10 Stratospheric ozone The ozone layer protects us

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10. Stratospheric ozone

10. Stratospheric ozone

The ozone layer protects us from UV radiation Visible UV Ozone layer Earth surface

The ozone layer protects us from UV radiation Visible UV Ozone layer Earth surface Ozone absorbs UV radiation while letting visible radiation through Ozone (O 3) is produced naturally in the stratosphere from molecular oxygen (O 2)

The ozone layer Dobson unit: physical thickness (0. 01 mm) of ozone layer if

The ozone layer Dobson unit: physical thickness (0. 01 mm) of ozone layer if compressed to 1 atm, 0 o. C 1 DU = 2. 69 x 1016 molecules cm-2 Latest satellite ozone data (March 12): http: //www. temis. nl/protocols/O 3 total. html

Structure of the natural ozone layer Ozone number density, 1012 molecules cm-3 Based on

Structure of the natural ozone layer Ozone number density, 1012 molecules cm-3 Based on ozonesonde observations in the 1970 s

The natural ozone layer: chemical production (P) and loss (L) Ozone number density, 1012

The natural ozone layer: chemical production (P) and loss (L) Ozone number density, 1012 molecules cm-3 L>P P>L L>P Brewer-Dobson stratospheric circulation

Chapman mechanism for stratospheric ozone (1930) Sydney Chapman (1888 -1970)

Chapman mechanism for stratospheric ozone (1930) Sydney Chapman (1888 -1970)

Calculation of photolysis rates k is the photolysis rate constant (also called photolysis frequency)

Calculation of photolysis rates k is the photolysis rate constant (also called photolysis frequency) radiation flux [photons cm-2 s-1] quantum yield: probability that photon absorption causes photolysis absorption cross-section [cm 2 molecule-1] Probability of absorption for incoming photons = σ/A photon is not absorbed Absorption cross-section s photon is absorbed Molecular cross-section A

Solar spectrum and absorption cross-sections O 2+hv O 3+hv

Solar spectrum and absorption cross-sections O 2+hv O 3+hv

Calculation of 3 -body reaction rates A and B are reactants; AB* is the

Calculation of 3 -body reaction rates A and B are reactants; AB* is the activated product; AB is the stable product; M is the “third body” (N 2, O 2 ) General solution: Low-pressure limit (Rate(2) >> Rate (3)): High-pressure limit (Rate(2) << Rate (3)):

Ozone photolysis produces the excited state O(1 D), which then relaxes thermally to the

Ozone photolysis produces the excited state O(1 D), which then relaxes thermally to the ground state O(3 P) • Spectroscopic notation is generally omitted for ground state: we write O instead of O(3 P) • But formation of the O(1 D) intermediate is important to recognize for later on because a small fraction of O(1 D) atoms can oxidize other chemicals rather than thermally relax to O(3 P) • For now we will only focus on the net reaction, which describes the dominant pathway

Energy states of the O atom (1 s 22 p 4) determined by the

Energy states of the O atom (1 s 22 p 4) determined by the arrangement of the four electrons in the 2 p orbitals multiplicity total electronic orbital angular momentum number Multiplicity = 2 S+1, where S is the spin. The spin of an electron is ± 1/2. Energy O(1 S) O(1 D) O(3 P) . . Hund’s Rule: lowest-lying energy state is the one of maximum multiplicity : : O O(3 P) is a biradical

Chemical radicals • Chemical radicals are atoms or molecules with unpaired electrons Example: hydroxyl

Chemical radicals • Chemical radicals are atoms or molecules with unpaired electrons Example: hydroxyl radical (OH) • The unpaired electron makes radicals highly reactive, with atmospheric lifetimes of generally less than a minute. One can reliably assume chemical steady state for radicals (chemical production = chemical loss) • To determine whether a species is a radical, add up the electrons contributed by each atom of the molecule. If the total number is odd, then the species is a radical; if it is even the species is a non-radical. • Very exceptionally, a species with an even number of electrons may be a biradical and hence very reactive, or in a singlet state and even more reactive. O(3 P) and O(1 D) are the only such cases that we will encounter.

