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REVIEW: Geoff showed something about the types of compounds: CH 4 CH 3 -CH(CH 3 ) 2

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

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REVIEW: Geoff showed something about the types of compounds: CH 4 CH 3 -CH(CH 3 ) 2

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  1. ATMOSPHERIC CHEMISTRYOF ORGANIC COMPOUNDSLecture for NC A&T (part 2)March 9, 2011John Orlandoorlando@ucar.edu

  2. REVIEW: Geoff showed something about the types of compounds: CH4 CH3-CH(CH3)2 CH3-CH=CH-CH3 CH3CH2CH2C(=O)CH3 CH3CH2CH2OH CH3CH2-O-CH2CH3

  3. REVIEW:Where they come from: Biogenic sources the largest – isoprene, terpenes,etc. Isoprene CH2=CH-C(CH3)=CH2 But also anthropogenic emissions, mostly the types of things we just saw on the previous page (fossil fuel combustion, industrial…) Alkanes Alkenes Alcohols Etc. 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 HO2 CH4 CO2 + H2O Isoprene Other by-products, such as O3, particles, acids, DMS, NH3 nitrates, etc. (2ry POLLUTANTS) NO NO2

  8. THE TROPOSPHERIC “ENGINE”: Now the “Odd Hydrogen” Family:Consider first OH and HO2: Production: O3 + hn O(1D) + O2 O(1D) + H2O  OH + OH Conversion of OH to HO2: OH + CO (+O2)  HO2 + CO2 dominant (when all ‘fuel’ considered) OH + O3 HO2 + O2, usually minor Conversion of HO2 back to OH: HO2 + O3 OH + 2 O2 HO2 + NO  OH + NO2, (followed by NO2 + hn  NO + O, O + O2 + M  O3 + M, which generates O3 !!) Losses of HOx via two processes: HO2 + HO2 + M  HOOH + O2 + M OH + NO2 + M = HNO3 + 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, CH4). • CH4

  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, CH4). • CH4 • Starts with reaction with OH: OH • CH3 + H2O

  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, CH4). • CH4 • Starts with reaction with OH: OH • CH3 + H2O • The alkyl radical adds O2, to make a peroxy radical. O2 • CH3O2

  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, CH4). • CH4 • Starts with reaction with OH: OH • CH3 + H2O • The alkyl radical adds O2, to make a peroxy radical. O2 • CH3O2 • Peroxy radical often reacts with NO, making an alkoxy NO • radical. (There are other pathways, see later). • CH3O + NO2

  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, CH4). • CH4 • Starts with reaction with OH: OH • CH3 + H2O • The alkyl radical adds O2, to make a peroxy radical. O2 • CH3O2 • Peroxy radical often reacts with NO, making an alkoxy NO • radical. (There are other pathways, see later). • CH3O + NO2 • 4. Alkoxy radical reacts with O2, to make a carbonyl O2 • compound. (There are other pathways, see later). • CH2O + HO2

  14. 1 CH3CH2CH2CH2CH3 + OH CH3CH2CH2CH()CH3+ H2O 2 + O2 3 CH3CH2CH2CH(OO)CH3 + NO CH3CH2CH2CH(O)CH3+ NO2 4 + O2 CH3CH2CH2C(=O)CH3+ HO2

  15. 1 CH3CH2CH2CH2CH3 IN GENERAL, REFER TO THE PARENT COMPOUND AS R-H + OH CH3CH2CH2CH()CH3+ H2O 2 REFER TO THE ALKYL RADICAL AS R• + O2 3 REFER TO THE PEROXY RADICAL AS RO2• CH3CH2CH2CH(OO)CH3 REFER TO THE ALKOXY RADICAL AS RO• + NO CH3CH2CH2CH(O)CH3+ NO2 4 + O2 NOTE ALSO: THESE BASIC REACTIONS PROPOGATE RADICALS !! We will refer to this again from time to time, noting that other pathways DO NOT PROPOGATE CH3CH2CH2C(=O)CH3+ HO2

  16. 1 CH3CH2CH2CH2CH3 + OH CH3CH2CH2CH()CH3+ H2O 2 + O2 3 Ea = 13 kcal CH3CH2CH2CH(OO)CH3 Ea = 8 kcal + NO CH3CH2CH2 + CH3CHO CH3CH2CH2CH(O)CH3 + NO2 4 CH2CH2CH2CH(OH)CH3 + HO2 + NO + O2 3b CH3CH2CH2C(=O)CH3 + HO2 CH3CH2CH2CH(OOH)CH3 CH3CH2CH2CH(ONO2)CH3

  17. OK, LET’S START WITH STEP #1 • – REACTION OF OH WITH HYDROCARBONS • (Also applies to NO3, and Cl-atoms) • CAN HAVE TWO KINDS OF REACTIONS – • ABSTRACTION: • OH + CH4 •CH3 + H2O • - Occurs when the hydrocarbon is “saturated” (no double bonds) • ADDITION: • OH + CH2=CH2 HOCH2-CH2•

  18. OK, LET’S START WITH STEP #1 – REACTION OF OH WITH HYDROCARBONS (Also applies to NO3, and Cl-atoms) Go back to our old friend, OH + Methane (CH4) REACTION DOES NOT OCCUR ON EVERY COLLISION!!! Ea k = A * exp(-Ea/RT) A is the pre-exponential factor, and accounts for the geometry limitations. Ea is activation energy. From Wikipedia

  19. REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.) ELEMENTARY REACTIONS (BIMOLECULAR) k = A * exp(-Ea/RT) So, Let’s go back to the OH / CH4 reaction. IF REACTION OCCURRED ON EVERY COLLISION, k = 2 x 10-10 cm3 molecule-1 s-1 Turns out that k = 2.45 x 10-12 * exp(- 3525 cal / RT) k = 6.3 x 10-15 cm3 molecule-1 s-1 at 298 K k = 5.2 x 10-16 cm3 molecule-1 s-1 at 210 K Only about 1 in 30000 OH/CH4 collisions results in reaction at 298 K.

