Chemistry of the Stratosphere

2/20/2021

Introduction Chemical Introduction Composition A Brief History… • Ozone 10-50 km above the surface Stratospheric Ozone layer • Temperature constant or INCREASING with Chapman Cycle O3 Destruction height Stratospheric O3 Production and Loss • Stable (not a lot of vertical mixing) and dry NOx Hydroxyl Radical • Only occasionally get overshooting cloud Back to our ozone problem tops from convection pushing into this layer Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone • “Oxygen-only chemistry…”

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Introduction Chemical Chemical Composition Composition A Brief History… • Earliest measured components include He, Ar and Ne Ozone Stratospheric Ozone layer Chapman Cycle O3 Destruction Stratospheric O3 Production and Loss [Chackett et al, Nature, 164, 128-129 (1949)] NOx Hydroxyl Radical • Ozone is the main component of the stratosphere Back to our ozone problem • “Ozone Chemistry is Stratospheric Chemistry” Chronology of Antarctic Ozone Hole Policy Solution to The Naked Gun II ozone hole problem "A love affair is like the ozone layer," says Lieut. Frank Drebin. "You Recovery of stratospheric ozone only miss it when it's gone."

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Introduction Chemical A Brief History… Composition A Brief History… • 1840’s Ozone first discovered and measured in air by Schonbein (from Greek Ozone Stratospheric ozein = smell) Ozone layer Chapman Cycle • 1880-1900’s: Hartley postulates the existence of a layer above the O3 Destruction Stratospheric O3 troposphere, where ozone is responsible for the absorption of solar UV Production and Loss between 200 and 300 nm NOx Hydroxyl Radical • 1913: Fabry and Buisson used UV measurements to estimate that if brought Back to our ozone problem down to the surface at STP, O3 would form a layer ~ 3 mm thick Chronology of Antarctic Ozone Hole Policy Solution to • 1920-25: Dobson first shows that T(z) in stratosphere not constant but ozone hole problem Recovery of increases with height (z) and implicates O3 absorption. Makes first extensive stratospheric ozone set of O3 column measurements with his spectrophotometer

• 1930: Chapman proposed that O3 is produced continually in a cycle initiated by O2 photolysis

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Introduction • Absorption of solar radiation by the stratospheric ozone layer Chemical Composition primarily responsible for vertical temperature profile A Brief History… Ozone Stratospheric Ozone layer Chapman Cycle O3 Destruction Stratospheric O3 Production and Loss NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone

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Ozone

3 2/20/2021

Introduction Chemical Composition A Brief History… Ozone • Most O3 (90%) is in the stratosphere Stratospheric Ozone layer • Remaining 10% is in the troposphere Chapman Cycle O3 Destruction • Ozone layer (15 – 30 km) is 3000 – Stratospheric O3 Production and Loss 5000 ppb in O3 NOx Hydroxyl Radical • Ozone at surface of Earth ~ 10 – 50 Back to our ozone problem ppb Chronology of Antarctic Ozone Hole • Sustains terrestrial life by absorbing Policy Solution to ozone hole problem UV radiation Recovery of stratospheric ozone • Impacts the atmospheric vertical temperature profile • Important role in Earth’s overall radiative balance

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Introduction Chemical Stratospheric Ozone layer Composition A Brief History… Ozone • Ozone is naturally produced photochemically via Chapman Cycle Stratospheric Ozone layer Chapman Cycle O3 Destruction Stratospheric O3 <240 nm Production and 푂2 + ℎ휈 푂 + 푂 푗푂2 Loss NOx 푂 + 푂 → 푂 + 푀 푘 Hydroxyl Radical 2 3 푏 Back to our ozone problem <300 nm Chronology of 푂3 + ℎ휈 푂2 + 푂 푗푂3 Antarctic Ozone Hole Policy Solution to 푂 + 푂 → 2푂 푘 ozone hole problem 3 2 4 Recovery of stratospheric ozone

