1. ATMOSPHERIC CHEMISTRY OF ORGANIC COMPOUNDS Lecture for NC A&T (part 2) March 9, 2011 John Orlando orlando@ucar.edu

2. REVIEW: Geoff showed something about the types of compounds: CH 4 CH 3 -CH(CH 3 ) 2 CH 3 -CH=CH-CH 3 CH 3 CH 2 CH 2 C(=O)CH 3 CH 3 CH 2 CH 2 OH CH 3 CH 2 -O-CH 2 CH 3

3. REVIEW: Where they come from: Biogenic sources the largest – isoprene, terpenes,etc. Isoprene CH 2 =CH-C(CH 3 )=CH 2 But also anthropogenic emissions, mostly the types of things we just saw on the previous page (fossil fuel combustion, industrial…) Alkanes Alkenes AlcoholsEtc. Etc. etc. Ethers

4. REVIEW: How they are distributed (and how we know - measurements): T. Karl et al. (ACD), J. Geophys. Res., 112, D18302, 2007.

5. REVIEW: What are the impacts? Ozone “Chemical Weather” – From Louisa Emmons (ACD), Mozart-4 Global CTM

6. REVIEW: What are the impacts? Secondary Organic Aerosol From Alma Hodzic (ACD) et al., Atmos. Chem. Phys., 9, 6949, 2009.

7. SO NOW LET’S TALK ABOUT THE CHEMISTRY: RECALL: The atmosphere (particularly the troposphere) acts as a low-temperature, slow-burning combustion engine. Takes all the emissions (reduced compounds) and ‘burns’ (oxidizes) them: OH HO 2 CH 4 CO 2 + H 2 O IsopreneOther by-products, such as O 3, particles, acids, DMS, NH 3 nitrates, etc. (2 ry POLLUTANTS) NO NO 2

8. THE TROPOSPHERIC “ENGINE”: Now the “Odd Hydrogen” Family: Consider first OH and HO 2 : Production:O 3 + h  O( 1 D) + O 2 O( 1 D) + H 2 O  OH + OH Conversion of OH to HO 2 : OH + CO (+O 2 )  HO 2 + CO 2 dominant (when all ‘fuel’ considered) OH + O 3  HO 2 + O 2, usually minor Conversion of HO 2 back to OH: HO 2 + O 3  OH + 2 O 2 HO 2 + NO  OH + NO 2, (followed by NO 2 + h  NO + O, O + O 2 + M  O 3 + M, which generates O 3 !!) Losses of HO x via two processes: HO 2 + HO 2 + M  HOOH + O 2 + M OH + NO 2 + M = HNO 3 + M

9. OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail: Talked about this in my seminar at NC A&T: What are the basic steps (there are four)? (Let’s start with methane, CH 4 ). CH 4

10. OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail: Talked about this in my seminar at NC A&T: What are the basic steps (there are four)? (Let’s start with methane, CH 4 ). CH 4 1.Starts with reaction with OH: OH CH 3 + H 2 O

11. OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail: Talked about this in my seminar at NC A&T: What are the basic steps (there are four)? (Let’s start with methane, CH 4 ). CH 4 1.Starts with reaction with OH: OH CH 3 + H 2 O 2.The alkyl radical adds O 2, to make a peroxy radical. O 2 CH 3 O 2

12. OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail: Talked about this in my seminar at NC A&T: What are the basic steps (there are four)? (Let’s start with methane, CH 4 ). CH 4 1.Starts with reaction with OH: OH CH 3 + H 2 O 2.The alkyl radical adds O 2, to make a peroxy radical. O 2 CH 3 O 2 3.Peroxy radical often reacts with NO, making an alkoxy NO radical. (There are other pathways, see later). CH 3 O + NO 2

13. OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail: Talked about this in my seminar at NC A&T: What are the basic steps (there are four)? (Let’s start with methane, CH 4 ). CH 4 1.Starts with reaction with OH: OH CH 3 + H 2 O 2.The alkyl radical adds O 2, to make a peroxy radical. O 2 CH 3 O 2 3.Peroxy radical often reacts with NO, making an alkoxy NO radical. (There are other pathways, see later). CH 3 O + NO 2 4.Alkoxy radical reacts with O 2, to make a carbonyl O 2 compound. (There are other pathways, see later). CH 2 O + HO 2

14. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 + NO + O 2 + OH 1 2 3 4

15. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 + NO + O 2 + OH 1 2 3 4 IN GENERAL, REFER TO THE PARENT COMPOUND AS R-H REFER TO THE ALKYL RADICAL AS R REFER TO THE PEROXY RADICAL AS RO 2 NOTE ALSO: THESE BASIC REACTIONS PROPOGATE RADICALS !! We will refer to this again from time to time, noting that other pathways DO NOT PROPOGATE REFER TO THE ALKOXY RADICAL AS RO

16. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 CH 3 CH 2 CH 2 + CH 3 CHO + NO + O 2 E a = 13 kcal  CH 2 CH 2 CH 2 CH(OH)CH 3 E a = 8 kcal CH 3 CH 2 CH 2 CH(OOH)CH 3 CH 3 CH 2 CH 2 CH(ONO 2 )CH 3 + HO 2 + NO + OH 1 2 3 4 3b

17. OK, LET’S START WITH STEP #1 – REACTION OF OH WITH HYDROCARBONS (Also applies to NO 3, and Cl-atoms) CAN HAVE TWO KINDS OF REACTIONS – 1)ABSTRACTION: OH + CH 4  CH 3 + H 2 O - Occurs when the hydrocarbon is “saturated” (no double bonds) 2)ADDITION: OH + CH 2 =CH 2  HOCH 2 -CH 2

18. OK, LET’S START WITH STEP #1 – REACTION OF OH WITH HYDROCARBONS (Also applies to NO 3, and Cl-atoms) Go back to our old friend, OH + Methane (CH 4 ) From Wikipedia REACTION DOES NOT OCCUR ON EVERY COLLISION!!! E a k = A * exp(-E a /RT) A is the pre-exponential factor, and accounts for the geometry limitations. E a is activation energy.

19. REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.) ELEMENTARY REACTIONS (BIMOLECULAR) k = A * exp(-E a /RT) So, Let’s go back to the OH / CH 4 reaction. IF REACTION OCCURRED ON EVERY COLLISION, k = 2 x 10 -10 cm 3 molecule -1 s -1 Turns out that k = 2.45 x 10 -12 * exp(- 3525 cal / RT) k = 6.3 x 10 -15 cm 3 molecule -1 s -1 at 298 K k = 5.2 x 10 -16 cm 3 molecule -1 s -1 at 210 K Only about 1 in 30000 OH/CH 4 collisions results in reaction at 298 K.

20. FOR OH + CH 4 : [ HO…H-CH 3 ] E a = 3525 calories OH + CH 4  H r = - 13900 calories HOH + CH 3

21. FOR OH + CH 4 : FOR OH + C 2 H 6 : (CH 3 -CH 3 ) [ HO…H-CH 3 ] E a = 3525 calories E a = 2100 calories OH + CH 4  H r = - 13900 calories OH + CH 3 -CH 3  H r = - 17800 calories HOH + CH 3 HOH + CH 3 -CH 2

22. SO, IN GENERAL: The more substituted (complicated) the molecule, the weaker the C-H bond, and the faster the rate coefficient n-PENTANE: CH 3 CH 2 CH 2 CH 2 CH 3 DIETHYL ETHER :CH 3 CH 2 -O-CH 2 CH 3 2-PROPANOL:CH 3 CH(OH)CH 3 2-PENTANONE: CH 3 CH 2 C(=O)CH 2 CH 3 COMPOUND A-Factor (cm 3 molecule -1 s -1 ) Activation Energy (calories) Rate Constant at 298 K (cm 3 molecule -1 s -1 ) Approx. Lifetime (OH = 10 6 molecule cm -3 ) METHANE 1.85  10 -12 3360 6.4  10 -15 8.4 years ETHANE 8.61  10 -12 2080 2.6  10 -13 45 days n-PENTANE 1.81  10 -11 900 3.9  10 -12 3 days 2-PROPANOL 2.7  10 -12 -190 5.1  10 -12 2 days DIETHYL ETHER 4.6  10 -12 -290 1.2  10 -11 1 days 2-PENTANONE 3.2  10 -13 -1430 3.6  10 -12 3 days CH 3 CF 3 1.06  10 -12 39751.3  10 -15 > 25 years

