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The Atmospheric Oxidation System: Ox, HOx, NOx, etc. Sasha Madronich and Gabi Pfister National Center for Atmospheric Research Boulder, Colorado USA 10.

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Presentation on theme: "The Atmospheric Oxidation System: Ox, HOx, NOx, etc. Sasha Madronich and Gabi Pfister National Center for Atmospheric Research Boulder, Colorado USA 10."— Presentation transcript:

1 The Atmospheric Oxidation System: Ox, HOx, NOx, etc. Sasha Madronich and Gabi Pfister National Center for Atmospheric Research Boulder, Colorado USA 10 March 2011

2 Atmospheric Life Cycle 2 photo-oxidation formation of intermediates transport Emissions: VOCs, NOx, SO 2 solar UV radiation Products: CO 2,H 2 O, CO O 3, H 2 O 2, CH 2 O H 2 SO 4, HNO 3 SOA Increasing solubility dry and wet deposition

3 Energetics of Oxygen in the Atmosphere  H f (298K) kcal mol -1 Excited atomsO*( 1 D)104.9 Ground state atomsO ( 3 P)59.6 OzoneO 3 34.1 Normal moleculesO 2 0 3 Increasing stability

4 Atmospheric Oxygen Thermodynamic vs. Actual 4 O3O3 O O*

5 Photochemistry  Thermodynamics alone cannot explain atmospheric amounts of O 3, O, O*  Need –energy input, e.g. O 2 + h  O + O ( < 240 nm) –chemical reactions, e.g. O + O 2 (+ M)  O 3 (+ M) = Photochemistry 5

6 Stratospheric Ozone Chemistry The Only Production: O 2 + h ( < 242 nm)  O + O Chapman 1930 O + O 2 + M  O 3 + M Several Destruction Reactions: Pure oxygen chemistry:O 3 + h ( < 800 nm)  O + O 2 Chapman 1930 O + O 3  2 O 2 Catalytic Cycles: Odd hydrogen (HOx = OH + HO 2 )O 3 + OH  O 2 + HO 2 Bates and Nicolet 1950 O + HO 2  O 2 + OH O 3 + HO 2  2 O 2 + OH Odd nitrogen (NOx = NO + NO 2 )O 3 + NO  O 2 + NO 2 Crutzen 1970 O + NO 2  O 2 + NO Halogens (Cl, Br) O 3 + Cl  O 2 + ClO Rowland and Molina 1974 O + ClO  O 2 + Cl 6

7 Total O 3 Column 7 Fishman et al., 2008

8 COLUMN OZONE TRENDS, % 8 http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_bulletins/sh_09

9 9 Oxygen Species UNEP 2002

10 Tropospheric O 3 S. Chandra and J. Ziemke (NASA GSFC) TOMS-OMI, Oct 2007

11 Tropospheric OH Source O 3 + hv  O( 1 D) + O 2 O( 1 D) + N 2  O( 3 P) + N 2 O( 1 D) + O 2  O( 3 P) + O 2 O( 1 D) + H 2 O  OH + OH j(effective) ~ 10% of jO 3 11 Shetter et al., 1996

12 Tropospheric Ozone Formation - How?  Laboratory studies show that O 3 is made almost exclusively by the reaction: O 2 + O + M  O 3 + M  But no tropospheric UV-C radiation to break O 2 O 2 + h ( < 242 nm)  O + O  Haagen-Smit(1950s) - Los Angeles smog: Urban ozone (O 3 ) is generated when air containing hydrocarbons and nitrogen oxides (NOx = NO + NO 2 ) is exposed to tropospheric UV radiation

13 The Nitrogen Family N nitrogen atoms – negligible at room temperature T N 2 molecular nitrogen Nitrogen oxides : NOx ≡ NO + NO 2 NOnitric oxide is 90-95% of direct emissions NO 2 nitrogen dioxide is 5-10% of direct emissions, but more is made from NO + oxidants in the atmosphere Zeldovich mechanism at high T (flames, engines, lightning): O 2 + heat  O + O O + N 2  N + NO N + O 2  O + NO (NO is the cross-product of scrambling N 2 and O 2 at high T) 13