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Introduction Chemical Chapman Cycle Composition Net O formation A Brief History… 3 O2 Ozone h휈 Slow Slow (R4) Stratospheric Ozone layer (j ) (R1) O2 (k ) Chapman Cycle Net O loss 4 O3 Destruction 3 Stratospheric O3 O Production and Odd Oxygen Chemical Loss Family NOx h휈 Hydroxyl Radical O2 Ox = O3 + O Back to our ozone Fast Fast problem Chronology of Antarctic Ozone Hole Policy Solution to O3 ozone hole problem Recovery of Mass balance for [O]: stratospheric ozone dO[] =+−−2~jOjOk 0  OO Mk OO       dt OO22332243 small small

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Introduction Chemical Composition A Brief History… Ozone 푑 푂푥 Stratospheric  Mass balance for [O ]: = 2푗 푂 − 2푘 푂 푂 Ozone layer x 푑푡 푂2 2 4 3 Chapman Cycle O3 Destruction  [O3] is controlled by slow net production and loss via O2 + h휈 (R1) and O + Stratospheric O3 Production and O3 (R4) NOT by the fast production and loss of O3 via O + O2 (R2) and O3 + Loss NOx h휈 (R3) Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone

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5 2/20/2021

Introduction Chemical Composition A Brief History… • The Chapman Cycle describes the natural production and loss mechanisms of Ozone Stratospheric O Ozone layer 3 Chapman Cycle • Works well for upper Stratosphere (>40 km), but not for middle/lower O3 Destruction Stratospheric O3 Stratosphere Production and Loss • Something was missing from the cycle! NOx Hydroxyl Radical • Also didn’t explain “localized” O loss over South Pole Back to our ozone 3 problem Chronology of • So, what was going on? Antarctic Ozone Hole Policy Solution to ozone hole problem • Lots of debate over causes, including… Recovery of stratospheric ozone

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Introduction Chemical Stratospheric Ozone Loss Composition A Brief History… Ozone Stratospheric Ozone layer Chapman Cycle O3 Destruction 1986! Stratospheric O3 Loss NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone

• There was also a real scientific debate over the relative roles of chemistry and meteorology …Turns out to be both chemistry and meteorology (of course!), but mainly because the meteorology facilitates the chemistry ☺

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Introduction Chemical Halides Composition A Brief History… Ozone Stratospheric Ozone layer Chapman Cycle O3 Destruction Halides NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone

Mario Molina and F. Sherwood Rowland at UC, Irvine

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Introduction Chemical Composition • XOx = X + XO (X = Cl, Br) A Brief History… Ozone • Main source of Stratospheric halogens include CFCs and methyl bromide (CH3Br) Stratospheric Ozone layer Chapman Cycle O3 Destruction • Initiation: CF2Cl2 + h휈 → CF2Cl + Cl Halides NOx • Propagation: Hydroxyl Radical 푑 푂3 Back to our ozone Cl + O3 → ClO + O2 O loss rate: − = 2[퐶푙푂][푂] problem 3 푑푡 Chronology of ClO + O → Cl + O Antarctic Ozone Hole 2 Policy Solution to ozone hole problem Net O3 + O → 2O2 Recovery of stratospheric ozone • Termination: Recycling:

Cl + CH4 → HCl + CH3 HCl + OH → Cl + H2O

ClO + NO2 + M → ClONO2 + M ClONO2 + h휈 → ClO + NO2

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Location of the Start of Polar Vortex & O Hole Introduction 3 Chemical Composition A Brief History… Ozone Stratospheric Ozone layer Chapman Cycle O3 Destruction Halides NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone • Unfortunately, halogens didn’t give us a complete picture. ClO did not describe the extensive O3 loss in the middle Stratosphere.

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Introduction Chemical Stratospheric O3 Production and Loss Composition A Brief History… • Considering only homogeneous, gas-phase chemistry Ozone Stratospheric Ozone layer Chapman Cycle

O3 Destruction Halides NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone

• NOx is the most important Ox sink throughout the middle Stratosphere (20-40 km) • Although, it sequesters Cl in the upper stratosphere, reducing catalytic O3 destruction • Halogens become important at higher altitudes

Portman, Daniel and Ravishankara, Philos. Trans. R. Soc. Lond. B Biol. Sci., 367(1593), 1256-1264, 2012.