Diagram of Chapman mechanism slow O 2 R 1 O R 2 fast R

Diagram of Chapman mechanism slow O 2 R 1 O R 2 fast R 3 Odd oxygen family [Ox] ≡ [O 3] + [O] O 3 R 4 slow In atmospheric chemistry, a “chemical family” is simply an accounting device. Here [O] << [O 3], so [O 3] ≈ [Ox]. The budget of ozone is actually that of odd oxygen: ozone is effectively produced by (R 1) and lost by (R 4)

Steady-state analysis of Chapman mechanism Steady state for O atoms: …so the budget of

Steady-state analysis of Chapman mechanism Steady state for O atoms: …so the budget of O 3 is controlled by the budget of Ox. Lifetime of Ox: Steady state for Ox: τOx

Photolysis rate constants: dependence on altitude quantum yield k 1(z) and k 3(z) depend

Photolysis rate constants: dependence on altitude quantum yield k 1(z) and k 3(z) depend on column of O 3 overhead absorption x-section photon flux

Chapman mechanism vs. observations log n. O 2 shape determined by k 1 n.

Chapman mechanism vs. observations log n. O 2 shape determined by k 1 n. O 2 -3 Chapman mechanism reproduces shape, but is too high by factor 2 -3 e missing sink!

General schematic of radical-assisted reaction chains Initiation: non-radical + radical photolysis thermolysis oxidation by

General schematic of radical-assisted reaction chains Initiation: non-radical + radical photolysis thermolysis oxidation by O(1 D) Propagation: radical + non-radical + radical Termination: radical + M non-radical + M bimolecular redox reaction 3 -body reaction

Water vapor in stratosphere H 2 O mixing ratio Source: transport from troposphere, oxidation

Water vapor in stratosphere H 2 O mixing ratio Source: transport from troposphere, oxidation of methane (CH 4)

Ozone loss catalyzed by hydrogen oxide radicals (HOx) Initiation: Propagation: Termination: slow H 2

Ozone loss catalyzed by hydrogen oxide radicals (HOx) Initiation: Propagation: Termination: slow H 2 O OH fast HO 2 slow HOx radical family: HOx ≡ OH + HO 2

Supersonic aircraft (Concorde) cruising at 60, 000’

Supersonic aircraft (Concorde) cruising at 60, 000’

Ozone loss catalyzed by nitrogen oxide radicals (NOx) Initiation: conversion of air to NO

Ozone loss catalyzed by nitrogen oxide radicals (NOx) Initiation: conversion of air to NO radicals in combustion engine NO ≡ nitric oxide NO 2 ≡ nitrogen dioxide Propagation: cycling of NOx radicals (NOx ≡ NO + NO 2) Termination: oxidation of NOx to HNO 3 (nitric acid) R 6 is rate-limiting step for catalytic ozone loss: -d[O 3]/dt = 2 k 6[NO 2][O] OH is a very strong oxidant (we will encounter it often) Recycling: conversion of HNO 3 back to NOx HNO 3 is called a ‘reservoir’ for NOx

WHAT IS A RATE-LIMITING STEP? • From IUPAC: “A rate-controlling (rate-determining or 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. ” It is not necessarily the slowest reaction in the sequence! Example: Steady-state for NO (or NO 2): Replace:

Computing the NOx-catalyzed ozone loss rate Total nitrogen oxides Conserved in stratosphere Loss rate

Computing the NOx-catalyzed ozone loss rate Total nitrogen oxides Conserved in stratosphere Loss rate depends on: 1. NOx emission rate E 2. [NOx]/[NOy] ratio (< 0. 1) 3. [NO 2]/[NOx] ratio (≈ 1) 4. Ozone loss rate given by (R 6) 5. Residence time τ of air in stratosphere (≈1 year) Typically [NOx]/[NOy] < 0. 1; importance of reservoirs! tropopause

Nitrous oxide: natural source of NOx in stratosphere H 2 O mixing ratio Source:

Nitrous oxide: natural source of NOx in stratosphere H 2 O mixing ratio Source: nitrogen cycling in biosphere, N 2 O is emitted as a byproduct Sinks: photolysis, oxidation by O(1 D) τN 2 O = 120 years

Rising concentration of N 2 O due to agriculture IPCC [2014]

Rising concentration of N 2 O due to agriculture IPCC [2014]

Computing the NOx-catalyzed ozone loss rate Same as before but with NOx source from

