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Chemistry of the Troposphere

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Introduction Photochemical Smog Introduction Nitrogen Oxide Chemistry Reservoir species Tropospheric ozone Ozone photochemistry Atmospheric OH radicals Reduced oxidation oxidized HOx cycle in the atmospheric atmospheric troposphere Other tropospheric species species reservoirs of HOx CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” emission Deposition/uptake Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Earth’s surface Nighttime Oxidation O3 Oxidation of VOCs

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Introduction Photochemical Smog Nitrogen Oxide Chemistry • John Evelyn, English author and founding member of the Royal Society, published Fumigufium in 1661, Reservoir species Tropospheric ozone describing London smog and offering suggestions for its mitigation Ozone photochemistry “…a cloud of sea-coal, as if there be a semblance of hell upon Earth…” Atmospheric OH radicals • Haagen-Smit, Ind. Eng. Chem. 44:1342 (1952): Smog from photochemical oxidation of hydrocarbons in the HOx cycle in the presence of NOx; description of ozone, aerosol pollution: troposphere Other tropospheric “Photochemical and other reactions change normally harmless compounds into objectionable ones. On the reservoirs of HOx CH4 oxidation other hand, substances irritating when released may soon be converted into harmless ones. A proper ORVOCs evaluation of the contribution of air pollutants to the smog nuisance must include not only the time and OH Oxidation of VOCs “Bipolar Ozone” place of their emission, but also their fate in the air.” Overview of Tropospheric O3 • Haagen-Smit, Ind. Eng. Chem. 45:2086 (1953): Ozone from HCs and NOx Formation Mitigation of “The release of large quantities of hydrocarbons to the air and the simultaneous presence of nitrogen oxides Tropospheric O3 from combustion processes explains the relatively high ozone content.” Nighttime Oxidation O3 Oxidation of VOCs

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Introduction Photochemical Smog Nitrogen Oxide Chemistry “To study those reactions further a fumigation room was built from Reservoir species Tropospheric ozone Plexiglas…Observers sensitive to eye irritation on smog days compared Ozone photochemistry their reaction in these fumigations with those experienced under conditions Atmospheric OH radicals of natural smog. ” HOx cycle in the troposphere Other tropospheric reservoirs of HOx CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

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Introduction Photochemical Smog Nitrogen Oxide Chemistry • Cadle and Allen, Science 167:3916 (1970): Troposphere is relatively inert Reservoir species Tropospheric ozone (really!?!); only photolysis or reactions with O or O2 matter Ozone photochemistry “The chemistry of the troposphere is mainly that of a large number of atmospheric Atmospheric OH radicals constituents and their reactions with molecular oxygen.” HOx cycle in the troposphere • Other tropospheric Robbins and Robbins, “Sources, Abundance, and Fate of Gaseous reservoirs of HOx Atmospheric Pollutants”, SRI report, 1967: Lifetime of CO estimated at CH4 oxidation ORVOCs 2.7 years (loss by soil) OH Oxidation of VOCs “Bipolar Ozone” 14 Overview of • Weinstock, Science 166:224 (1969): CO measurements → lifetime of Tropospheric O3 Formation 0.1 years (!), loss by reaction with OH Mitigation of Tropospheric O3 Nighttime Oxidation • Levi, Science 173:141 (1971): OH generated from ozone photolysis O3 Oxidation of VOCs

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Introduction Photochemical Smog Photochemical Smog Nitrogen Oxide Chemistry Reservoir species • Contains elevated levels of oxidants and carbon-containing reaction products Tropospheric ozone Ozone • Requires presence of both unburned hydrocarbons and nitrogen oxides (NO ) photochemistry x Atmospheric OH radicals • Requires specific climatic conditions and stable atmosphere HOx cycle in the troposphere Other tropospheric • Both thermal and photochemical reactions contribute to smog reservoirs of HOx • Aggravated by higher temperatures and solar spectral irradiance at surface CH4 oxidation ORVOCs OH Oxidation of VOCs • Favored when there is a temperature inversion (i.e., cooler, denser air remains “Bipolar Ozone” Overview of close to the surface preventing mixing/diluting of reactants) Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