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Introduction Chemical NOx Contribution to O3 Destruction Composition A Brief History… • NO = NO + NO + NO Ozone x 2 3 Stratospheric Ozone layer • In the Stratosphere, N2O is the main source of NOx Chapman Cycle O3 Destruction • N2O has both biogenic and anthropogenic sources, but… Halides NOx • Very long atmospheric lifetime (~120 years) results in efficient transport to Hydroxyl Radical Back to our ozone stratosphere problem Chronology of 1 Antarctic Ozone Hole 푁2푂 + ℎ휈 → 푁2 + 푂 퐷 Policy Solution to 1 ozone hole problem 푁2푂 + 푂 퐷 → 2푁푂 Recovery of stratospheric ozone 1 푁2푂 + 푂 퐷 → 푁2 + 푂2

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Introduction 1 Chemical • Initiation: N2O + O( D) →2NO (Biogenic and Anthropogenic sources!) Composition A Brief History… • Propagation: Ozone Stratospheric Ozone layer NO + O3 → NO2 + O2 Chapman Cycle O3 Destruction NO2 + O → NO + O2 Halides NOx Net: O + O → 2O Hydroxyl Radical 3 2 Back to our ozone problem • Termination: Recycling: Chronology of Antarctic Ozone Hole Policy Solution to NO2 + OH + M → HNO3 + M HNO3 + h → NO2 + OH ozone hole problem Recovery of stratospheric ozone NO3 + NO2 + M → N2O5 + M N2O5 + h →NO2 + NO3

Recall: NO2 captures Cl•, thereby removing it from cycle of catalytic destruction of O3. But, PSCs sequester HNO3, thereby reducing [NO2] and permitting higher [Cl•], which leads to enhanced O3 destruction.

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9 2/20/2021

Introduction Chemical Hydroxyl Radical Composition A Brief History… • HO = H + OH + HO Ozone x 2 Stratospheric 1 Ozone layer • Initiation: H2O + O( D) → 2OH Chapman Cycle O3 Destruction • Propagation: OH + O3 → HO2 + O2 Halides NOx HO + O → OH + 2O Hydroxyl Radical 2 3 2 Back to our ozone problem Net: 2O3 → 3O2 Chronology of Antarctic Ozone Hole • Termination: OH + OH → H O + O (for example) Policy Solution to 2 2 2 ozone hole problem Recovery of stratospheric ozone • So, HOx is also a catalyst for O3 loss, but not the only one…

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Introduction Chemical Back to our ozone problem Composition A Brief History… • Nothing we’ve discussed so far explains the cyclic behavior of the ozone hole Ozone Stratospheric • Why is O3 depletion enhanced in austral Spring Ozone layer Chapman Cycle O3 Destruction Halides • What about the role of PSCs in stratospheric chemistry? NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone

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Introduction Chemical Composition A Brief History… • PSCs form in the cold (T < 70 oC) winter season in the South Pole Ozone Stratospheric Ozone layer • They are composed primarily of nitric acid/water crystals Chapman Cycle • They sequester gas-phase stratospheric NO2 and, O3 Destruction • Provide a surface for heterogeneous chemistry Halides NOx • Recall the catalytic role of Cl atoms in stratospheric O destruction: Hydroxyl Radical 3 Back to our ozone problem Cl + O3 → ClO + O2 Chronology of Antarctic Ozone Hole ClO + O → Cl + O2 Policy Solution to ozone hole problem • Also recall that one possible termination pathway to removing Cl from this Recovery of stratospheric ozone chemistry is

ClO + NO2 + M → ClONO2 + M • ClONO2 is a reservoir species for Cl, trapping it • But…