23. Figure I-F-1g. The annual mean surface distribution of a synthetic alkane with a man-made source strength of 1 Tg yr -1 and an OH reaction rate coefficient of 1.0 ×10 -14 cm 3 molecule -1 s -1. (From Calvert et al., Mechanisms of the Atmospheric Oxidation of the Alkanes, OUP, 2008) 400 ppt 200 ppt

24. Figure I-F-1a. The annual mean surface distribution of a synthetic alkane with a man-made source strength of 1 Tg yr -1 and an OH reaction rate coefficient of 1.0 ×10 -11 cm 3 molecule -1 s -1. (From Calvert et al., Mechanisms of the Atmospheric Oxidation of the Alkanes, OUP, 2008) 50 ppt < 1 ppt

25. THERE ARE OTHER OXIDANTS BESIDES OH: - One of the them is the “NITRATE RADICAL”, NO 3 - Photolyzes rapidly, so only active at nighttime. - Can abstract, though energetics not as favorable. As an example, OH + Isobutane (C 4 H 10 )  C(CH 3 ) 3 + H 2 Ok = 7.0  10 -12 exp(-350/T) cm 3 molecule -1 s -1 NO 3 + Isobutane (C 4 H 10 )  C(CH 3 ) 3 + H 2 O k = 3.9  10 -12 exp(-3150/T) cm 3 molecule -1 s -1

26. Figure III-F-1. Plots of logarithm of the rate coefficients (cm 3 molecule -1 s -1 ) for reaction of Cl, O( 3 P) and NO 3 with the alkanes versus those for reaction of OH with the corresponding alkane. Solid lines are unweighted least-squares fits to the data. (From Calvert et al., Mechanisms of Atmospheric Oxidation of the Alkanes, OUP, 2008)

27. SO FAR, We have only dealt with abstraction. Can also have ‘addition’ reactions, when the hydrocarbon is ‘unsaturated’: (i.e., contains a C=C double bond, alkenes) Occurs for OH, NO 3, Cl-atoms too: Generally very fast reactions: OH + CH 2 =CH 2 (ethene)  HOCH 2 -CH 2 For OH + ethene, k = 8.1  10 -12 cm 3 molecule -1 s -1 Ethene lifetime  1.5 days = = Again, more substituted species react even faster. k(OH + isoprene) = 1.0  10 -10 cm 3 molecule -1 s -1 Isoprene lifetime  (1-2) hours

28. Generally, when multiple choices, addition will win over abstraction. CH 3 CH 2 -CH=CH-CH(CH 3 ) 2

29. Generally, when multiple choices, addition will win over abstraction. CH 3 CH 2 -CH=CH-CH(CH 3 ) 2 Addition reaction wins, k  6  10 -11 cm 3 molecule -1 s -1 Abstraction reactions, k  3  10 -12 cm 3 molecule -1 s -1

30. OZONE CAN ALSO ACT AS AN OXIDANT – Adds to double bonds: Chemistry is a bit weird, producing something called “Criegee Biradicals”: O - O O 3 + CH 2 =CH 2  CH 2 CH 2  CH 2 =O + CH 2 -OO O Chemistry of Criegee radicals is complex (and not totally understood): CH 2 -OO undergoes numerous types of reactions that form CO, CO 2, HCOOH

31. THERE ARE METHODS FOR ESTIMATING RATE COEFFICIENTS FOR REACTION OF VARIOUS OXIDANTS WITH HYDROCARBONS “STRUCTURE-REACTIVITY” RELATIONSHIPS (e.g., Kwok & Atkinson, Atm. Env., 1995) Consider only OH abstraction today, but they exist for addition reactions and also for other reactants (NO 3, Cl, O 3 ) How does it work? First: Assign ‘starting values’ for reaction of OH with a –CH 3 group, and –CH 2 - group, and a –CH< group (298 K): k(-CH 3 ) = 1.36  10 -13 cm 3 molecule -1 s -1 k(-CH 2 -) = 9.34  10 -13 cm 3 molecule -1 s -1 k(-CH<) = 19.4  10 -13 cm 3 molecule -1 s -1

