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