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Introduction Photochemical Smog Solar Spectral Irradiance Nitrogen Oxide Chemistry Reservoir species • Recall that wavelengths below ~350 are Tropospheric ozone Ozone absorbed by stratosphere photochemistry Atmospheric OH • Mainly visible wavelengths are radicals HOx cycle in the transmitted by the troposphere troposphere Other tropospheric reservoirs of HOx • Wavelengths correspond to bond energies CH4 oxidation of 4 x 10-19J or 240 kJ mol-1 (@ 500 nm) ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

2x10-18J 2x10-20J

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Introduction Photochemical Smog Smog Timeline Nitrogen Oxide Chemistry Reservoir species • A typical timeline for a photochemical Tropospheric ozone Ozone smog event includes: photochemistry • Elevated concentrations of VOCs and NO Atmospheric OH radicals • NO rapidly oxidized to NO2 HOx cycle in the troposphere • Solar flux increases during day Other tropospheric reservoirs of HOx • NO2 is photolyzed to NO + O CH4 oxidation • NO and NO react with VOCs to form OH and ORVOCs 2 OH Oxidation of VOCs other, organic radicals “Bipolar Ozone” Overview of • OH and other radicals react (mainly with VOCs) to Tropospheric O3 form smog (elevated O3 and aerosols) Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

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Introduction Photochemical Smog Nitrogen Dioxide Nitrogen Oxide Chemistry • One of a very few atmospheric molecules that absorb and photolyze in the visible Reservoir species Tropospheric ozone range Ozone λ <398푛푚 3 photochemistry 푁푂2 + ℎ휈 푁푂 + 푂 푃 Atmospheric OH radicals HOx cycle in the troposphere • Absorption cross-section (σNO2) is structured and has non-trivial dependence on T, P Other tropospheric reservoirs of HOx CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

• NO2 contributes to the brown color of air in very polluted cities • …but color due mostly to aerosols!

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Introduction Photochemical Smog Daytime Photochemistry Nitrogen Oxide Chemistry • Photolysis occurs with nearly 100% • Photolysis rates can be very large! Reservoir species Tropospheric ozone yield below 398 nm Ozone photochemistry Atmospheric OH radicals HOx cycle in the troposphere Other tropospheric reservoirs of HOx CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs Roehl et al, J. Phys. Chem., 98:1-5 (1994) Volz-Thomas et al, J. Geophys. Res., 101:18613 (1996)

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• NO and NO interconvert rapidly photolytically and by reaction with O Introduction 2 3 Photochemical Smog 푗 Nitrogen Oxide 푁푂2 Chemistry 휏푁푂푥 = 휏푁푂2 1 + 푘푁푂+푂3 푂3 Reservoir species • jNO2 is essentially constant with altitude Tropospheric ozone Ozone • [O3] decreases with altitude because of decreasing number concentration in air 푝푀 photochemistry • Concentration ∝ pressure: 푂 휇푔 푚−3 = 푖 휉 (푝푝푚) Atmospheric OH 3 8.314 푇 푂3 radicals • Note that mixing ratio stays constant though HOx cycle in the troposphere • Taking some typical values: Other tropospheric -11 3 -1 -1 kOH+NO3 ~ 1 x 10 cm molec s (NO3 + OH → HO2 + NO2) reservoirs of HOx 6 -3 CH4 oxidation [OH] ~ 10 molec cm ; ξO3 = 50 ppb ORVOCs -1 jNO2 ~ 0.015 s (at Earth’s surface, 298 K, noon) OH Oxidation of VOCs “Bipolar Ozone” z (km) T (K) [NO]/[NO2] 휏NOx (days) Overview of Tropospheric O3 0 288 0.72 1.8 Formation Mitigation of 10 256 2.6 4.2 Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs 50 223 12.6 18.6

• NO2 and NO interconvert rapidly photolytically and by reaction with O3 • Consider as “one chemical family”: NOx • Other important chemical families include HOx and SOx