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Introduction • NO2 can react with OH, forming HNO3, which then adsorbs onto PSCs Chemical • i.e., one of the mechanisms of Cl removal is slowed down Composition A Brief History… Ozone • PSCs provide an active surface for heterogeneous chemistry Stratospheric • In the Stratosphere, Cl can also react with methane (CH ) Ozone layer 4 Chapman Cycle O3 Destruction Cl• + CH4 → HCl(gas) + •CH3 Halides v. slow NOx HCl + ClONO Cl + HNO Hydroxyl Radical (gas) 2 (gas) 2 (g) 3 (g) Back to our ozone But problem ice surface Chronology of v. fast Antarctic Ozone Hole HCl + ClONO Cl + HNO Policy Solution to (adsorbed) 2 (adsorbed) 2 (g) 3 (adsorbed) ozone hole problem Recovery of stratospheric ozone Zondlo et al, Annu. Rev. Phys. Chem., 51, 473-499, 2000.

Ganske et al, J. Geophys. Res., 97(D7), 7651-7656, 1992.

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Chronology of Antarctic Ozone Hole

Introduction Chemical Composition Critical T A Brief History… for PSCs; Ozone T < 197 K Stratospheric Ozone layer Chapman Cycle O3 Destruction Halides NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone weak sunlight returns

Daniel J. Jacob, “Introduction to Atmospheric Chemistry,” Princeton University Press, 1999 (Fig. 10.13)

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Introduction Chemical Policy Solution to ozone hole problem Composition A Brief History… • Great success! Ozone Stratospheric Controlled Start of Complete Ozone layer Chapman Cycle Substance Control Phasing- O3 Destruction Out Halides NOx CFC-11 1999 2010 Hydroxyl Radical Back to our ozone Halon 2002 2010 problem Chronology of CH3Br 2002 2015 Antarctic Ozone Hole Policy Solution to Other 2003 2010 ozone hole problem Recovery of CFCs stratospheric ozone Cl3CCH3 2003 2015

CCl4 2005 2010 HCFC 2013 2030

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Introduction Chemical Recovery of stratospheric ozone Composition A Brief History… Ozone 1979 1989 Stratospheric Ozone layer Chapman Cycle O3 Destruction Halides NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to 2006 2010 ozone hole problem Recovery of stratospheric ozone

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Introduction Chemical Summary Composition A Brief History… Ozone Stratospheric Ozone layer Chapman Cycle O3 Destruction Halides NOx Hydroxyl Radical Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone Summary

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Introduction Chemical Composition A Brief History… Ozone Stratospheric Ozone layer Chapman Cycle O3 Destruction • Chemistry may be the “central science” to describe O depletion in the upper Halides 3 NOx atmosphere, but as is often the case with most questions of environmental Hydroxyl Radical importance, it is not the only player! Back to our ozone problem Chronology of Antarctic Ozone Hole Policy Solution to ozone hole problem Recovery of stratospheric ozone Summary

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Extra Slides

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1. The original Chapman mechanism included a fifth reaction:

O + O + M → O2 + M What would be the effect of this reaction on ozone? Is it more important in the lower or in the upper stratosphere?

2. [O]~109 molec/cm3 in the upper stratosphere and ~ 106 in the lower stratosphere, if the lifetime of O3 w.r.t. transport away from equator is ~ month, is it fair to assume O3 is in a chemical steady state throughout the stratosphere?

-15 3 -16 3 kO+O3 (US) ~ 4x10 cm /molec/s; kO+O3(LS) ~ 3x10 cm /molec/

Questions1. Of the ozone loss mechanisms we have examined so far, can any operate at night?

2. There are several variations possible for NOx and HOx catalyzed O3 destruction cycles. Given that NO3 photolysis can proceed via one of two channels

a) NO3 + hv→ NO2 + O

b) NO3 + hv→ NO + O2

Come up with at least one additional catalytic cycle for NOx.

3. A minor oxidation pathway for NO is

HO2 + NO -> OH + NO2

What is the net effect of this reaction on ozone?

