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Chemistry of the Troposphere.Pdf 3/3/2021 Chemistry of the Troposphere 1 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 2 3/3/2021 The Troposphere – Intro CHEM 196 Petrucci 2 1 3/3/2021 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 3 3/3/2021 The Troposphere – Intro CHEM 196 Petrucci 3 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 4 3/3/2021 The Troposphere – Intro CHEM 196 Petrucci 4 2 3/3/2021 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 5 3/3/2021 The Troposphere – Intro CHEM 196 Petrucci 5 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 6 3/3/2021 AddThe Tropospherea footer – Intro CHEM 196 Petrucci 6 3 3/3/2021 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 7 3/3/2021 The Troposphere – Intro CHEM 196 Petrucci 7 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 8 3/3/2021 AddThe Tropospherea footer – Intro CHEM 196 Petrucci 8 4 3/3/2021 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! 9 3/3/2021 The Troposphere – Chemistry CHEM 196 Petrucci 9 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) 10 3/3/2021 The Troposphere – Chemistry CHEM 196 Petrucci 10 5 3/3/2021 • 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 11 3/3/2021 The Troposphere – Chemistry CHEM 196 Petrucci 11 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
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