Tropospheric OH chemistry

Transcription

1 Tropospheric OH chemistry CO Oxidation mechanism: CO + OH CO 2 + H, H + O 2 + M HO 2 + M, HO 2 + NO OH + NO 2 NO 2 + hν (+O 2 ) NO + O 3 Initiation step Propagation Net: CO + 2 O 2 CO 2 + O 3 HO 2 + HO 2 H 2 O 2 + O 2 NO 2 + OH + M HNO 3 + M (irreversible loss of H 2 O 2 and HNO 3 by dry deposition, wet deposition, reaction with OH) Reformation of radicals through photolysis: H 2 O 2 + hν 2 OH HNO 3 + hν OH + NO 2 Termination steps HO x (= OH+HO 2 +H) and NO x (= NO+NO 2 ) catalyze O 3 production In the troposphere 1

2 Methane oxidation cycle (high NO x ) CH 4 + OH CH 3 + H 2 O CH 3 + O 2 + M CH 3 O 2 + M CH 3 O 2 + NO CH 3 O + NO 2 CH 3 O + O 2 CH 2 O + HO 2 HO 2 + NO OH + NO 2 2 (NO 2 + hν NO + O) 2 (O + O 2 + M O 3 + M) Net: CH O hν CH 2 O + 2O 3 + H 2 O O 3 and CH 2 O production Formaldehyde (CH 2 O) itself can photolyze (2 branches), or react with OH: CH 2 O + hν CHO + H (+2O 2 ) CO + 2HO 2 CH 2 O + hν CO + H 2 CH 2 O + OH (+O 2 ) CO + HO 2 + H 2 O, pathway I, pathway II, pathway III Methane oxidation cycle (high NO x ) cont. CH 4 + OH CH 3 + H 2 O CH 3 + O 2 + M CH 3 O 2 + M CH 3 O 2 + NO CH 3 O + NO 2 CH 3 O + O 2 CH 2 O + HO 2 HO 2 + NO OH + NO 2 2 (NO 2 + hν NO + O) 2 (O + O 2 + M O 3 + M) Pathway I: CH 2 O + hν (+2O 2 ) CO + 2HO 2 CO + OH (+O 2 ) HO 2 + CO 2 3( HO 2 + NO OH + NO 2 ) 3 (NO 2 + hν NO + O) 3 (O + O 2 + M O 3 + M) Net: CH O 2 CO 2 + 2OH + 5O 3 + H 2 O O 3 and OH production Pathway I is the most efficient at producing O 3 and OH 2

3 Methane oxidation cycle (no NO x ) CH 4 + OH CH 3 + H 2 O CH 3 + O 2 + M CH 3 O 2 + M CH 3 O 2 + HO 2 CH 3 OOH + O 2 Methylhydroperoxide may either react with OH (2 branches) or photolyze: CH 3 OOH + OH CH 2 O + OH + H 2 O, pathway a CH 3 OOH + OH CH 3 O 2 + H 2 O, pathway b CH 3 OOH + hν CH 3 O+ OH (+O 2 ) CH 2 O + HO 2, pathway c Let s follow pathway a and assume that CH 2 O will react with OH: CH 3 OOH + OH CH 2 O + OH + H 2 O CH 2 O + OH (+O 2 ) HO 2 + CO + H 2 O CO + OH (+O 2 ) HO 2 + CO 2 Net: CH 4 + 3OH + 2O 2 CO 2 + 3H 2 O + HO 2 No O 3 production and 2HO x molecules are lost Methane oxidation cycle - summary Seinfeld J. H. and S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change,

4 Generalized hydrocarbon oxidation cycle RH + OH R + H 2 O R+ O 2 + M RO 2 + M RO 2 + NO RO + NO 2 RO + O 2 R CHO + HO 2 HO 2 + NO OH + NO 2 2 (NO 2 + hν NO + O) 2 (O + O 2 + M O 3 + M) Net: RH + 4 O 2 R CHO + 2O 3 + H 2 O RH = organic hydrocarbon, with R = organic group RO 2 = organic peroxy radical, RO = alkoxy radical R CHO = aldehyde Non-methane hydrocarbon oxidation 4

5 Global emissions of volatile organic compounds (VOCs) ANTHROPOGENIC SOURCES Energy use and transfer Biomass burning Organic solvents ~100 TgC/yr 43 TgC/yr 45 TgC/yr 15 TgC/yr NATURAL SOURCES Emissions from vegetation isoprene (C 5 H 8 ) monoterpenes other VOC Oceanic emissions Brasseur et al., 1998 ~1170 TgC/yr 500 TgC/yr 125 TgC/yr 520 TgC/yr 6-36 TgC/yr Global distribution of isoprene emissions: July 1-2 hour lifetime Guenther et al., JGR, 1995 g C m -2 month -1 5