32. MODIFY THE INITIAL VALUE ACCORDING TO WHAT IS BONDED TO IT (“Substituent factors”) CH 3 – Xk = k(-CH 3 ) * F(X) Y – CH 2 – Xk = k(-CH 2 -) * F(X) * F(Y) Y – CH – Xk = k(-CH<) * F(X) * F(Y) * F(Z) Z

33. CONSIDER PROPANOL: HO – CH 2 – CH 2 CH 3 k = k(CH 2 ) * F(X) * F(Y) k(-CH 2 -) = 9.34  10 -13 cm 3 molecule -1 s -1 F(-OH) = 4.0 F(-CH 2 CH 3 ) = 1.23 So, estimated k for reaction at the one particular -CH 2 - group is: k = k(-CH 2 -) * F(X) * F(Y) = 9.34  10 -13 cm 3 molecule -1 s -1 * (4.0) * (1.23) = 4.2  10 -12 cm 3 molecule -1 s -1

35. Generally, when multiple choices, addition will win over abstraction. CH 3 CH 2 -CH=CH-CH(CH 3 ) 2 Addition reaction wins, k  6  10 -11 cm 3 molecule -1 s -1 Abstraction reactions, k  3  10 -12 cm 3 molecule -1 s -1

36. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 + NO + O 2 + OH 1 2 3 4 OK, READY FOR STEP #2

37. No worries, this one is EASY PEASY LEMON SQUEEZY Take alkyl radical, e.g., CH 3 -CH 2 And add O 2, CH 3 -CH 2 + O 2 + M  CH 3 -CH 2 O 2 + M Voila, instant peroxy radical !! Typical k = 7 x 10 -12 cm 3 molecule -1 s -1 [O 2 ] = 5 x 10 18 molecule cm -3 So, time scale for the reaction is about 30 ns at Earth’s surface !!! Nothing else has much of a chance, except in extremely rare circumstances that we will not pursue today.

38. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 + NO + O 2 + OH 1 2 3 4 OK, ON TO STEP #3 !!!

39. PEROXY RADICAL CHEMISTRY MAIN REACTION IS WITH NO, CONVERTS PEROXY TO ALKOXY RADICAL NO Reaction (MAIN PATHWAY): RO 2 + NO  RO + NO 2 CH 3 O 2 + NO  CH 3 O + NO 2 This reaction propogates radicals. 3

40. PEROXY RADICAL CHEMISTRY MAIN REACTION IS WITH NO, CONVERTS PEROXY TO ALKOXY RADICAL NO Reaction (MAIN PATHWAY): RO 2 + NO  RO + NO 2 CH 3 O 2 + NO  CH 3 O + NO 2 This reaction propogates radicals. BUT, ALSO ANOTHER MINOR CHANNEL THAT COMPETES: RO 2 + NO  RONO 2 CH 3 O 2 + NO  CH 3 ONO 2 CH 3 CH 2 CH 2 CH(OO)CH 3 + NO  CH 3 CH 2 CH 2 CH(ONO 2 )CH 3 The larger and more complex the peroxy radical, typically the higher the nitrate yield (up to about 40% in some cases). NB: This channel is a radical TERMINATION! 3

41. Rate coefficient independent of structure, all k  10 -11 cm 3 molecule -1 s -1 So what are typical lifetimes for an RO 2 (peroxy) radical in the real world (Earth’s surface)? [NO] (pptv)LOCATIONApprox. RO 2 LIFETIME 5 Very remote regions 800 sec. 1000Rural conditions 4 sec. 100000Mexico City (e.g.)0.04 sec.

42. PEROXY RADICAL CHEMISTRY MAIN REACTION IS WITH NO, CONVERTS PEROXY TO ALKOXY RADICAL. ALSO HAVE THE NITRATE FORMING CHANNEL, WHICH TERMINATES. ALSO, a reaction with HO 2, main channel RO 2 + HO 2  ROOH + O 2 Radical termination. 3

43. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 CH 3 CH 2 CH 2 + CH 3 CHO + NO + O 2 E a = 13 kcal  CH 2 CH 2 CH 2 CH(OH)CH 3 E a = 8 kcal CH 3 CH 2 CH 2 CH(OOH)CH 3 CH 3 CH 2 CH 2 CH(ONO 2 )CH 3 + HO 2 + NO + OH 1 2 3 4 3b

44. RATE CONSTANTS FOR REACTION OF PEROXY RADICALS WITH HO 2 (Boyd et al., JPCA, 107, 818, 2003) Similar values to RO 2 + NO reactions.

45. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 + NO + O 2 + OH 1 2 3 4 OK, ON TO STEP #4, WE CAN DO IT !!!

46. ALKOXY RADICAL CHEMISTRY MAIN REACTION IS WITH O 2, CONVERTS ALKOXY RADICAL TO A CARBONYL COMPOUND, ALSO GET HO 2 (a peroxy radical) formed. PROPOGATION!! CH 3 O + O 2  CH 2 O + HO 2 CH 3 CH 2 CH 2 CH(O)CH 3 + O 2  CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 Rate coefficient typically about 10 -14 cm 3 molecule -1 s -1 So lifetime is about 20  s For larger alkoxy radicals, like 2-pentoxy, can have competing reactions: Decomposition 4

47. H CH 3 CH 2 CH 2 C O  CH 3 CH 2 CH 2 C(=O)CH 3 + H CH 3 CH 3 CH 2 CH 2 CHO + CH 3 CH 3 CHO + CH 3 CH 2 CH 2 (Baldwin et al., 1977; Choo and Benson, 1981; Atkinson, 1999) Energy k = 5e13 * exp (-E a /RT) sec -1 4

48. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 CH 3 + CH 3 CH 2 CH 2 CHO H + CH 3 CH 2 CH 2 C(=O)CH 3 CH 3 CH 2 CH 2 + CH 3 CHO + NO + O 2 E a = 13 kcal E a > 20 kcal E a = 17 kcal + OH

49. HCH 3 CH 2 CH 2 C(=O)CH 3 + H CH 3 CH 2 CH 2 C O  CH 3 CH 3 CH 2 CH 2 CHO + CH 3 CH 3 CHO + CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH(OH)CH 3 (Isomerization via 6-Member Transition State)

51. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH(  )CH 3 + H 2 O CH 3 CH 2 CH 2 CH(OO  )CH 3 CH 3 CH 2 CH 2 CH(O  )CH 3 + NO 2 CH 3 CH 2 CH 2 C(=O)CH 3 + HO 2 CH 3 CH 2 CH 2 + CH 3 CHO + NO + O 2 E a = 13 kcal  CH 2 CH 2 CH 2 CH(OH)CH 3 E a = 8 kcal CH 3 CH 2 CH 2 CH(OOH)CH 3 CH 3 CH 2 CH 2 CH(ONO 2 )CH 3 + HO 2 + NO + OH 1 2 3 4

52. 2-Pentoxy Chemistry vs. Altitude Reaction with O 2 Isomerization Methyl Elimination Propyl elimination

53. CH 3 CH 2 -O-CH 2 CH 3 CH 3 CH 2 -O-CH(  )CH 3 + H 2 O CH 3 CH 2 -O- CH(OO  )CH 3 CH 3 CH 2 -O-CH(O  )CH 3 + NO 2 CH 3 CH 2 -O-C(=O)CH 3 + HO 2 CH 3 + CH 3 CH 2 -O-CHO H + CH 3 CH 2 -O-C(=O)CH 3 CH 3 CH 2 O + CH 3 CHO + NO + O 2 E a = 7 kcal? E a ≤ 11 kcal? E a = 15 kcal? Orlando, 2007; Cheema et al., 1999; Wallington and Japar, 1991; Eberhard et al., 1993 DIETHYL ETHER + OH 4

54. CH 3 CH 2 -O-CH 2 CH 3 CH 3 CH 2 -O-CH(  )CH 3 + H 2 O CH 3 CH 2 -O- CH(OO  )CH 3 CH 3 CH 2 -O-CH(O  )CH 3 + NO 2 CH 3 CH 2 -O-C(=O)CH 3 + HO 2 CH 3 + CH 3 CH 2 -O-CHO H + CH 3 CH 2 -O-C(=O)CH 3 CH 3 CH 2 O + CH 3 CHO + NO + O 2 E a = 7 kcal? E a ≤ 11 kcal? E a = 15 kcal? Orlando, 2007; Cheema et al., 1999; Wallington and Japar, 1991; Eberhard et al., 1993 DIETHYL ETHER + OH [CH 3 CH 2 OCH(O  )CH 3 ] ‡ 4