Computing the NOx-catalyzed ozone loss rate Same as before but with NOx source from N 2 O oxidation The natural ozone layer can be largely explained by production from O 2 photolysis (Chapman mechanism) balanced by NOx-catalyzed loss from biogenic N 2 O

Chlorofluorocarbons (CFCs) in the atmosphere (ppb) • Industrial gases valued for their inertness •

Chlorofluorocarbons (CFCs) in the atmosphere (ppb) • Industrial gases valued for their inertness • Most important are CFC-11 (CFCl 3) and CFC-12 (CF 2 Cl 2) • Large-scale production started in 1950 s, growth rate in 1970 s was 4%/year

STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980 s SPACE SHUTTLE OBSERVATIONS Salawitch et

STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980 s SPACE SHUTTLE OBSERVATIONS Salawitch et al. [1989]

Ozone loss catalyzed by chlorine radicals (Cl. Ox) Initiation: photolysis of CFCs Propagation: cycling

Ozone loss catalyzed by chlorine radicals (Cl. Ox) Initiation: photolysis of CFCs Propagation: cycling of Cl. Ox radicals (Cl. Ox ≡ Cl + Cl. O) R 5 is rate-limiting step for catalytic ozone loss Termination: conversion of Cl. Ox to HCl and Cl. NO 3 reservoirs Recycling: conversion of HCl and Cl. NO 3 back to NOx HCl ≡ hydrogen chloride Cl. NO 3 ≡ chlorine nitrate CH 4 ≡ methane NO 3 ≡ nitrate radical

Computing the Cl. Ox–catalyzed ozone loss rate Follow exactly the same approach as for

Computing the Cl. Ox–catalyzed ozone loss rate Follow exactly the same approach as for the NOx-catalyzed ozone loss rate

Chlorine partitioning in stratosphere >90% of Cly is locked up in reservoirs (HCl, Cl.

Chlorine partitioning in stratosphere >90% of Cly is locked up in reservoirs (HCl, Cl. NO 3) rather than radicals (Cl. O, Cl) WMO [2014]

CFC trends

CFC trends

Antarctica: the “scientific continent” 40 countries operate bases dedicated to scientific research

Antarctica: the “scientific continent” 40 countries operate bases dedicated to scientific research

Discovery of Antarctic ozone depletion at Halley Bay, Antarctica Ozone layer thickness (October) 1985:

Discovery of Antarctic ozone depletion at Halley Bay, Antarctica Ozone layer thickness (October) 1985: ozone hole first reported In Nature by British Antarctic Survey Ozone layer First NASA satellite observations of ozone layer spectrometer October 1985 1 Dobson Unit (DU) = 0. 01 mm pure ozone = 2. 69 x 1016 molecules cm-2

Satellite data show recurrence of ozone hole every austral spring http: //ozonewatch. gsfc. nasa.

Satellite data show recurrence of ozone hole every austral spring http: //ozonewatch. gsfc. nasa. gov/

Ozone hole is a seasonal phenomenon it develops in austral spring (September-October) and is

Ozone hole is a seasonal phenomenon it develops in austral spring (September-October) and is gone by December Isolated concentric region around Antarctic continent is called the polar vortex. Strong westerly winds, little meridional transport http: //ozonewatch. gsfc. nasa. gov/

Vertical structure of ozone hole: near-total depletion in lower stratosphere Ozone concentration (Pa) WMO

Vertical structure of ozone hole: near-total depletion in lower stratosphere Ozone concentration (Pa) WMO [2014]

What is the cause of the ozone hole? Spring 1987 NASA ER-2 mission from

What is the cause of the ozone hole? Spring 1987 NASA ER-2 mission from Punta Arenas, Chile Ozone-destroying chlorine produced from CFCs ozone hole boundary

SATELLITE OBSERVATIONS OF Cl. O IN THE SOUTHERN HEMISPHERE STRATOSPHERE Cl. O increases by

SATELLITE OBSERVATIONS OF Cl. O IN THE SOUTHERN HEMISPHERE STRATOSPHERE Cl. O increases by an order of magnitude in the ozone hole – reflects inability of the reservoir species (HCl, Cl. NO 3) to lock up the chlorine

The standard Cl. Ox-catalyzed ozone loss mechanism does not work in Antarctic spring Standard

