1

ESS200A

Prof. Jin-Y

i Yu

Anthropogenic C

limate C

hanges

Hum

an production of freons(C

FCs)

Ozone H

ole Depletion

Hum

an production of CO

2 and CH

4 G

lobal Warm

ing

Hum

an change of land use D

eforestation

CO

2

CH

4

CF

C

(from E

arth’s Clim

ate: Past and F

uture)E

SS200AP

rof. Jin-Yi Y

u

Standard Atm

osphere

(from U

nderstanding Weather &

Clim

ate)

ESS200A

Prof. Jin-Y

i Yu

Measurem

ents of Ozone

(1) Num

ber Density

Nu

mb

er of molecu

les per cubic centimeter

(molecules/cm

3).

The num

ber density is typically about 5x1012

molecules/cm

3near 20 to 25 km

altitude, near the peak of ozone concentration in the stratosphere.

(2)L

ayer Th

ickness

The th

ickness of pure ozon

e wou

ld have at 1 atm

pressu

re.

One atm

osphere-centimeter (1 atm

-cm) is equal

to 2.687x1019

molecules/cm

2.

One D

obson Unit (D

U) is equivalent to a layer of

pure ozone 0.001 cm thick at 1 atm

pressure.

A typical m

idlatitudeozone colum

n depth is about 300 D

U (0.3 atm

-cm).

(from W

MO

Report 2003)

ESS200A

Prof. Jin-Y

i Yu

Why is O

zone Important?

The stratospheric ozone layer

reduces the amount of U

V-B

radiation from

the sun reaching E

arth’s surface.

UV

-B exposure can dam

age hum

an’s imm

une system,

increase risk of skin cancer, and dam

age terrestrial plant life, single-cell organism

s, and aquatic ecosystem

.

(from W

MO

Report 2003)

2

ESS200A

Prof. Jin-Y

i Yu

How

Is Ozone Form

ed?

(from W

MO

Report 2003)

ESS200A

Prof. Jin-Y

i Yu

Ozone D

istribution

The greatest production of ozone occurs in the tropics, w

here the solar UV

flux is the highest.

How

ever, the general circulation in the stratosphere transport ozone-rich air from

the tropical upper stratosphere to mid-to-high latitudes.

Ozone colum

n depths are highest during springtime at m

id-to-high latitudes.

Ozone colum

n depths are the lowest over the equator.

(from T

he Earth System

)

Antarctic

Ozone

Hole

ESS200A

Prof. Jin-Y

i Yu

Polar Vortex

ESS200A

Prof. Jin-Y

i Yu

Ozone Production and D

estruction

Photodissociation

(or photolysis)

visible light

(from T

he Earth System

)

destroy O3 perm

anently

3

ESS200A

Prof. Jin-Y

i Yu

Radiation and O

zone

Ozone P

roductionU

V photons are required for ozone production:

O2

+ UV

photon O

+ O

Below

about 20 km, U

V photons are absorbed by ozone. T

herefore, O2 can be

photolyzedonly above 20 km

. This is w

hy the ozone layer is located in thestratosphere and not near E

arth’s surface.

Ozone D

estructionO

3 can be split by radiation in the visible-light range:O

3+ photon

O2

+ O

Because m

any more visible photons than U

V photons are available, O

3 is photolyzed

(ozone destruction) much faster than O

2(ozone production).

Also, O

3 can be photolyzed all the way sow

n to Earth’s surface.

ESS200A

Prof. Jin-Y

i Yu

Other O

zone Destruction Processes

Other atm

ospheric 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 w

ould otherwise be.

These radicals include: nitric oxide (N

O), atom

ic chlorine (Cl), brom

ine (Br)

radicals, and hydroxyl (OH

) radicals.

These radicals can destroy stratospheric ozone through “catalytic cycles”.

A catalytic cycle is a set of chem

ical reactions facilitated by the presence of a catalyst.

A catalyst is a substance that increases the rate of a chem

ical reaction but is itself unchanged by the reaction.

