1. Atmospheric Chemistry Sasha Madronich National Center for Atmospheric Research Boulder, Colorado USA Boulder, 1 June 2009

2. Earth’s Atmosphere  Composition 78% nitrogen 21% oxygen 1-2% water (gas, liquid, ice) trace amounts (<< 1%) of many other species, some natural and some “pollutants”  Reactivity dominated by oxygen chemistry solar photons  To understand fate of pollutants, must first understand oxygen photochemistry 2

3. Pure Oxygen Species 3

4. 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 4 Increasing stability

5. Atmospheric Oxygen Thermodynamic vs. Actual 5 O3O3 O O*

6. Photochemistry  Thermodynamics alone cannot explain atmospheric amounts of O 3, O, O*  Need –energy input, e.g. O 2 + h  O + O ( < 242 nm) –chemical reactions, e.g. O + O 2 (+ M)  O 3 (+ M) = Photochemistry 6

8. 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 8

9. COLUMN OZONE TRENDS, % 9 http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_bulletins/sh_07/Fig_5.gif

10. 10 SOLAR SPECTRUM UNEP, 2002 O 2 and O 3 absorb all UV-C ( <280 nm) before it reaches the troposphere

11. 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

12. The Nitrogen Family N nitrogen atoms – negligible at room T N 2 molecular nitrogen 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) 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 12

13. (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 13

14. 14 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

15. CALCULATION OF 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.

16. NO 2 + h ( < 420 nm)  NO + O 16 Mexico City, surface, March 2006

17. 17 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

18. 18 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

19. 19 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

20. 20 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

21. NOx Photo-stationary State NO 2 + h  NO + O 3 J NO2 NO + O 3  NO 2 + O 2 k 1 NO + HO 2  NO 2 + OHk 2 NO + RO 2  NO 2 + ROk 3 d[NO]/dt = +J NO2 [NO 2 ] – [NO](k 1 [O 3 ]+k 2 [HO 2 ]+k 3 [RO 2 ]) ~ 0 at steady state  ≡ J NO2 [NO 2 ] / (k 1 [NO][O 3 ]) ~ 1 + (k 2 [HO 2 ]+k 3 [RO 2 ]) /k 1 [O 3 ] Can use measurements of  to estimate [HO 2 ] + [RO 2 ] and instantaneous O 3 production 21

22. DIURNAL AND WEEKLY VARIATIONS Surface network in Mexico City 22 Stephens et al., ACP 2008

23. 23

24. 24 Tropospheric Chemical Mechanisms  This talk: 10 reactions  Typical 3D model used for air quality: 100 - 200 reactions  Typical 0D (box) models used for sensitivity studies: 5,000 - 10,000 reactions  Fully explicit (computer-generated) mechanisms: 10 6 - 10 7 reactions

25. Atmospheric Volatile Organic Compounds (VOCs): Hydrocarbons  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 …. 25

26. 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 … 26

27. 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 27 Ehhalt, 1999 CH 4 ~ 500 – 600 Tg CH 4 yr -1 [IPCC, 2001]

28. 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 ) 28

29. 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 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. 31 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

32. 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 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. 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

40. 40 Consequences of tropospheric chemistry - 1  Formation of O 3 Urban: 100-500 ppb Regional: 50-100 ppb Global background increase 10-20 ppb  35-45 ppb in NH 10-20 ppb  25-35 ppb in SH  Damage to health and vegetation e.g. $3.5B-6.1B /yr in US for 8 major crops (Murphy, 1999)  Greenhouse role of O 3  Changes in global oxidation capacity

41. EPA, 2004

42. 42 Consequences of tropospheric chemistry - 2  Formation of peroxides and acids: HO 2 + HO 2  H 2 O 2 + O 2 OH + NO 2 + M  HNO 3 + M OH + SO 2  …  H 2 SO 4 H 2 O 2 (aq) + SO 2 (aq)  …  H 2 SO 4 (aq)  Damage to vegetation, lakes, and buildings (acid precipitation)  Sulfate aerosol formation (visibility, precipitation, direct and indirect radiative forcing of climate)

43. 43 Consequences of tropospheric chemistry - 3  Products of hydrocarbon oxidation CO 2 (minor compared to direct emissions) CO (~ 1/3 of total emissions) Oxygenated organics:aldehydes, ketones, alcohols, organic acids, nitrates, peroxides  Damage to health, vegetation  Secondary organic aerosol formation (health, visibility, meteorology, direct and indirect climate forcing)  Changes in global oxidation capacity

44. Organic aerosol > Sulfate in most observations Zhang et al., GRL 2007

45. Human Health Impacts of Particles  For 2002 (World Health Organization, 2007): World: 865,000 deaths per year 1.0 DALY* /1000 capita per year * DALY = Disability-Adjusted Lost Years U.S.: 41,200 deaths per year 0.8 DALY /1000 capita per year

46. 46 Global Oxidation (self-cleaning) Capacity 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

47. 47 Consequences of tropospheric chemistry - 4  Oxidizing Capacity: Increase because of increasing emissions of NOx? Increase because of increasing UV radiation? or Decrease because of increasing emissions of CO, C m H n, SO 2, and other reduced compounds?  Decreased OH (oxidizing capacity) implies generally higher amounts of most pollutants including: Higher amounts of greenhouse gases Higher amounts of substances that deplete the ozone layer More global spread

48. 48 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?

49. FUTURE TROPOSPHERIC O 3 : MODELS DISAGREE IPCC 2001

50. SUMMARY Stratospheric chemistry is relatively simple: Oxygen photo-dissociation Ozone catalytic destruction Impacts: climate, surface UV radiation Tropospheric chemistry is complex, non-linear: Ozone made from UV, NOx, and HCs Many hydrocarbons (biogenic and anthropogenic) Aerosols: most are made in atmosphere by condensation of gas phase species Many impacts: health, ecosystems, meteorology, climate 50