6 VOC lifetimes Reactions with OH, O 3 COMPOUND LIFETIME ALKANES Ethane, C2H6 56 d Propane, C3H8 12 d n-butane, C4H10 6 d ALKENES, ALKADIENES, ALKYNES Ethene, C2H4 1.4 d Propene, C3H6 6 h 1-Butene, C4H8 6 h Isoprene, C5H8 2 h Acetylene, C2H2 18 d AROMATIC COMPOUNDS Benzene, C6H6 12 d Toluene, C6H5CH3 2.4 d Xylenes 1 d MONOTERPENES, C10H16 Alpha-pinene 2 h Limonene 1 h VOC lifetimes Reactions with OH, O 3 and photolysis COMPOUND LIFETIME ALDEHYDES AND KETONES Formaldehyde, HCHO 6 h Acetaldehyde, CH3CHO 1 d Acetone, CH3COCH3 18 d Ethyl vinyl ketone 10 h ALCOHOLS Methanol, CH3OH 15 d Ethanol, C2H5OH 4.5 d CARBOXYLIC ACIDS Formic acid 30 d Acetic acid 20 d 6

7 VOC oxidation and atmospheric chemistry OH sink Source of secondary organic aerosols Ozone production hν HO x source Reservoirs for long-range transport of NO x Seinfeld & Pandis, 1998

8 VOC oxidation mechanism Observed diurnal dependence of OH O 3 + hν O 2 + O( 1 D) λ<330 nm [R1, J(O1D)] O( 1 D) + M O( 3 P) + M [R2] O( 3 P) + O 2 + M O 3 + M [R3] O( 1 D) + H 2 O OH + OH [R4] 7

9 Observed dependence of OH on NO 2 Modeled Dependence of OH and HO 2 on NO Logan et al.,

10 Model-calculated OH concentrations in January and July GEOS-CHEM global model of tropospheric chemistry Generate your own plots at: Model calculated OH for January and July: zonal average 9

11 Model calculated OH for January and July: 5 km altitude Model calculated OH for January and July: surface 10

12 Measurement methods for atmospheric OH The challenges: High reactivity in gas phase and on surfaces severe constraints on sampling inlets Small concentrations (<10 7 molecules/cm 3 ) high detection sensitivity required need high specificity to avoid interference with more abundant gases An OH calibration standard is not available for instruments that require calibration Three methods for measuring OH Absorption spectroscopy Laser-induced fluorescence (LIF) Chemical conversion methods (Mass spectrometry, Radiocarbon method, Liquid phase scrubbing) New York City Measurements of OH Ren et al., Atmos. Env. 37, Upper troposphere Wennberg et al., Science, 279,

13 Modeling OH in the troposphere Why do we need models? *** Short lifetime of OH (1 second) OH concentrations are very variable in time and space would need very dense network of observations to know [OH] throughout the troposphere instead we use models, which can be divided in 3 classes: Models constrained by emission sources and atmospheric transport, Models constrained by observed distributions of OH precursors, Models constrained by observed distributions of OH proxies. Models constrained by emission sources and atmospheric transport Global 3-D models simulating emissions, chemistry, transport, deposition of OH precursors (NO x, CO, hydrocarbons). sensitivity of OH to changes in sources 2

14 Models constrained by observed distributions of OH precursors OH = f(solar radiation, T, H 2 O, CO, O 3, CH 4, NO x, hydrocarbons, etc ) Simultaneous measurements of all these parameters along with OH provides a test of the photochemistry of OH: test for local environment. Use 0-D models (no transport) Obtain global scale prediction of OH (variations with latitude, longitude, altitude, season ) by fixing tracers and meteorological parameters to our best present knowledge. Models constrained by observed distributions of OH proxies Use tracers with known sources, dominant sink by OH oxidation: CH 3 CCl 3, HCFC-22, CH 2 Cl 2, C 2 Cl 4, 14 CO observations of CH 3 CCl 3 combined with a mass balance analysis yield global mean OH concentration (weighted by atmospheric mass and by the temperature dependence of CH 3 CCl 3 +OH reaction) Global mean OH used to calculate lifetimes of other long-lived tracers (CO, CH 4, HCFCs) Global mean OH also used as a standard test for models of tropospheric chemistry 3

15 CH 3 CCl 3 emissions d[ch 3 CCl 3 ]/dt = Prod(t) Loss(t) Loss = (k[oh]+k stratos + k ocean )[CH 3 CCl 3 ] (1/ k stratos ~40 years, 1/k ocean ~90 years) Globally averaged [OH] ~ 1x10 6 molec/cm 3 Prinn et al.,

16 hν, H 2 O troposphere Have human activities changed the oxidizing capacity of the atmosphere? O 3 NO OH HO 2 CO, CH 4 hydrocarbons Decrease in OH away from sources driven by increases of CO (~months) Since pre-industrial times CO has increased by factors of 3-4; CH 4 increased by ~150%; O 3 increased by %; NO increased by factors of 2-8. What has happened to OH? O 3 increase in OH NO increase in OH CO+CH 4 decrease in OH Little change in OH globally (buffering) Wang & Jacob, 1999 Increase in OH near sources driven by increases of NO x (~days) 5