14 hydrocarbon reservoir vaporizer O 2 entrainment black body radiation from soot (~1700 K) HC + O 2  partly oxidized fragments soot fragments CO 2 +H 2 O+heat N + O 2  O + NO O + N 2  N + NO O 2 + heat  O + O heat, light H 2 O, CO 2 fragments soot NO Hydrocarbon Oxidation

15 (some other nitrogen species) NO 3 nitrate radical N 2 O 5 dinitrogen tetroxide HONOnitrous acid HONO 2 nitric acid CH 3 ONO 2 methyl nitrate N 2 Onitrous oxide (laughing gas) NH 3 ammonia NH 2 CH 3 methyl amine 15

16 16 Tropospheric O 3 Formation - 2 NO 2 photo-dissociation is the source of O atoms that make tropospheric O 3 NO 2 + h ( < 420 nm)  NO + O O + O 2 + M  O 3 + M _____________________________________________ Net: NO 2 + h + O 2  NO + O 3

17 PHOTODISSOCIATION COEFFICIENTS J (s -1 ) =  F( )  d F( ) = spectral actinic flux, quanta cm -2 s -1 nm -1  probability of photon near molecule.  absorption cross section, cm 2 molec -1  probability that photon is absorbed.  photodissociation quantum yield, molec quanta -1  probability that absorbed photon causes dissociation.

18 NO 2 + h ( < 420 nm)  NO + O 18 Mexico City, surface, March 2006

19 19 Tropospheric O 3 Formation - 3  NO 2 photo-dissociation makes some O 3, but not enough. Two problems: Usually O 3 ~ 20 - 500 ppb >> NO 2 ~ 1 – 10 ppb Reversal by the reaction: NO + O 3  NO 2 + O 2

20 20 Tropospheric O 3 Formation - 4  Initiation by UV radiation (Levy, 1970): O 3 + h ( < 330 nm)  O*( 1 D) + O 2 O*( 1 D) + H 2 O   OH +  OH  Hydrocarbon consumption (oxygen entry point):  OH + RH  R  + H 2 O R  + O 2 + M  ROO  + M  Single-bonded oxygen transferred to NOx: ROO  + NO  RO  + NO 2  NOx gives up oxygen atoms (as before): NO 2 + h ( < 420 nm)  NO + O O + O 2 + M  O 3 + M

21 21 Tropospheric O 3 Formation - 5  Propagation RO  + O 2  R’CO + HOO  HOO  + NO   OH + NO 2 every NO  NO 2 conversion makes O 3 except NO + O 3  NO 2 + O 2  Termination  OH + NO 2 + M  HNO 3 + M HOO  + HOO  + M  H 2 O 2 + M HOO  + O 3   OH + 2 O 2

22 22 Initiation by photo-dissociation O 3 + h + H 2 O  2  OH + O 2 Oxidation of hydrocarbons  OH + RH + O 2 + M  ROO  + H 2 O + M NO  NO 2 conversions ROO  + NO  RO  + NO 2 O 3 + NO  NO 2 + O 2 Actual O 3 formation NO 2 + h + O 2  O 3 + NO Propagation RO  + O 2  HOO  + R’CO HOO  + NO   OH + NO 2 Termination  OH + NO 2 + M  HNO 3 + M HOO  + HOO  + M  H 2 O 2 + O 2 + M HOO  + O 3   OH + 2 O 2 Summary of Key Steps In Tropospheric O 3 Formation An aside: How do you know which reactions can happen, and which ones cannot? Myth: Obvious to chemists Truth: - Not obvious at all - Quantum theory pretty good only for H + H 2  H 2 + H - Most knowledge is semi- empirical, from lab experiments and from statistical thermodynamics