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M Introduction Photochemical Smog NOx and NOy families Nitrogen Oxide OH Chemistry

Reservoir species HNO3 Tropospheric ozone Ozone photochemistry • Nighttime behavior Atmospheric OH radicals • No photolysis of NO2 HOx cycle in the troposphere • Any NO present reacts rapidly with O3, so almost all NOx is converted to NO2 Other tropospheric -13 3 -1 -1 • NO2 then reacts with O3 to form NO3 (k = 1.2 x 10 exp(-2450/T) cm molec s reservoirs of HOx CH4 oxidation • Only direct source of NO3 radical in the atmosphere ORVOCs • NO + NO combine to form N O OH Oxidation of VOCs 3 2 2 5 “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 푁2푂5 −27 3 −1 Nighttime Oxidation 퐾1,2 = = 3.0푥10 푒푥푝 10,990/푇 푐푚 푚표푙푒푐 O3 Oxidation of VOCs 푁푂2 푁푂3

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Introduction Photochemical Smog Lifetime of NO3 Nitrogen Oxide Chemistry • N O can react heterogeneously (with surface water) to form HNO thermally, thereby Reservoir species 2 5 3 Tropospheric ozone sinking NOx Ozone 푘3 photochemistry 푁2푂5 + 퐻2푂 2퐻푁푂3 Atmospheric OH radicals HOx cycle in the • Only other sink for NOx is OH + NO2 troposphere Other tropospheric • These two sinks dominate lifetime of NO3 reservoirs of HOx CH4 oxidation ORVOCs 1 1 OH Oxidation of VOCs 휏푁푂3 = 1 + “Bipolar Ozone” 푘3 퐾1,2 푁푂2 Overview of Tropospheric O3 Formation 3 -1 Mitigation of z (km) T (K) K1,2 (cm molec ) τNO3 (h) Tropospheric O3 Nighttime Oxidation 0 288 1.1 x 10-10 1.1 O3 Oxidation of VOCs 10 256 1.3 x 10-8 0.93

50 223 7.6 x 10-6 0.93

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Introduction Photochemical Smog Global NO2 Nitrogen Oxide Chemistry Reservoir species Tropospheric ozone Ozone photochemistry Atmospheric OH radicals HOx cycle in the troposphere Other tropospheric reservoirs of HOx CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

http://www.temis.nl/airpollution/no2.html From SCIAMACHY (SCanning Imaging Absorpton spectroMeter for Atmospheric CartograpHY

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Introduction Photochemical Smog PAN: Tropospheric reservoir of NOx Nitrogen Oxide Chemistry Tropospheric ozone • Peroxyacetyl Nitrates (PANs) Ozone photochemistry Atmospheric OH radicals HOx cycle in the troposphere Other tropospheric reservoirs of HOx CH4 oxidation ORVOCs • Loss mechanisms include OH Oxidation of VOCs “Bipolar Ozone” • Thermal decomposition important below 7 km altitude (few hrs) Overview of Tropospheric O3 • Photodecomposition important above 7 km (2 weeks) Formation Mitigation of • PAN is easily transported into the upper troposphere (where lifetime is long), Tropospheric O3 Nighttime Oxidation resulting in long range transport and global distribution O3 Oxidation of VOCs • Result: NOx (and its chemistry) is not associated only with polluted areas

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Introduction Photochemical Smog Tropospheric ozone Nitrogen Oxide Chemistry Tropospheric ozone • Only significant source is photochemistry of NO2 Ozone 1 photochemistry 3 푁푂2 + ℎ휈 푁푂 + 푂 푃 Atmospheric OH 2 radicals 푂 3푃 + 푂 + 푀 푂 + 푀 HOx cycle in the 2 3 troposphere Other tropospheric reservoirs of HOx CH4 oxidation • Once formed, O3 can be consumed by reaction with NO to reform NO2 ORVOCs OH Oxidation of VOCs 3 “Bipolar Ozone” 푁푂 + 푂3 푂2 + 푁푂2 Overview of Tropospheric O3 Formation • Note that net result is production of O3 Mitigation of Tropospheric O3 • For example, if [O3]0 = [NO]0 = 0 Nighttime Oxidation O3 Oxidation of VOCs

