Chapter 8: Stratospheric chemistry

The ozone layer protects us from UV radiation

Earth surface

Ozone layer

UV

Visible

Ozone absorbs UV radiation

while letting visible radiation through

A brief history of ozone discovery

1839: Schonbein discovers a smell from electrolysis of water, calls it ozone (Greek: to smell)

1865: Soret establishes that ozone is O₃

1913: Fabry discovers atmospheric ozone at high altitude by observing solar absorption spectrum

1927: Gotz infers vertical distribution of ozone by observing solar radiation near the horizon: peak at 25 km

1930: Dobson maps global distribution of ozone in stratosphere

1931: Piccard does first manned ascent into stratosphere

1934: Regener measures ozone vertical profile up to 34 km from hydrogen balloon

Dobson’s global observations of the ozone column

Spectrograph at Earth’s surface measures solar radiation at two neighboring UV wavelengths; one absorbs ozone and other doesn’t

λ₁

λ₂

Ozone layer

Ozone

absorption

spectrum

λ₁

λ₂

Sun (I_∞)

Surface (I_o)

Beer’s law:

from which we get the ozone column:

θ

Total atmospheric optical depth at wavelength λ:

Assume

Dobson’s observations (1930)

Data show thickest ozone columns at high latitudes in spring, thinnest in tropics

Ozone columns reported in units of 0.01 mm pure ozone at 1 atm and 0^oC (STP),

now known as Dobson unit (DU): 1 DU = 2.69x10¹⁶ molecules cm^-2

​

Present-day satellite observations of ozone column follow Dobson’s method

Satellite ozone column data (March 24)

https://ozoneaq.gsfc.nasa.gov/data/ozone

Chapman mechanism for stratospheric ozone (1930)

Sydney Chapman

(1888-1970)

Energy states of the O atom (1s²2s²2p⁴)

multiplicity

total electronic

orbital angular

momentum number

Multiplicity = 2S+1, where S is the spin. The spin of an electron is ±1/2.

Hund’s Rule: lowest-lying energy state is the one of highest multiplicity

Energy

O(¹ S)

O(¹D)

O(³P)

determined by the arrangement of the four electrons in the 2p orbitals

Solar spectrum and absorption cross-sections

O₂+hv

O₃+hv

O(¹D)

O(³P)

Ozone photolysis produces the excited state O(¹D), �which then relaxes thermally to the ground state O(³P)

• But formation of the O(¹D) intermediate is important to recognize because a small fraction of O(¹D) atoms can oxidize other chemicals rather than thermally relax to O(³P)

​

• For now we will only focus on the net reaction, which describes the dominant pathway

• Spectroscopic notation is generally omitted for ground state:

we write O instead of O(³P)

Chapman mechanism for stratospheric ozone (1930)

Sydney Chapman

(1888-1970)

Ground state:

O ≡ O(³P)

O O₃

O₂

R2

R3

R4

R1

Steady-state analysis of Chapman mechanism

Odd oxygen family [O_x] ≡ [O₃] + [O]

In atmospheric chemistry, a “chemical family” is simply an accounting device.

Here [O] << [O₃], so [O₃] ≈ [O_x]. The budget of ozone is actually that of odd oxygen:

ozone is effectively produced by (R1) and lost by (R4)

Steady-state analysis of Chapman mechanism

Steady state for O atoms:

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

Lifetime of O_x:

Steady state for O_x:

τ_Ox

O_x

Strong dependence of photolysis rate constants on altitude

k₁(z) and k₃(z) depend on overhead columns of O₃ and O₂

Strong attenuation of photolysis rate constants k₁ and k₃ by ozone overhead; steady-state [O₃] must be computed numerically starting from top of atmosphere

-3

Chapman mechanism reproduces shape, but is too high by factor 2-3 🢫 missing sink!

log n_O2

Lola Deming (1936)

Compatibility of Chapman mechanism �with high-latitude ozone maximum

Ozone number density, 10¹² molecules cm^-3

P > L

P > L

Brewer-Dobson circulation: ozone is produced as air rises in equatorial stratosphere, accumulates as air travels to poles, strongest in winter and in northern hemisphere

Energy from breaking gravity waves drives meridional circulation in stratosphere

P > L

Alan Brewer (1949)

Vertical profiling of ozone layer with ozonesondes starting in late 1950s

• Ozone is maximum at about 20 km altitude
• Concentrations are highest in polar spring, lowest in the tropics

Ozone number density, 10¹² molecules cm^-3

Planning Committee,

International Geophysical Year (1958)

Photolysis of water vapor drives fast catalytic ozone loss above 70 km, �explaining low ozone levels in mesosphere (Bates and Nicolet, 1950)

Initiation:

Propagation:

Termination:

H OH

H₂O

slow

slow

fast

HO_x radical family:

HO_x ≡ OH + H

Bates Nicolet

In stratosphere, water vapor is oxidized by O(¹D)…�

Initiation:

Propagation:

Termination:

OH HO₂

H₂O

slow

slow

fast

HO_x radical family:

HO_x ≡ H + OH + HO₂

hydroperoxy radical

But kinetic data in the 1960s found that it was too slow to explain the missing O₃ sink

Supersonic stratospheric aircraft (SSTs) and their effect on ozone (Johnston, 1971)

Harold Johnston

Ozone loss catalyzed by nitrogen oxide radicals (NO_x)

Initiation: conversion of air to NO radicals

in combustion engine

Propagation: cycling of NO_x radicals (NO_x ≡ NO + NO₂)

R6 is rate-limiting step

for catalytic ozone loss:

-d[O₃]/dt = 2k₆[NO₂][O]

Termination: oxidation of NO_x to HNO₃ (nitric acid)

Recycling: conversion of HNO₃ back to NO_x

HNO₃ is called a ‘reservoir’ for NO_x

OH is a very strong oxidant

(we will encounter it often)

NO ≡ nitric oxide

NO₂ ≡ nitrogen dioxide

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 NO₂):

Replace:

Computing the NO_x-catalyzed ozone loss rate

tropopause

1. NO_x emission rate E

​

• [NO_x]/[NO_y] ratio (< 0.1)

​

• [NO₂]/[NO_x] ratio (≈ 1)

​

• Ozone loss rate given by (R6)

​

• Residence time τ of air in stratosphere (≈1 year)

Loss rate depends on:

Typically [NO_x]/[NO_y] < 0.1; importance of reservoirs!

Total nitrogen oxides

Conserved in stratosphere

Natural source of stratospheric NO_x from nitrous oxide (Crutzen, 1970)

Source: nitrogen cycling in biosphere

Sinks: photolysis, oxidation by O(¹D)

τ_N2O = 120 years

90%

5%

5%

Paul Crutzen

Stratospheric ozone budget �inferred from 1980s space shuttle observations

Salawitch et al. [1989]

Ross Salawitch

NO_x catalysis is the main ozone sink, closes the stratospheric ozone budget

Rise in atmospheric N₂O �driven by agriculture

IPCC [2022]; Global Carbon Budget [2024]

Atmospheric lifetime of 120 years

Chlorine chemistry as driver for stratospheric ozone loss

Initiation: volcanic injection

Propagation: cycling of ClO_x radicals (ClO_x ≡ Cl + ClO)

R5 is rate-limiting step

for catalytic ozone loss

Termination: conversion of ClO_x to HCl and ClNO₃ reservoirs

Recycling: conversion of HCl and ClNO₃ back to ClO_x

HCl ≡ hydrogen chloride

ClNO₃ ≡ chlorine nitrate

NO₃ ≡ nitrate radical

Cicerone and Stolarski (1973)

Cicerone Stolarski

Molina and Rowland (1974): �CFCs as anthropogenic sources �of stratospheric chlorine

Sherry Rowland

Mario

Molina

Rapid growth of CFCs �in the 1960s and 70s

(ppb)

• Industrial gases valued for their inertness…but they photolyze in stratosphere
• Most important are CFC-11 (CFCl₃) and CFC-12 (CF₂Cl₂)
• Large-scale production started in 1950s, growth in 1970s was 10%/year

Ban on CFC use in aerosol spray cans

Montreal protocol

Computing the ClO_x–catalyzed ozone loss rate

Follow exactly the same approach as for the NO_x-catalyzed ozone loss rate

Observed chlorine partitioning in stratosphere

WMO [2014]

>90% of Cl_y is locked up in reservoirs (HCl, ClNO₃) rather than radicals (ClO, Cl)

Stratospheric ozone budget �inferred from 1980s space shuttle observations

Salawitch et al. [1989]

Ross Salawitch

Chlorine was a minor ozone sink but rapidly rising, motivated Montreal Protocol (1987) to stabilize CFC production

By the early 1980s, stratospheric ozone was thought to be very well understood, readily measured

NASA TOMS satellite instrument launched in 1979

First satellite instrument to measure atmospheric composition: main purpose was to study stratospheric weather based on variations in ozone

Satellite ozone column data

https://ozoneaq.gsfc.nasa.gov/data/ozone

The Antarctic ozone hole bombshell

Antarctica: the “scientific continent”

40 countries operate bases dedicated to scientific research

Discovery of Antarctic ozone depletion at Halley Bay, Antarctica

First NASA satellite observations of ozone layer

1985: ozone hole first reported

by British Antarctic Survey

(Farman et al., 1985)

Ozone layer thickness

(October)

Ozone layer

​

spectrometer

1 Dobson Unit (DU) = 0.01 mm pure ozone = 2.69x10¹⁶ molecules cm^-2

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 gone by December

http://ozonewatch.gsfc.nasa.gov/

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

Strong westerly winds, little meridional transport

Vertical structure of ozone hole: �near-total depletion in lower stratosphere

WMO [2022]

What causes the ozone hole?