23 23 Initiation by photo-oxidation O 3 + h + H 2 O  2  OH + O 2 -1 +2 na  OH + RH + O 2 + M  ROO  + H 2 O + M 0 0 na Partitioning by NOx ROO  + NO  RO  + NO 2 +1 0 0 NO 2 + h + O 2  O 3 + NO 0 na 0 NO + O 3  NO 2 + O 2 0 na 0 Propagation RO  + O 2  HOO  + R’CO 0 0 na HOO  + NO   OH + NO 2 +1 0 0 Termination  OH + NO 2 + M  HNO 3 + M -1 -1 -1 HOO  + HOO  + M  H 2 O 2 + O 2 + M 0 -2 na HOO  + O 3   OH + 2 O 2 -1 -1 na  Ox  ROx  NOx _______________________________________ Ox ≡ NO 2 + O 3 ROx ≡ OH + HOO + RO + ROO NOx ≡ NO + NO 2 The 1-slide Mechanism

24 24 Initiation by photo-oxidation O 3 + h + H 2 O  2  OH + O 2 -1 +2 na  OH + RH + O 2 + M  ROO  + H 2 O + M +1 0 na Partitioning by NOx ROO  + NO  RO  + NO 2 0 0 0 NO 2 + h + O 2  O 3 + NO 0 na 0 NO + O 3  NO 2 + O 2 0 na 0 Propagation RO  + O 2  HOO  + R’CO +1 0 na HOO  + NO   OH + NO 2 0 0 0 Termination  OH + NO 2 + M  HNO 3 + M -1 -1 -1 HOO  + HOO  + M  H 2 O 2 + O 2 + M -2 -2 na HOO  + O 3   OH + 2 O 2 -2 -1 na  Oy  ROx  NOx _______________________________________ Oy ≡ ROO + HOO + NO 2 + O 3 ROx ≡ OH + HOO + RO + ROO NOx ≡ NO + NO 2 The 1-slide Mechanism

25 Global Hydrocarbon Emissions Tg C yr -1 IsopreneTerpenesC2H6C2H6 C3H8C3H8 C 4 H 10 C2H4C2H4 C3H6C3H6 C2H2C2H2 BenzeneToluene Fossil fuel - - 4.8 4.9 8.3 8.6 2.3 4.6 13.7 Biomass burning - - 5.6 3.3 1.7 8.6 4.3 1.8 2.8 1.8 Vegetation 503 123 4.0 4.1 2.5 8.6 - - - Oceans - - 0.8 1.1 - 1.6 1.4 - - - TOTAL 503 123 15.2 13.4 12.5 27.4 22.9 4.1 7.4 15.5 25 Ehhalt, 1999 CH 4 ~ 500 – 600 Tg CH 4 yr -1 [IPCC, 2001]

26 26 Tropospheric Chemical Mechanisms  Heuristic: ~10 reactions[here, or Seinfeld and Pandis, 1997]  Typical 3D model used for air quality: 100 - 300 reactions CB-IV, CB-V[Gery, 1989] RADM, RACM [Stockwell, 1990; 1997] SAPRC99 [Carter, 2000]  Typical 0D (box) models used for sensitivity studies: 3,000 - 10,000 reactions NCAR Master Mechanism [Madronich and Calvert, 1990] Leeds Master Chemical Mechanism [Jenkin et al. 1997]  Fully explicit (computer-aided) mechanisms: 10 6 - 10 7 reactions GECKO-A [Aumont et al. 2005]

27 Atmospheric VOC’s: Hydrocarbons - 1  Alkanes CH 4 CH 3 CH 3 CH 2 CH 3 C 4 H 10 (2 isomers) C 5 H 12 (3 isomers) C 6 H 14 (5 isomers) C 7 H 16 (9 isomers) C 8 H 18 (18 isomers) …. methane ethane propane butane pentane hexane heptane octane …. 27