[NO2]0, ppb [O3], ppb 100 27 1000 95

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Ozone photochemistry Introduction Photochemical Smog Nitrogen Oxide 1푎 3 3 Chemistry 푂3 + ℎ휈 푂2 + 푂 푃 휙 푃 ≈ 10% 푏푒푙표푤 305 푛푚 Tropospheric ozone Atmospheric OH 1푏 radicals 푂 +푂 1퐷 휙 1퐷 ≈ 90% 푏푒푙표푤 305 푛푚 HOx cycle in the 2 troposphere 3 Other tropospheric 푂 1퐷 + 푀 푂 3푃 + 푀 퐶표푙푙푖푠푖표푛푎푙 푞푢푒푛푐ℎ푖푛푔 reservoirs of HOx CH4 oxidation 2 ORVOCs 푂 3푃 + 푂 + 푀 푂 + 푀 OH Oxidation of VOCs 2 3 “Bipolar Ozone” Overview of Tropospheric O3 Formation 1 4 Mitigation of 푂 퐷 + 퐻2푂 2푂퐻 휙 푂퐻 ≈ 10% 푎푡 푠푢푟푓푎푐푒 Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

1 λthreshold( D) ~ 310 nm

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Atmospheric OH radicals Introduction Photochemical Smog 1 Nitrogen Oxide • 푂 퐷 is sufficiently reactive that we can assume pseudo-steady-state Chemistry Tropospheric ozone Atmospheric OH 1 푗 푂3 = 푘3 푀 + 푘4 퐻2푂 푂 퐷 At SS: production = consumption radicals 푂3՜푂 1퐷 HOx cycle in the troposphere Other tropospheric reservoirs of HOx and CH4 oxidation 푗 푂3 푂3՜푂 1퐷 Recall: ORVOCs 푂 1퐷 = OH Oxidation of VOCs 푘 푀 + 푘 퐻 푂 1푎 “Bipolar Ozone” 3 4 2 푂 + ℎ휈 푂 + 푂 1퐷 Overview of 3 2 Tropospheric O3 4 Formation 1 Mitigation of At 298K, the rate coefficient values are 푂 퐷 + 퐻2푂 2푂퐻 Tropospheric O3 -11 3 -1 -1 Nighttime Oxidation k3 (M=O2)4.0 x 10 cm molecule s O3 Oxidation of VOCs -11 3 -1 -1 k3 (M=N2)2.6 x 10 cm molecule s k4 = 2.2 x 10-10 cm3 molecule-1 s-1

-11 3 -1 -1 For the atmospheric N2/O2 ratio, k3,net = 2.9 x 10 cm molecule s

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Introduction Photochemical Smog Nitrogen Oxide The photolysis rate constant (푗 ) at the Earth’s surface and a solar zenith Chemistry 푂3՜푂 1퐷 Tropospheric ozone 0 -5 -1 Atmospheric OH angle (θ ) of zero degrees is 6 x 10 s . radicals HOx cycle in the troposphere We can re-write the rate equation in terms of mixing ratios of O and H O: Other tropospheric 3 2 reservoirs of HOx CH4 oxidation 푗 1 휉푂3 푀 ORVOCs 1 푂3՜푂 퐷 OH Oxidation of VOCs 푂 퐷 = “Bipolar Ozone” 푘3 푀 + 푘4휉퐻2푂 푀 Overview of Tropospheric O3 Formation 푗 휉 1 푂3 Mitigation of 푂3 푂 퐷 Tropospheric O3 = Nighttime Oxidation 푘3+푘4휉퐻2푂 O3 Oxidation of VOCs • Under typical conditions at mid-latitudes,

