The depletion is too great and too sudden. These observations can not be explained by catalytic O3 destruction by ClOx alone.
The discovery of the ozone hole(Polar Ozone Depletion)
• The British Antarctic Survey has been monitoring, for many years, the total column ozone levels at its base at Halley Bay in the Antarctica.
• Monitoring data indicate that column ozone levels have been decreasing since 1977.
• This observation was later confirmed by satellite data (TOMS-Total Ozone Mapping Spectrometer)
– Initially satellite data were assumed to be wrong with values lower than 190 DU
Polar Ozone Depletion
The polar ozone holes
are caused by a
different mechanism, in
which polar
stratospheric clouds
provide a catalytic
surface for the reaction
of chlorine carriers
(HCl and ClONO2)
TOMS-Total Ozone Mapping Spectrometer
Springtime development of ozone hole (Jul-Dec 2011)
Evidence linking ClO generation and O3 loss
Features of the ozone hole
• Ozone depletion occurs at altitudes between 10 and 20 km
– If O3 depletion resulted from the ClOx cycle, the depletion would occur at middle and lower latitude and altitudes between 35 and 45 km.
– The ClOx cycle requires O atom, but in the polar stratosphere, the low sun elevation results in essentially no photodissociation of O2.
– The above observation could not be explained by the ClOx destruction mechanism alone.
• Depletion occurs in the Antarctic spring
Special Features of Polar Meteorology
• During the winter polar night, sunlight does not reach the south pole.
• A strong circumpolar wind develops in the middle to lower stratosphere; These strong winds are known as the 'polar vortex'.
• The air within the polar vortex can get very cold.
• Once the air temperature gets to below about -80°C (193K), Polar Stratospheric Clouds (or PSCs for short) are formed.
Polar vortex
What do these clouds look like?
Polar Stratospheric Clouds
Polar Stratospheric Clouds (PSCs)
PSCs promote the conversion of inorganic Cl and Cl reservoir species to active Cl
• Heterogeneous reaction of gaseous ClONO2 with HCl on the PSC particles
HCl(s) + ClONO2 HNO3 (s) + Cl2
where s denotes the PSC surface
)PSC(
Note: The gas phase reaction between HCl and ClONO2 is extremely slow.
Active Cl species can rapidly yield Cl atoms when light is available
Active Cl species Cl2
Active Cl species readily photolyze to yield Cl atoms when daylight returns in the springtime.
Cl2 + hv 2Cl
Polar ClOx cycle to remove O3
• Polar regions: lack of O atom because of low sun elevation The ordinary ClOx cycle is not operative since it requires the presence of O atom.
• Under polar atmospheric conditions, the reaction sequence to remove O3 is as follows
Cl + O3 ClO + O2
ClO + ClO ClO-OCl
ClO-OCl + hv ClOO + Cl
ClOO + hv Cl + O2
2 [Cl + O3 ClO + O2]
Net of the last FOUR reactions: 2O3 3O2
Chlorine gas
Cold Temperatures
T~-80C
Polar
Stratospheric
Clouds
Ozone
destroying
chemicals
Everywhere in Atmosphere
Sunlight
Ozone
Hole
produces
and
and
produces
Ozone Hole Formation
Summary of the roles played by PSCs
• Provide surface for the conversion of inactive Cl species into active species
http://www.atm.ch.cam.ac.uk/tour/index.html
Courtesy of the Centre for
Atmospheric Sciences,
Cambridge University
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