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Stratospheric Chemistry EPS 133 28 March – 04 April 2011. Polar Stratospheric Clouds. ATMOSPHERIC ATTENUATION OF SOLAR RADIATION. Solar UV radiation reaching the top of the atmosphere is absorbed by ozone. Based on ozonesonde observations in the 1970s. THE NATURAL OZONE LAYER.

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atmospheric attenuation of solar radiation
ATMOSPHERIC ATTENUATION OF SOLAR RADIATION

Solar UV radiation reaching the top of the atmosphere is absorbed by ozone

slide4

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at stp;

the ozone layer over Labrador is ~300 DU.

Mean ratio,

column O3: air = 5 x 10-7

chapman mechanism for stratospheric ozone 1930
CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930)

Odd oxygen family [Ox] = [O3] + [O]

slow

R2

R1

O2

O O3

fast

R3

R4

slow

steady state analysis of chapman mechanism

Lifetime of O atoms:

STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM

…is sufficiently short to assume steady state for O:

…so the budget of O3 is controlled by the budget of Ox.

Lifetime of Ox:

τOx

Steady state for Ox:

photolysis rate constants vertical dependence
PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE

quantum

yield

absorption

X-section

photon

flux

chapman mechanism vs observation

shape

determined

by k1nO2

CHAPMAN MECHANISM vs. OBSERVATION

-3

Chapman mechanism reproduces shape, but is too high by factor 2-3

e missing sink!

radical reaction chains in the atmosphere

non-radical

radical + radical

RADICAL REACTION CHAINS IN THE ATMOSPHERE

photolysis

thermolysis

oxidation by O(1D)

Initiation:

bimolecular

redox reactions

Propagation:

radical + non-radical

non-radical + radical

radical redox

reaction

Termination:

non-radical + non-radical

radical + radical

non-radical + M

3-body recombination

radical + radical + M

water vapor in stratosphere

H2O mixing ratio

WATER VAPOR IN STRATOSPHERE

Source: transport from troposphere, oxidation of methane (CH4)

ozone loss catalyzed by hydrogen oxide ho x h oh ho 2 radicals
Ozone loss catalyzed by hydrogen oxide (HOx ≡ H + OH + HO2) radicals

Initiation:

Propagation:

Termination:

slow

H2O

OH HO2

fast

HOx radical family

slow

slide15

Rate limiting step: Example

OH + O3 -> HO2+ + O2k1

HO2 + O3 -> OH + O2k2

HO2 + NO ->->-> OH + NO + O3k3

{ + O2 + hν … }

d[OH] / dt = -d[HO2] / dt = - k1[OH][O3] + k2[O3][HO2] + k3*[NO][HO2] ≈ 0 A

d[O3] / dt = -k1[OH][O3] – k2[HO2][O3] + k3*[NO][HO2] B

To B,add (-1)xA ≈ 0

d[O3] / dt = - 2 k2 [HO2][O3]

OH + O3

HO2 + O3

HO2 + NO

Rate limiting step for removal of ozone by Reactions 1, 2, 3

stratospheric ozone budget for midlatitudes constrained from 1980s space shuttle observations
STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS
slide19

Rate limiting step, NOx: Example

NO + O3 -> NO2+ + O2k1

NO2 + hν -> NO + O -> O3k2

NO2 + O -> NO + O2k3

d[NO] / dt = -d[NO2] / dt = - k1[NO][O3] + k2[NO2] + k3[NO2][O] ≈ 0 A

d[O3] / dt = -k1[NO][O3] + k2[NO2] - k3[NO2][O] B

To B,add (-1)xA ≈ 0

d[O3] / dt = - 2 k3 [NO2][O]

NO + O3

NO2 + O

NO2 + hv

Rate limiting step for removal of ozone by Reactions 1, 2, 3

what is a rate limiting step
WHAT IS A RATE-LIMITING STEP?
  • From IUPAC: “A rate-controlling (rate-determining or rate-limiting) step in a reaction occurring by a composite reaction sequence is an elementary reaction the rate constant for which exerts a strong effect — stronger than that of any other rate constant — on the overall rate.”
slide25

altitude

Latitude

Latitude

http://ccmc.gsfc.nasa.gov/modelweb/atmos/msise.html

ftp://hanna.ccmc.gsfc.nasa.gov/pub/modelweb/atmospheric/msis/msise90/

slide26

altitude

Latitude

Latitude

slide28

Stratospheric Circulation

Prof. James R. Holton

ozone trend at halley bay antarctica october
OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER)

Farman et al. paper

published in Nature

1 Dobson Unit (DU) = 0.01 mm O3 STP = 2.69x1016 molecules cm-2

spatial extent of the ozone hole
SPATIAL EXTENT OF THE OZONE HOLE

Mean October

data

Isolated concentric region around Antarctic continent is called the polar vortex.

Strong westerly winds, little meridional transport

vertical structure of the ozone hole near total depletion in lower stratosphere
VERTICAL STRUCTURE OF THE OZONE HOLE:near-total depletion in lower stratosphere

Argentine Antarctic station southern tip of S. America

association of antarctic ozone hole with high levels of clo
ASSOCIATION OF ANTARCTIC OZONE HOLEWITH HIGH LEVELS OF CLO

Sept. 1987 ER-2 aircraft measurements at 20 km altitude south of Punta Arenas

O3

ClO

O3

Sep. 16

Edge of

Polar

vortex

ClO

Sep. 2, 1987

20 km altitude

Measurements by Jim Anderson’s group (Harvard)

slide36
WHY THE HIGH ClO IN ANTARCTIC VORTEX?Release of chlorine radicals from reactions of reservoir species in polar stratospheric clouds (PSCs)
psc formation at cold temperatures
PSC FORMATION AT COLD TEMPERATURES

PSC formation

Frost point of water

how do pscs start forming at 195k hno 3 h 2 o phase diagram
HOW DO PSCs START FORMING AT 195K?HNO3-H2O PHASE DIAGRAM

Antarctic

vortex

conditions

PSCs are not water but nitric acid trihydrate (NAT) clouds

long term cooling of the stratosphere
LONG-TERM COOLING OF THE STRATOSPHERE

Sep 21-30, 25 km, 65-75˚S

Increasing CO2 is expected to cool the stratosphere

slide44
TRENDS IN POLAR OZONECould greenhouse-induced cooling of stratosphereproduce an Arctic ozone hole over the next decade?

Race between chlorine decrease and climate change

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