• ξO3 ≅ 50 ppb 푝퐻2푂 0 o • ξH2O ≅ = RH x 푝퐻2푂 = 0.0083 at 50% RH and 15 C (0.015 at 90% RH) 푝푎푡푚 • Even at high RH then, the 2nd term in the denominator is <10x the 1st term, so can be neglected

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Introduction Photochemical Smog Nitrogen Oxide • …and finally…the production rate of OH (POH) from O3 photolysis is Chemistry 푗 1 푘4 Tropospheric ozone 푂3՜푂 퐷 퐻2푂 Atmospheric OH 푃푂퐻 = 2 푂3 radicals 푘3 푀 HOx cycle in the troposphere 1 Other tropospheric • How important is quenching of O( D) (by collisions with M) compared to reaction with reservoirs of HOx H2O to form OH radical? CH4 oxidation 2푘 휉 ORVOCs 4 퐻2푂 ∈푂퐻= OH Oxidation of VOCs 푘3 “Bipolar Ozone” Overview of RH (%) 10 25 50 80 Tropospheric O3 Formation Mitigation of ∈푂퐻 0.047 0.12 0.23 0.38 Tropospheric O3 Nighttime Oxidation 1 O3 Oxidation of VOCs • At 80% RH, approx. 40% of the O( D) formed leads to OH radicals!

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HO cycle in the troposphere Introduction x Photochemical Smog Nitrogen Oxide Chemistry Tropospheric ozone Atmospheric OH radicals HOx cycle in the troposphere Other tropospheric reservoirs of HOx CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

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Other tropospheric reservoirs of HO Introduction x Photochemical Smog Nitrogen Oxide • HO2: H2O2, CH3OOH, hydroperoxides (ROOH, HOOH), acetone Chemistry Tropospheric ozone Atmospheric OH • OH: HONO (through photolysis) radicals HOx cycle in the troposphere CH4 oxidation ORVOCs Primary tropospheric sink of OH radicals is reaction with CO and CH4. OH Oxidation of VOCs

“Bipolar Ozone” Overview of Tropospheric O3 Formation CO Mitigation of CH4 Tropospheric O3 H2 Nighttime Oxidation O3 Oxidation of VOCs O3 9%

HCHO

81% 2% 5% 3%

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CH oxidation Introduction 4 Photochemical Smog Nitrogen Oxide • Most abundant atmospheric hydrocarbon and GHG Chemistry Tropospheric ozone Atmospheric OH • Only significant removal pathway is through reaction with OH radical radicals HOx cycle in the troposphere CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

• Net loss of radicals by reaction: CH3O2H + H2O → deposition

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ORVOCs Introduction Photochemical Smog Nitrogen Oxide • Other Reactive Volatile Organic Compounds in the atmosphere Chemistry Tropospheric ozone Atmospheric OH • Although minor components, they play outsized role in aerosol production radicals HOx cycle in the • Biogenic and anthropogenic sources troposphere CH4 oxidation ORVOCs • Unlike CH4, also highly reactive with O3 and other atmospheric oxidants OH Oxidation of VOCs “Bipolar Ozone” Overview of • OH and O3 oxidation occurs by very different pathways. Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

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OH Oxidation of VOCs Introduction Photochemical Smog Nitrogen Oxide • ALL VOCs are subject to OH oxidation Chemistry Tropospheric ozone Atmospheric OH • General reaction steps include: radicals HOx cycle in the troposphere CH4 oxidation ORVOCs • Initiation involves a hydrogen abstraction, leaving a carbon-centered OH Oxidation of VOCs “Bipolar Ozone” radical Overview of Tropospheric O3 • Molecular oxygen adds to the radical, creating an alkylperoxy radical Formation Mitigation of • Alkylperoxy radical transfers O-atom to NO molecule Tropospheric O3 Nighttime Oxidation • Hydroperoxyl radical and aldehyde products result O3 Oxidation of VOCs • Aldehydes react with NO2 to form PANs • PAN decomposition products propagate chain reaction