32 32

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Nitrogen Oxide (NOx) Family – move to • NOx = NOTropospheric + NO2 Chemistry topic • NO2 is one of a very few atmospheric molecules that absorb and photolyze in the visible range

λ <398푛푚 • Photolysis occurs with nearly 100% yield below 398 nm 3 푁푂2 + ℎ휈 푁푂 + 푂 푃

• Photolysis rates can be very large!

Roehl et al, J. Phys. Chem., 98:1-5 (1994) • This will become especially important when we talk about Tropospheric chemistry

Volz-Thomas et al, J. Geophys. Res., 101:18613 (1996)

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The Chapman Model • Valid for mid-Stratosphere<240 nm 푂2 + ℎ휈 푂 + 푂 푗푂2

푂 + 푂2 → 푂3 + 푀 푘푏 <300 nm 푂3 + ℎ휈 푂2 + 푂 푗푂3

푂 + 푂3 → 2푂2 푘4

• Underestimates ξO3 in other layers of Stratosphere • Overestimates ξO3 in middle layer by factor of 2

• What’s missing chemically?

<220 푛푚 푁 푂 + ℎ휈 푁 + 푂(1퐷) Role of NO2 x in O3 Destruction2 1 푂 퐷 + 푁2푂 → 푁2 + 푂2 → 2푁푂 From jO3 below 300 nm Reactive NOx

푁푂 + 푂 → 푁푂 + 푂 3 2 2 Rapid interconversion of NO <395 푛푚 and NO2 (i.e., NOx) 푁푂2 + ℎ휈 푁푂 + 푂

1 O( D) can react with both NO2 and O3

푂 + 푂3 → 2푂2

푂 + 푁푂2 → 푁푂 + 푂2

However, even though ξO3 ~ 100 x ξNO2, the rate of reaction of O with NO2 is much faster and reaction of O-atom with NO2 is favored.

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• NO2 andNO NO 2interconvert-NO Interconversion rapidly photolytically and by reaction with O3

푗푁푂2 • j is essentially constant with휏푁푂푥 altitude= 휏푁푂2 1 + NO2 푘푁푂+푂3 푂3 • [O3] decreases with altitude because of decreasing number concentration in air 푝푀 • Concentration ∝ pressure: 푂 휇𝑔 푚−3 = 푖 휉 (푝푝푚) 3 8.314 푇 푂3 • Note that mixing ratio stays constant though • Taking some typical values: -11 3 -1 -1 kOH+NO3 ~ 1 x 10 cm molec s (NO3 + OH → HO2 + NO2) 6 -3 [OH] ~ 10 molec cm ; ξO3 = 50 ppb -1 jNO2 ~ 0.015 s (at Earth’s surface, 298 K, noon)

z (km) T (K) [NO]/[NO2] 휏NOx (days) 0 288 0.72 1.8

10 256 2.6 4.2

50 223 12.6 18.6

• Rate of O3 destruction is a second order reaction:

푂3 + ℎ휈 < 푥푥푥 푛푚 → 푂 + 푂2

푑 O 푅 = − 3 = −푘[푂][푂 ] 푑푡 3

https://www.esrl.noaa.gov/csd/assessments/ozone/2010/twentyquestions/Q9.pdf

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• WhatMain are Questions the species We(gases Seek & particles) To Answer present in Part in the 1 of atmosphere? the Course What are their natural and anthropogenic sources?

• What chemical reactions do these species undergo in the atmosphere?

• What are the products from the atmospheric transformations of these species?

• What effects do the presence of these species and their chemical transformation products have on the atmosphere, climate, and human health?