The standard Cl. Ox-catalyzed ozone loss mechanism does not work in Antarctic spring Standard mechanism: R 5 is rate-limiting step for catalytic ozone loss -d[O 3]/dt = 2 k 5[Cl. O][O] But the O atom concentration is determined by Weak photon flux in Antarctic spring O is low R 5 is slow

Alternative mechanism for ozone depletion in Antarctic spring requires high Cl. O Ozone loss

Alternative mechanism for ozone depletion in Antarctic spring requires high Cl. O Ozone loss rate: Cl. O dimer Cl-O-O-Cl is weakly bound so can be easily photolyzed

Conversion of Cl reservoirs to Cl. O in polar stratospheric clouds (PSCs) HCl and

Conversion of Cl reservoirs to Cl. O in polar stratospheric clouds (PSCs) HCl and Cl. NO 3 stick to PSC particles and then react Cl. NO 3 HCl Cl. NO 3 PSC HCl particle Cl. NO 3 Cl. NO HCl 3 Polar stratospheric clouds over Mc. Murdo, Antarctica

Polar stratospheric clouds require very cold conditions, found mainly in Antarctic stratosphere in winter

Polar stratospheric clouds require very cold conditions, found mainly in Antarctic stratosphere in winter Observed PSC formation Frost point of water WMO [2014]

HOW DO PSCs START FORMING AT 195 K? HNO 3 -H 2 O PHASE

HOW DO PSCs START FORMING AT 195 K? HNO 3 -H 2 O PHASE DIAGRAM Antarctic vortex conditions PSCs are not water but nitric acid trihydrate (NAT) clouds

Polar stratospheric clouds occur in Arctic too but are nowhere as extensive as in

Polar stratospheric clouds occur in Arctic too but are nowhere as extensive as in Antarctica

No Arctic ozone depletion last year http: //ozonewatch. gsfc. nasa. gov/

No Arctic ozone depletion last year http: //ozonewatch. gsfc. nasa. gov/

Large Arctic ozone depletion this year http: //ozonewatch. gsfc. nasa. gov/

Large Arctic ozone depletion this year http: //ozonewatch. gsfc. nasa. gov/

Large year-to-year variability in Arctic ozone depletion http: //ozonewatch. gsfc. nasa. gov/

Large year-to-year variability in Arctic ozone depletion http: //ozonewatch. gsfc. nasa. gov/

March polar cap average Only three years (1997, 2011, 2020) have seen large ozone

March polar cap average Only three years (1997, 2011, 2020) have seen large ozone depletion – all associated with strong polar vortices and hence low temperatures Paul Newman, NASA

Year-to-year variability in Arctic ozone depletion is driven by temperature Paul Newman, NASA

Year-to-year variability in Arctic ozone depletion is driven by temperature Paul Newman, NASA

Even in cold winter, ozone depletion in Arctic is much less than in Antarctic

Even in cold winter, ozone depletion in Arctic is much less than in Antarctic WMO [2018]

What about future? CO 2 warms surface but cools stratosphere Solar UV absorption emission

What about future? CO 2 warms surface but cools stratosphere Solar UV absorption emission O 3 CO 2 stratosphere tropopause Troposphere (TT) Terrestrial IR IPCC [2014]

Global distribution of ozone WMO [2018]

Global distribution of ozone WMO [2018]

Global perspective on ozone depletion Pinatubo eruption WMO [2018]

Global perspective on ozone depletion Pinatubo eruption WMO [2018]

Montreal Protocol (1987 -1996): global ban of CFC production …but CFCs have long lifetimes

Montreal Protocol (1987 -1996): global ban of CFC production …but CFCs have long lifetimes in the atmosphere so ozone hole will remain for decades Atmospheric CFC concentrations (NOAA ESRL data)

There is tentative evidence that Antarctic ozone recovery has begun http: //ozonewatch. gsfc. nasa.

There is tentative evidence that Antarctic ozone recovery has begun http: //ozonewatch. gsfc. nasa. gov/

Montreal Protocol as model for successful global environmental action Montreal Protocol and its amendments

Montreal Protocol as model for successful global environmental action Montreal Protocol and its amendments have reversed the stratospheric chlorine trend Ozone recovery may take another decade to confirm beause of meteorological year-to-year variability WMO [2014]

Montreal Protocol avoided the Arctic ozone hole WMO [2018]

Montreal Protocol avoided the Arctic ozone hole WMO [2018]