28 Atmospheric VOC’s: Hydrocarbons - 2  Alkenes CH 2 =CH 2 CH 2 =CHCH 3 … CH 2 =C(CH 3 )CH=CH 3  Aromatics C 6 H 6 C 6 H 5 CH 3 C 6 H 5 (CH 3 ) 2 (3 isomers) …  Terpenes C 10 H 16 ethene (ethylene) propene (propylene) … 2-methyl 1,3 butadiene (isoprene) Benzene Toluene Xylenes …  -pinine,  -pinine … 28

29 Atmospheric VOC’s: Substituted Hydrocarbons  Alcohols, -OH –methanol, CH 3 OH –ethanol, CH 3 CH 2 OH  Aldehydes, -CHO –formaldehyde,CH 2 O –acetaldehyde, CH 3 CHO  Ketones, -CO- –acetone, CH 3 COCH 3 –MEK, CH 3 COCH 2 CH 3  Carboxylic acids, -CO(OH) –formic, HCO(OH) –acetic, CH 3 CO(OOH)  Organic hydroperoxides, -OOH –methyl hydroperoxide, CH 3 (OOH)  Organic peroxy acids, -CO(OOH) –peracetic, CH 3 CO(OOH)  Organic nitrates, -ONO 2 –methyl nitrate, CH 3 (ONO 2 ) –Ethyl nitrate, CH 3 CH 2 (ONO 2 )  Peroxy nitrates, -OONO 2 –methyl peroxy nitrate, CH 3 (OONO 2 )  Acyl peroxy nitrates, -CO(OONO 2 ) –PAN, CH 3 CO(OONO 2 ) 29

30 General Hydrocarbon Reaction Patterns  Short-chain compounds tend to have unique behavior, and must be considered individually.  Longer-chain compounds are quite alike within each family (e.g. all aldehydes). Kinetics and mechanisms can be adjusted for chain length and substitutions (structure-activity relations). 30

31 OH + Hydrocarbon Reactions  Abstraction of H OH + CH 3 CH 3  CH 3 CH 2  …followed immediately by CH 3 CH 2  + O 2 + M  CH 3 CH 2 OO  + M  Addition to double bonds OH + CH 2 =CH 2  CH 2 (OH)CH 2  …followed immediately by CH 2 (OH)CH 2  + O 2 + M  CH 3 (OH)CH 2 OO  + M 31

32 Atmospheric Organic Radicals  Alkyl (carbon-centered)  CH 3 methyl  CH 2 CH 3 ethyl  CH 2 CH 2 CH 3 propyl  Peroxy, -OO  CH 3 OO  methyl peroxy CH 3 CH 2 OO  ethyl peroxy  Alkoxy, -O  CH 3 O  methoxy CH 3 CH 2 O  ethoxy  Acyl, CO(OO  ) CH 3 CO(OO  )acetyl  Criegee,  C(OO  )  CH 2 OO  from O 3 + C 2 H 4 CH 3  CHOO  from O 3 + C 3 H 6 32

33 O 3 + Hydrocarbon Reactions  Ozone addition across double bond O 3 + CH 2 =CH 2  CH 2 – CH 2  CH 2 O + (  CH 2 OO  )* Fate of excited Criegee diradical: (  CH 2 OO  )*  CO + H 2 O  CO 2 + H 2  CO 2 + 2 H  … + M   CH 2 OO  (stabilized Criegee diradical)  CH 2 OO  + (H 2 O, NO, NO 2, SO 2 )  Products 33 O OO

34 NO 3 + VOC Reactions  H atom abstraction: CH 3 CHO + NO 3  CH 3 CO  + HNO 3 CH 3 CO  + O 2 + M  CH 3 CO(OO  ) + M  Addition to double bond: CH 2 =CH 2 + NO 3 + M  CH 2 (ONO 2 )CH 2  + M CH 2 (ONO 2 )CH 2  + O 2 + M  CH 2 (ONO 2 )CH 2 (OO  ) + M 34