• Three gases make up ~99% of total atmospheric mass (~5.14 x 1018 kg)

• N2 (78%) – diatomic nitrogen • O2 (21%) – diatomic oxygen • Ar (1%) – argon • These 3 gases are relatively un-reactive and their mean residence times are much longer that the rate of atmospheric mixing • As a result, their concentrations are relatively uniform globally • Water vapor is the next most abundant constituent • Found in the lowest part of the atmosphere • Concentrations are variable • Can be as high as 3% in some areas • Remaining gaseous constituents, trace gases, represent <1% of atmosphere, but play critical role in Earth’s radiative balance

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• DefinedStructure by temperature of Earth’s Atmosphere

Chapman Cycle (responsible for steady-state conc. O3):

Rxn 1: O2 + h (< 240 nm) → 2O

Rxn 2: O2 + O + M → O3 + M + heat

Rxn 3: O3 + O → 2O2

Rxn 4: O3 + h → O + O2

Stable Air – Vertical Mixing Slow Ozone Layer Stratospheric Aerosol Lifetimes ~ 1 – 2 years

Contains ~ 80% of total atmospheric mass Unstable Air – Rapid Vertical Mixing (all weather here)

Most of the H2O (gas) here Contains Most Atmospheric Aerosol Mass (< ~ 10km) Tropospheric Aerosol Lifetimes ~ hours – 2 weeks

Vertical Transport of Air • Exchange of air between the troposphere and stratosphere is considerably slower than mixing of the troposphere because of the temperature inversion in the stratosphere • Air is transported from the troposphere to the stratosphere principally in the tropics.

free troposphere

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• Average Tropopause Heights (defined as the lowest level at which temp lapse rate decreasesConnection to 2 K km of-1: Tropospheric Emissions to Stratosphere • Equator ~ 18 km • Poles ~ 8 km • Tropopause slopes downwards from equator to poles

Source: http://earthobservatory.nasa.gov/Features/Aura/Aura_2.php

How Does Air Circulate Globally? • At equator air is heated and rises and water is evaporated • As air rises it cools producing large amounts of precipitation in equatorial regions • In N.H. air moves north and then sinks around 30° N (where deserts are) • At 40-60°N westerly winds and storms setup

Flaw: No Coriolis Effect Considered More Realistic: Coriolis Effect Considered 44

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https://www.metoffice.gov.uk/learning/atmosphere/global-circulation-patterns

Horizontal Time Scales for Transport of Air

• The Intertropical Convergence Zone (ITCZ) – persistent convergence near equator associated with rain and clouds; location varies by season

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Units of Atmospheric푐푖 푝푖 Concentration • Mixing ratio: 휉푖 = = 푐푡표푡푎푙 푝

• independent of P and T • total pressure includes water vapor, so 휉푖 depends on humidity • convenient units of 휉푖 include: • parts per million (ppm) 10-6 µmol mol-1 (106 molecules of “i” (total molecules)-1) • parts per billion (ppb) 10-9 nmol mol-1 • parts per trillion (pptr) 10-12 pmol mol-1

• sometimes specify “by volume (ppmv)” or “by mass (ppmm)” • if not specified, assume ppmv

Chemical Composition of the Atmosphere

0.039% (fraction = → x106 = 390 ppm) 100

0.000179% (fraction = → x106 = 1.79 ppm) 100

7푥10−6 % (fraction = → x106 = 0.07 ppm) 100 → x109 = 70 ppb

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• Concentration (mol m-3) • depends on pressure and temperature (through IGL) • Mass loading or concentration (µg m-3)

푝푀 휇𝑔 푚−3 = 푖 휉 (푝푝푚) 8.314푇 푖

For example, at STP, 390 ppm CO2 in atmosphere corresponds to

1.013푥105푃푎 44 𝑔 푚표푙−1 푐 = 390 푝푝푚 = 7.7푥105휇𝑔 푚−3 퐶푂2 푚3푃푎 8.314 ൗ푚표푙 퐾 273퐾

So, while CO2 comprises only 0.039% of the atmosphere by volume, its fraction by mass is 0.062%.

• Water vapor is especially important in atmospheric chemistry • Concentrations units include: • volume mixing ratio (ppm)

• mass mixing ratio (g H2O/g air) • specific humidity (g H2O/kg air) 푝퐻2푂 • relative humidity, RH ൗ 0 푝퐻2푂

0 2 3 4 푝퐻2푂 푇 = 1013.25 exp 13.3185푎 − 1.97푎 − 0.6445푎 − 0.1299푎

373.15 푤ℎ푒푟푒 푎 = 1 − 푇