55. 10-15 % 35-40 % CH 3 CH 2 OC(=O)CH 3 + H CH 3 CH 2 OCH=O + CH 3 deactivation (50%) CH 3 CH 2 OCH(O  )CH 3 dissoc., minor E A ~ 6 kcal, major + O 2 CH 3 CH 2 OC(=O)CH 3 + H CH 3 CH 2 OCH=O + CH 3 CH 3 CH 2 OC(=O)CH 3 + HO 2 [Orlando, 2007]

56. CHEMICAL ACTIVATION: About 20 occurrences documented ! (alkenes, halogenates, ketones, ethers, esters, even alkanes !!!)

57. SOME GENERALITIES ON ALKOXY RADICALS 1.There is almost always a reaction with O 2 to produce HO 2 and a carbonyl, time constant about 20  s. 2.There can be competing unimolecular reactions – decompositions and isomerizations. 3.Chemical activation might also be important (if barrier is low enough).

58. OK, Let’s step back a minute and review: We have a set of four reactions that occur for essentially every organic species. E.g., we saw methane (CH 4 ) get converted to CH 2 O. Also, pentane to 2-pentanone. CH 4 1.Starts with reaction with OH: OH CH 3 + H 2 O 2.The alkyl radical adds O 2, to make a peroxy radical. O 2 CH 3 O 2 3.Peroxy radical often reacts with NO, making an alkoxy NO radical. (There are other pathways, see later). CH 3 O + NO 2 4.Alkoxy radical reacts with O 2, to make a carbonyl O 2 compound. (There are other pathways, see later). CH 2 O + HO 2

59. OK, Let’s step back a minute and review: We have a set of four reactions that occur for essentially every organic species. E.g., we saw methane (CH 4 ) get converted to CH 2 O. Also, pentane to 2-pentanone. So, what happens to the CH 2 O, and to the 2-pentanone. Well, they go through the same processes: e.g., OH + CH 2 O  HCO + H 2 O HCO + O 2  HO 2 + CO

60. Figure V-B-10. Main routes in the OH-initiated oxidation mechanism of 2-pentanone under high NO x conditions. (From “The Mechanisms of Atmospheric Oxidation of the Oxygenates, J. Calvert et al., Oxford Univ. Press, 2011)

61. BUT, ONE OTHER THING CAN HAPPEN IN THE GAS-PHASE: Photolysis !! Because in general carbonyl compounds (species containing C=O double bonds) absorb near-UV photons !! From Sasha’s Lecture: Photolysis frequency (s -1 ) J =  F ( )  d

62. (From “The Mechanisms of Atmospheric Oxidation of the Oxygenates, J. Calvert et al., Oxford Univ. Press, 2011)

63. So, photolysis of CH 3 CHO to CH 3 and HCO occurs at a rate of about 10 -5 sec -1 for overhead sun. (From “The Mechanisms of Atmospheric Oxidation of the Oxygenates, J. Calvert et al., Oxford Univ. Press, 2011)

64. AND, ONE OTHER THING CAN HAPPEN :Deposition !! RECALL: We are converting an emitted hydrocarbon (say pentane, CH 3 CH 2 CH 2 CH 2 CH 3 ) to oxidized products, CH 3 CH 2 CH 2 C(=O)CH 3. As the process continues, the partially-oxidized products become increasingly SOLUBLE, and also LESS VOLATILE. So, they are more prone to uptake into clouds, into aqueous aerosols, to deposition to the ground, etc… Big issue these days: Formation of secondary organic aerosol !! Species like CH 3 (CH 2 ) 15 C(=O)CH 3 actually form aerosol !

65. OH HO 2 CH 4 CO 2 + H 2 O IsopreneOther by-products, such as O 3, particles, acids, DMS, NH 3 nitrates, etc. (2 ry POLLUTANTS) NO NO 2

66. OHHO 2 RO  RO 2  RR Parent NMHC In + Oxidized Species Out Nitrates, Peroxides Out NO, HO 2 O2O2 NO O2O2 Unimolecular Reaction OZONE PRODUCTION NO 2 HONO 2 OZONE PRODUCTION