35 Peroxy Radical Reactions - 1  with NO ROO  + NO  RO  + NO 2 ROO  + NO + M  RONO 2 + M  with NO 2 ROO  + NO 2 + M  ROONO 2 + M RCO(OO  ) + NO 2 + M  RCO(OONO 2 ) + M 35

36 Peroxy Radical Reactions - 2  with HO 2 ROO  + HOO   ROOH + O 2 RCO(OO  ) + HOO   RCO(OOH) + O 2  with other organic peroxy radicals, e.g. CH 3 CH 2 OO  + CH 3 OO   radical channel  CH 3 CH 2 O  + CH 3 O  + O 2 molecular channel 1  CH 3 CH 2 OH + CH 2 O + O 2 molecular channel 2  CH 3 CHO + CH 3 OH + O 2 36

37 Alkoxy Radical Reactions  with O 2, e.g. CH 3 CH 2 O  + O 2  CH 3 CHO + HOO  CH 3 CH(O  )CH 3 + O 2  CH 3 COCH 3 + HOO   thermal decomposition, e.g. CH 2 CH(O  )CH 2 OH + M  CH 3 CHO +  CH 2 OH + M  isomerization, e.g. CH 3 CH(O  )CH 2 CH 2 CH 2 CH 3   CH 3 CH(OH)CH 2 CH 2  CHCH 3 37

38 Reactions of Partly Oxidized Species  OH, O 3, and NO 3 reactions as with precursor hydrocarbons  photolysis important for –aldehydes –ketones –peroxides –alkyl nitrates –but not for alcohols or carboxylic acids  thermal decomposition for peroxy nitrates 38

39 RH RR ROO  RO  R’CHO CO 2 + H 2 O ROOHRONO 2 … OH, O 3, NO 3 O2O2 NO HO 2 h h O 2, heat OH, O 3, NO 3 OH Generalized Oxidation Sequence of Hydrocarbons

40 Simplified Mechanism for Pentane (C 5 H 12 ) Multiple NO  NO 2 conversions produce O 3 Organic nitrates allow long-range transport of NOx Radical sinks: Some are temporary, producing HOx later Some have low vapor pressures, can make organic aerosols

41 41 All C7 species 1 function 2 functions 3 functions 4 functions Time Integration with Two-step Iterative Solver Products of n-heptane oxidation (high NOx case)

42 42 All C7 species 1 function 2 functions 3 functions 4 functions 2 functional groups : typical species OH O ONO 2 O OH O O O ONO 2 O

43 43 All C7 species 1 function 2 functions 3 functions 4 functions 3 functional groups : typical species OH O O ONO 2 O OH O O O ONO 2 O O O O O

44 44 All C7 species 1 function 2 functions 3 functions 4 functions 4 functional groups : typical species O OH ONO 2 O O OH O ONO 2 O OH O O OOO ONO 2 OH O ONO 2

45 Growth of Mechanisms 45 Aumont et al., 2007

46 46 Initiation by photo-oxidation O 3 + h + H 2 O  2  OH + O 2 -1 +2 na  OH + RH + O 2 + M  ROO  + H 2 O + M 0 0 na Partitioning by NOx ROO  + NO  RO  + NO 2 +1 0 0 NO 2 + h + O 2  O 3 + NO 0 na 0 NO + O 3  NO 2 + O 2 0 na 0 Propagation RO  + O 2  HOO  + R’CO 0 0 na HOO  + NO   OH + NO 2 +1 0 0 Termination  OH + NO 2 + M  HNO 3 + M -1 -1 -1 HOO  + HOO  + M  H 2 O 2 + O 2 + M 0 -2 na HOO  + O 3   OH + 2 O 2 -1 -1 na  Ox  ROx  NOx _______________________________________ Ox ≡ NO 2 + O 3 ROx ≡ OH + HOO + RO + ROO NOx ≡ NO + NO 2 The 1-slide Mechanism

