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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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Stratospheric chemistry eps 133 28 march 04 april 2011
Stratospheric ChemistryEPS 133 28 March – 04 April 2011

Polar Stratospheric Clouds


Atmospheric attenuation of solar radiation
ATMOSPHERIC ATTENUATION OF SOLAR RADIATION

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



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) stp;

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: stp;

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 stp;

quantum

yield

absorption

X-section

photon

flux


Chapman mechanism vs observation

shape stp;

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 stp;

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

H stp; 2O 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 stp; (HOx ≡ H + OH + HO2) radicals

Initiation:

Propagation:

Termination:

slow

H2O

OH HO2

fast

HOx radical family

slow


Rate limiting step: Example stp;

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 stp; CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS


Nitrous oxide in the stratosphere

H stp; 2O mixing ratio

NITROUS OXIDE IN THE STRATOSPHERE



Rate limiting step, NOx: Example stp;

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




Source gas contributions to stratospheric chlorine 2004
SOURCE GAS CONTRIBUTIONS TO stp; STRATOSPHERIC CHLORINE (2004)



What is a rate limiting step
WHAT IS A RATE-LIMITING STEP? stp;

  • 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.”


altitude stp;

Latitude

Latitude

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

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


altitude stp;

Latitude

Latitude


Stratospheric Circulation stp;

Prof. James R. Holton


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

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 stp;

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: stp; 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 HOLE stp; WITH 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)


Satellite observations of clo in the southern hemisphere stratosphere
SATELLITE OBSERVATIONS OF ClO stp; IN THE SOUTHERN HEMISPHERE STRATOSPHERE


WHY THE HIGH ClO IN ANTARCTIC VORTEX? stp; Release of chlorine radicals from reactions of reservoir species in polar stratospheric clouds (PSCs)


Psc formation at cold temperatures
PSC FORMATION AT COLD TEMPERATURES stp;

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? stp; HNO3-H2O PHASE DIAGRAM

Antarctic

vortex

conditions

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


Denitrification in the polar vortex sedimentation of pscs
DENITRIFICATION IN THE POLAR VORTEX: stp; SEDIMENTATION OF PSCs



Trends in global ozone
TRENDS IN GLOBAL OZONE stp;

Mt. Pinatubo


Long term cooling of the stratosphere
LONG-TERM COOLING OF THE STRATOSPHERE stp;

Sep 21-30, 25 km, 65-75˚S

Increasing CO2 is expected to cool the stratosphere


TRENDS IN POLAR OZONE stp; Could greenhouse-induced cooling of stratosphereproduce an Arctic ozone hole over the next decade?

Race between chlorine decrease and climate change



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