Spring 1987 NASA ER-2 mission from Punta Arenas, Chile

Very high ClO in ozone hole

ozone

hole

boundary

Satellite observations of ClO (southern polar view)

ClO increases by an order of magnitude in the ozone hole – reflects inability of the reservoir species (HCl, ClNO₃) to lock up the chlorine

Joe Waters

The standard ClO_x-catalyzed ozone loss mechanism�does not work in Antarctic lower stratosphere in spring

Standard mechanism:

R5 is rate-limiting step

for catalytic ozone loss

-d[O₃]/dt = 2k₂[ClO][O]

But the O atom concentration is determined by

O is very low because hv is low, air density is high R2 is slow

Mechanism for ozone depletion in Antarctic spring�(Molina and Molina, 1989)

Ozone loss rate:

requires high ClO

ClO dimer Cl-O-O-Cl is weakly bound so easily photolyzed

Mario and Luisa Molina

But why is ClO so high in Antarctic spring?�Answer is in polar stratospheric clouds (PSCs)

PSCs forming at very cold temperatures in stratosphere above McMurdo station

Conversion of Cl reservoirs to ClO in polar stratospheric clouds (PSCs)�Solomon et al. (1986)�

Polar stratospheric clouds

over McMurdo, Antarctica

PSC

particle

ClNO₃

HCl

HCl

ClNO₃

ClNO₃

ClNO₃

ClNO₃

ClNO₃

HCl

HCl

HCl

HCl and ClNO₃ stick to PSC particles and then react

Susan

Solomon

PSC formation requires very cold conditions, �found mainly in Antarctic stratosphere in winter

Observed

PSC formation threshold (195 K)

Frost point of water

(188 K)

WMO [2014]

…but PSC formation temperature is much higher than the frost point of water – why?

HOW DO PSCs START FORMING AT 195K?�HNO₃-H₂O PHASE DIAGRAM

​

Antarctic

vortex

conditions

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

• Phase rule: n = c + 2 - p

with c = 2 components (H₂O, HNO₃)

• Maximum of 3 degrees of freedom (p = 1):

p_HNO3, p_H2O, T

• Diagram as shown here assumes at least one condensed phase at equilibrium with the gas (p = 2) and tells us the conditions for that phase to form
• Lose 1 degree of freedom for each additional condensed phase at equilibrium

Summary: chronology of Antarctic ozone hole

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

Large Arctic ozone depletion in 2020…but none since

http://ozonewatch.gsfc.nasa.gov/

Large year-to-year variability in Arctic ozone depletion

http://ozonewatch.gsfc.nasa.gov/

March polar cap average

Paul Newman, NASA

• Only three years (1997, 2011, 2020) have seen large ozone depletion – all associated with strong polar vortices and hence low temperatures
• Still much less than in Antarctica

Stratosphere

CO₂

IPCC [2014]

O₃

Solar UV

absorption

emission

Increasing CO₂ radiates more of the absorbed solar UV

Rising CO₂ warms troposphere but COOLS stratosphere

emission

absorption

T_T

T_S

T_S

T_T < T_S

Global distribution of ozone

WMO [2018]

Global perspective on ozone depletion

Pinatubo eruption

WMO [2018]

Large source of aqueous sulfuric acid aerosol from volcanic eruptions

tropopause

SO₂

sulfur dioxide

H₂SO₄

H₂O

H₂SO₄●H₂O

aqueous

aerosol

Kremser et al. [2016]

N₂O₅ hydrolysis in volcanic aerosols is source of HO_x radicals

N₂O₅(g)

N₂O₅(aq)

NO₃^- + NO₂⁺

2HNO₃(aq)

2HNO₃(g)

drives catalytic ozone loss

Montreal Protocol: originally to level CFC production (1987), later amended to total ban (1999)

…but CFCs have long lifetimes in the atmosphere so ozone hole will remain for decades

Atmospheric CFC concentrations

(NOAA ESRL data)

Τ = 100 years

Τ = 70 years

ODP = ozone depletion potential

Montreal protocol and its subsequent amendments

Original 1987 protocol called only for leveling of CFC emissions;

discovery of CFC’s role in ozone hole led to subsequent amendments for total ban.

But CFCs have a lasting legacy for ozone depletion

WMO [2018]

Chlorine takes a long time to decrease because of the long lifetime of CFCs…

…and therefore the recovery of the ozone layer will take the rest of this century

Latest data confirm that it is still too early to see Antarctic ozone recovery

http://ozonewatch.gsfc.nasa.gov/

Montreal Protocol avoided the Arctic ozone hole

WMO [2018]