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“Bipolar Ozone” Introduction Photochemical Smog Nitrogen Oxide • In the stratosphere, ozone is important in blocking harmful UV radiation that Chemistry Tropospheric ozone would otherwise reach the Earth’s surface (“Good Ozone”) Atmospheric OH radicals HOx cycle in the • In the troposphere, ozone is considered a pollutant and is detrimental to human troposphere CH4 oxidation health (“Bad Ozone”) ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of • Tropospheric ozone is a secondary product of emissions from fossil fuel Tropospheric O3 Formation combustion Mitigation of Tropospheric O3 Nighttime Oxidation 퐻푂2 O3 Oxidation of VOCs hν ,푂2 NO + 푅퐶퐻2푂2 ՜ 푁푂2 푁푂 + 푂3 푅퐶푂2

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Overview of Tropospheric O3 Formation

Introduction • Photochemistry in polluted planetary boundary layers Photochemical Smog Nitrogen Oxide Chemistry Tropospheric ozone Atmospheric OH radicals HOx cycle in the troposphere CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Overview of Tropospheric O3 Formation Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

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Mitigation of Tropospheric O Introduction 3 Photochemical Smog Nitrogen Oxide • Dependence of Ozone Production on NOx and VOCs Chemistry Tropospheric ozone Atmospheric OH radicals HOx cycle in the troposphere CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

VOC-limited regime NOx-limited regime

2푘4푃퐻푂푥 푅퐻 푃퐻푂푥 푃푂3 = 푃푂3 = 2푘7 푁푂 푘9 푁푂2 푀 2푘8

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Introduction Photochemical Smog Nitrogen Oxide • 푃푂 ends up being a complex function of 푁푂 and 푉푂퐶 Chemistry 3 Tropospheric ozone Atmospheric OH • Ozone isopleths for a typical urban airshed radicals HOx cycle in the troposphere CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

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Nighttime Oxidation Introduction Photochemical Smog Nitrogen Oxide • In the absence of sunlight (i.e., no photolysis of NO2): Chemistry Tropospheric ozone Atmospheric OH 푁푂 + 푂 ՜ 푁푂 + 푂 radicals 2 3 3 2 HOx cycle in the troposphere CH4 oxidation ORVOCs • NO3 reacts fast with NO and NO2, which limits NO3 concentrations OH Oxidation of VOCs “Bipolar Ozone” Mitigation of • NO3 is a strong oxidant reacting with some organic compounds in a Tropospheric O3 somewhat similar way as OH radicals. Nighttime Oxidation O3 Oxidation of VOCs • NO3 only reacts fast with specific compounds, which is different to OH.

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O Oxidation of VOCs Introduction 3 Photochemical Smog Nitrogen Oxide • O3 oxidation most important for unsaturated VOCs (i.e., containing at least Chemistry Tropospheric ozone one double/triple bond) Atmospheric OH radicals • Most abundant class of unsaturated compounds in our atmosphere is terpenes HOx cycle in the troposphere CH4 oxidation ORVOCs OH Oxidation of VOCs • General reaction steps include: “Bipolar Ozone” Mitigation of Tropospheric O3 Nighttime Oxidation • Addition of O across double bond to form primary ozonide O3 Oxidation of VOCs 3 • Decomposition of ozonide to form 2 peroxy Criegee Intermediates and two aldehydes • Reaction of Criegee Intermediates with themselves and other oxygenated products • Rearrangement of Criegee Intermediates to produce carbon centered radical • Continue as in case of OH oxidation

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General Chemical Mechanism Introduction Photochemical Smog Nitrogen Oxide Chemistry Tropospheric ozone Atmospheric OH radicals HOx cycle in the troposphere CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

• What happens to first-generation oxidation products?

• Smaller oxidizable VOCs ultimately become CO2 • Larger oxidizable VOCs lead to atmospheric aerosol

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Introduction Photochemical Smog Nitrogen Oxide Chemistry Tropospheric ozone Atmospheric OH radicals HOx cycle in the troposphere CH4 oxidation ORVOCs OH Oxidation of VOCs “Bipolar Ozone” Mitigation of Tropospheric O3 Nighttime Oxidation O3 Oxidation of VOCs

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