ESS200A

Prof. Jin-Y

i Yu

The C

hlorine Catalytic C

ycle

The C

atalyst

ESS200A

Prof. Jin-Y

i Yu

The C

hlorine Catalytic C

ycle

(from W

MO

Report 2003)

4

ESS200A

Prof. Jin-Y

i Yu

The N

itrogen Catalytic C

ycle

The C

atalyst

(from T

he Earth System

)

ESS200A

Prof. Jin-Y

i Yu

Where do those ozone-depleting catalysts com

e from?

(1) Cl–

related to natural and man-m

ade substances

(2) Br –

related to natural and man-m

ade substances

(3) NO

2 reacts with C

land Br to produce reservoirs for C

land B

r.

ESS200A

Prof. Jin-Y

i Yu

The O

dd Nitrogen C

ycleN

2O: from

Earth’s surface, w

here it is produced by m

icrobial activity in soil and in the ocean.

In the stratosphere, some of N

2O

reacts to form N

O, the rest is

photolyzedback to N

2 and O2.

The N

O so produced participates

in the ozone-destroying nitrogen catalytic cycle.

Once for a w

hile, the resulting N

O2 m

olecule interacts with a

hydroxyl radical (OH

), producing nitric acid (H

NO

3).

Nitric A

cid (HN

O3) then diffuses

down into the troposphere and

dissolves into rain.

(from T

he Earth System

)

from U

V photolysis

of ozone

ESS200A

Prof. Jin-Y

i Yu

Other Sources of N

O

N2O

is currently the largest source of stratospheric odd nitrogen (NO

and N

O2).

How

human activity affect N

O?

high-flying, supersonic transport airplanes

Jet plane-produced high temperatures from

combustion.

combine N

2 and O2 to form

NO

.

injected NO

goes to the stratosphere and destroy ozone

This is w

hy jet plans can affect stratospheric ozone layer

(even though jet plans fly in the upper troposphere).

5

ESS200A

Prof. Jin-Y

i Yu

The C

hlorine Cycle

Natural sources of chlorine:

(1) Methyl ch

loride (CH

3Cl):

produced by m

arine plankton. (2) Hyd

rogen chloride

(HC

l):produced by volcanic eruption and by

evaporation of sea spray.

These natural-generated chlorine are m

ost rem

oved by precipitation before they reach the stratosphere.

The largest sources of stratospheric chlorine

today are freons(C

FCs), w

hich are thropogeniccom

pounds.

CFC

s do not react in either the troposphere or low

er stratosphere

CFC

s go all the way to the upper stratosphere

and are photolyzedby U

V radiation.

The C

lso produced proceeds to destroy ozone.

Eventually, C

lreact with C

H4 and is diffused

to the troposphere, where it is precipitated out.

(from T

he Earth System

)E

SS200AP

rof. Jin-Yi Y

u

Chlorine Sources

(from W

MO

Report 2003)

ESS200A

Prof. Jin-Y

i Yu

Man-M

ade Sources for CFC

s

There are tw

o kinds of CFC

s: freon-11 (C

Cl3F) and freon-12 (C

Cl2F2).

Freon-11 has been used:

(1) as a propellant in spray cans

(2) as a blowing agent for producing foam

s

(3) to clean semiconductor chips.

Freon-12 has been used as

(1) a refrigerant

(2) working fluid in m

ost car air conditioners.

(from T

he Earth System

)E

SS200AP

rof. Jin-Yi Y

u

The B

romine C

ycleB

r is also a ozone-depleting catalyst

The brom

ine cycle is similar to the chlorine cycle.

The natural source of brom

ine is Methyl B

romine (C

h3Br), w

hich is a byproduct of biological activity in the ocean. T

hese natural source reacts in the troposphere.

The m

an-made source of brom

ine is two chem

ical compounds: H

alon-1211 (C

F2ClB

r) and Halon-1301 (C

F3Br).

These tw

o halonsare used in certain types of fire extinguishers.

Halons

diffuse up to the stratosphere, where they are photolyzed

into brom

ine atmos. T

hey eventually rain out.

6

ESS200A

Prof. Jin-Y

i Yu

Brom

ine Sources

(from W

MO

Report 2003)

ESS200A

Prof. Jin-Y

i Yu

Coupling B

etween O

dd Nitrogen and C

hlorine Cycles

Throughout m

ost of the lower stratosphere, the nitrogen and chlorine

cycles are coupled by the above chemical reaction.

