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Chemistry of the Stratosphere

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Chemistry of the Stratosphere 2/20/2021 Chemistry of the Stratosphere 1 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…” 2 2/20/2021 The Stratosphere CHEM 196 Petrucci 2 1 2/20/2021 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." 3 2/20/2021 The Stratosphere-Composition CHEM 196 Petrucci 3 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 4 2/20/2021 The Stratosphere-Composition CHEM 196 Petrucci 4 2 2/20/2021 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 5 2/20/2021 The Stratosphere-Composition CHEM 196 Petrucci 5 Ozone 6 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 7 2/20/2021 The Stratosphere – Ozone CHEM 196 Petrucci 7 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 8 2/20/2021 The Stratosphere – Ozone CHEM 196 Petrucci 8 4 2/20/2021 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[] =2jO + jO − kOOMkOO − ~ 0 dt OO2 2 3 3 2 2 4 3 small small 9 2/20/2021 The Stratosphere – Ozone Production CHEM 196 Petrucci 9 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 10 2/20/2021 The Stratosphere – Ozone Production CHEM 196 Petrucci 10 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 11 2/20/2021 The Stratosphere – Ozone Production CHEM 196 Petrucci 11 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 ☺ 12 2/20/2021 The Stratosphere – Ozone Destruction CHEM 196 Petrucci 12 6 2/20/2021 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 13 2/20/2021 The Stratosphere – Ozone Destruction CHEM 196 Petrucci 13 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 14 2/20/2021 The Stratosphere – Ozone Destruction CHEM 196 Petrucci 14 7 2/20/2021 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. 15 2/20/2021 The Stratosphere – Ozone Destruction CHEM 196 Petrucci 15 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. 17 2/20/2021 The Stratosphere – Ozone Destruction CHEM 196 Petrucci 17 8 2/20/2021 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
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