47 47 Initiation by photo-oxidation O 3 + h + H 2 O  2  OH + O 2 -1 +2 na  OH + RH + O 2 + M  HOO  + H 2 O + M +1 0 na Partitioning by NOx HOO  + NO   OH+ NO 2 0 0 0 NO 2 + h + O 2  O 3 + NO 0 na 0 NO + O 3  NO 2 + O 2 0 na 0 Termination  OH + NO 2 + M  HNO 3 + M -1 -1 -1 HOO  + HOO  + M  H 2 O 2 + O 2 + M -2 -2 na  Oy  ROx  NOx _______________________________________ Oy ≡ HOO + NO 2 + O 3 ROx ≡ OH + HOO + RO + ROO NOx ≡ NO + NO 2 The Half-slide Mechanism

48 NOx-VOC Regimes NOx-limited VOC-limited NOx-inhibited Low NOx High NOx

49

50 DIURNAL AND WEEKLY VARIATIONS Surface network in Mexico City 50 Stephens et al., 2008

51 51

52 Mexico City’s O 3 Production Is VOC-limited, NOx-inhibited 3NOx 3VOC 3NOx 3VOC Tie et al., 2007 WRF-Chem model --- sensitivity studies ● observations

53 Aerosol Yield is a function of VOC/NOx 53 Camredon et al., 2007

54 California EPA, 2004

55 Health Impacts of Air Quality Largely due to carbon-containing particles and O 3 WHO estimates for 2002: –World: 865,000 deaths per year – 1.0 DALY* /1000 capita per year –U.S.: 41,200 deaths per year – 0.8 DALY /1000 capita per year * DALY = Disability-Adjusted Lost Years WHO, 2007

56 Air Quality in CA has improved over the past years, but ozone values exceeding health standards are still frequent. Local Ozone Frequency of hourly surface O 3 for 1994-2008 (U.S. EPA Surface Monitoring; AQS Datamart) site categories: all urban suburban rural

57 Air Quality has improved over the past years, but ozone values exceeding health standards are still frequent. As local sources are reduced and health standards get tighter, the influence of "background" gains more importance. Local Ozone - and Background O. Cooper et al., Nature, 2009 “Increasing springtime ozone mixing ratios in the free troposphere over western North America” 0.76 ± 0.29 ppbv/year

58 WRF-Chem Regional O 3 Prediction Observed daily 1-h maximum O3 for all EPA AIRNOW surface stations in the model domain, 21 July - 4 August 2002. Grell et al., 2005

59 Surface Ozone 14 June – 15 July 2008 Surface Ozone: Monitoring and Modeling Average for Local AfternoonAverage for Nighttime

60 FUTURE TROPOSPHERIC O 3 : MODELS DISAGREE IPCC 2001

61 Advances in modeling 61 Jonson et al., 2010

62 Global Oxidation (self-cleaning) Capacity 62 Solar UV radiation Oxidation, e.g.: CH 4 + OH  …  CO 2 + H 2 O Insoluble  Soluble Emissions CH 4 CmHnCmHn SO 2 NO CO NO 2 Halocarbons Deposition (dry, wet) HNO 3, NO 3 - H 2 SO 4, SO 4 = HCl, Cl - Carboxylic acids

63 63 TROPOSPHERIC OXIDIZING (SELF-CLEANING) CAPACITY Log 10 [OH] - Global Box Model Different OH regimes 10 6 10 5 10 4 10 3 10 2 10 7 10 1 10 0 F CH4, cm -3 s -1 F NO, cm -3 s -1 F O3 =5e4 cm -3 s -1, F CO =1e5 cm -3 s -1 ~current Madronich and Hess, 1993 pre- industrial future?

64 64 O 3 +NO NO 2 +h O 3 +h OH+CO OH HO 2 +NO HO 2 OH NO 2 NO O3O3 O3O3 NO 2 OH+NO 2 HO 2 +HO 2 HO 2 OH NO 2 OH Ox-HOx-NOx-CO Processing HO 2 +HO 2 HO 2 OH OH+NO 2 NO 2 OH


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