The chlorine nitrate (C

lON

O2) form

ed in this reaction does not react directly w

ith either ozone or atomic oxygen. T

herefore, this coupling reaction keeps chlorine from

being in its reactive forms (C

land ClO

) (w

hich can destroy ozone).

reactiveunreactive

ESS200A

Prof. Jin-Y

i Yu

Polar Stratospheric Clouds (P

SCs)

In winter the polar stratosphere is so

cold (-80°C

or below) that certain

trace atmospheric constituents can

condense.

These clouds are called “polar

stratospheric clouds”(P

SC

s).

The particles that form

typically consist of a m

ixture of water and

nitric acid (HN

O3).

The P

SC

salter the chem

istry of the low

er stratosphere in two w

ays:

(1) by slow dow

n the coupling betw

een the odd nitrogen and chlorine cycles

(2) by providing surfaces on which

heterogeneous reactions can occur.

(Sw

eden, January 2000; from N

AS

A w

ebsite)

ESS200A

Prof. Jin-Y

i Yu

How

PSCs

Affect C

hlorine?In m

ost of the seasons, there are always abundant N

O2 in the stratosphere to

tie up a significant fraction of the available chlorine in the form of chlorine

nitrate.

In the wintertim

e Antarctic stratosphere, N

O2 concentrations are

low, because

most of the odd nitrogen has been converted into H

NO

3 and become

droplets in P

SC

s(polar stratospheric clouds).

Therefore, the form

ation of PS

Cs

allow reactive chlorine concentration to

increases.

The P

SC

sparticles also help convert unreactive

forms of chlorine into reactive

chlorine by providing surfaces on which heterogeneous reaction can occur.

On P

CSs

destroy ozone

7

ESS200A

Prof. Jin-Y

i Yu

Three Factors for the O

zone Hole

Chem

ical Reactions –

polar stratospheric clouds (PSCs)

Atm

ospheric Circulation –

stratospheric polar vortex

Sunlight –spring season

In Winter !!

ESS200A

Prof. Jin-Y

i Yu

The Polar V

ortexT

he wintertim

e circulation over the S

outh Pole is characterized

by a gigantic whirlpool of cold

and dense air, called the polar vortex.

The cold and dense cold air in

the middle of the vortex is

subsiding.

The sinking air carries cloud

particles along with it.

Rem

ove odd nitrogen from the

stratosphere.

Very little ozone and odd

nitrogen can be brought into the south pole.

ESS200A

Prof. Jin-Y

i Yu

Antarctic O

zone Hole

The decrease in ozone near the

South P

ole is most striking near

the spring time (O

ctober).

During the rest of the year, ozone

levels have remained close to

normal in the region.

(from T

he Earth System

)

Mean T

otal Ozone O

ver Antarctic in O

ctober

ESS200A

Prof. Jin-Y

i Yu

Satellite View

of the Ozone H

ole

September 6, 2000

8

ESS200A

Prof. Jin-Y

i Yu

The 1997 O

zone Hole

ESS200A

Prof. Jin-Y

i Yu

Ozone H

ole Depletion

Long A

ntarctic winter (M

ay through Septem

ber)

The stratosphere is cold enough to form

PSC

s

PS

Cs

deplete odd nitrogen (NO

)

Help convert unreactive

forms of chlorine (C

lON

O2 and H

Cl) into m

ore reactive form

s (such as Cl2).

The reactive chlorine rem

ains bound to the surface of clouds particles.

Sunlight returns in springtim

e (Septem

ber)

The sunlight releases reactive chlorine from

the particle surface.

The chlorine destroy ozone in O

ctober.

Ozone hole appears.

At the end of w

inter, the polar vortex breaks down.

Allow

fresh ozone and odd nitrogen to be brought in from low

latitudes.

The ozone hole recovers (disappears) until next O

ctober.

ESS200A

Prof. Jin-Y

i Yu

Global T

otal Ozone C

hange

(from W

MO

Report 2003)

ESS200A

Prof. Jin-Y

i Yu

Why N

o Ozone H

ole in Artic?

(from W

MO

Report 2003)