  20. FOR OH + CH4: [ HO…H-CH3 ] Ea = 3525 calories OH + CH4 DHr = - 13900 calories HOH + CH3

  21. FOR OH + CH4: FOR OH + C2H6: (CH3-CH3) [ HO…H-CH3 ] Ea = 3525 calories Ea = 2100 calories OH + CH4 DHr = - 13900 calories OH + CH3-CH3 DHr = - 17800 calories HOH + CH3 HOH + CH3-CH2

  22. SO, IN GENERAL: The more substituted (complicated) the molecule, the weaker the C-H bond, and the faster the rate coefficient n-PENTANE: CH3CH2CH2CH2CH3 DIETHYL ETHER : CH3CH2-O-CH2CH3 2-PROPANOL:CH3CH(OH)CH3 2-PENTANONE: CH3CH2C(=O)CH2CH3

  23. 400 ppt 200 ppt 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 cm3 molecule-1 s-1. (From Calvert et al., Mechanisms of the Atmospheric Oxidation of the Alkanes, OUP, 2008)

  24. 50 ppt < 1 ppt 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 cm3 molecule-1 s-1. (From Calvert et al., Mechanisms of the Atmospheric Oxidation of the Alkanes, OUP, 2008)

  25. THERE ARE OTHER OXIDANTS BESIDES OH: - One of the them is the “NITRATE RADICAL”, NO3 - Photolyzes rapidly, so only active at nighttime. - Can abstract, though energetics not as favorable. As an example, OH + Isobutane (C4H10)  •C(CH3)3 + H2O k = 7.0  10-12 exp(-350/T) cm3 molecule-1 s-1 NO3 + Isobutane (C4H10)  •C(CH3)3 + H2O k = 3.9  10-12 exp(-3150/T) cm3 molecule-1 s-1

  26. Figure III-F-1. Plots of logarithm of the rate coefficients (cm3 molecule-1 s-1) for reaction of Cl, O(3P) and NO3 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, NO3, Cl-atoms too: Generally very fast reactions:OH + CH2=CH2 (ethene)  HOCH2-CH2• For OH + ethene, k = 8.1  10-12 cm3 molecule-1 s-1 Ethene lifetime  1.5 days = = = = Again, more substituted species react even faster. k(OH + isoprene) = 1.0  10-10 cm3 molecule-1 s-1 Isoprene lifetime  (1-2) hours

  28. Generally, when multiple choices, addition will win over abstraction. CH3CH2-CH=CH-CH(CH3)2

  29. Generally, when multiple choices, addition will win over abstraction. CH3CH2-CH=CH-CH(CH3)2 Addition reaction wins, k  6  10-11 cm3 molecule-1 s-1 Abstraction reactions, k  3  10-12 cm3 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 O3 + CH2=CH2 CH2 CH2  CH2=O + •CH2-OO• O Chemistry of Criegee radicals is complex (and not totally understood): •CH2-OO• undergoes numerous types of reactions that form CO, CO2, 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 (NO3, Cl, O3) How does it work? First: Assign ‘starting values’ for reaction of OH with a –CH3 group, and –CH2- group, and a –CH< group (298 K): k(-CH3) = 1.36  10-13 cm3 molecule-1 s-1 k(-CH2-) = 9.34  10-13 cm3 molecule-1 s-1 k(-CH<) = 19.4  10-13 cm3 molecule-1 s-1

  32. MODIFY THE INITIAL VALUE ACCORDING TO WHAT IS BONDED TO IT (“Substituent factors”) CH3– X k = k(-CH3) * F(X) Y – CH2– X k = k(-CH2-) * F(X) * F(Y) Y – CH – X k = k(-CH<) * F(X) * F(Y) * F(Z) Z

  33. CONSIDER PROPANOL: HO – CH2 – CH2CH3k = k(CH2) * F(X) * F(Y) k(-CH2-) = 9.34  10-13 cm3 molecule-1 s-1 F(-OH) = 4.0 F(-CH2CH3) = 1.23 So, estimated k for reaction at the one particular -CH2- group is: k = k(-CH2-) * F(X) * F(Y) = 9.34  10-13 cm3 molecule-1 s-1* (4.0)* (1.23) = 4.2  10-12 cm3 molecule-1 s-1

  34. Generally, when multiple choices, addition will win over abstraction. CH3CH2-CH=CH-CH(CH3)2 Addition reaction wins, k  6  10-11 cm3 molecule-1 s-1 Abstraction reactions, k  3  10-12 cm3 molecule-1 s-1

  35. 1 CH3CH2CH2CH2CH3 + OH CH3CH2CH2CH()CH3+ H2O OK, READY FOR STEP #2 2 + O2 3 CH3CH2CH2CH(OO)CH3 + NO CH3CH2CH2CH(O)CH3+ NO2 4 + O2 CH3CH2CH2C(=O)CH3+ HO2

  36. No worries, this one is EASY PEASY LEMON SQUEEZY Take alkyl radical, e.g., CH3-CH2• And add O2, CH3-CH2 + O2 + M  CH3-CH2O2 + M Voila, instant peroxy radical !! Typical k = 7 x 10-12 cm3 molecule-1 s-1 [O2] = 5 x 1018 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.

  37. 1 CH3CH2CH2CH2CH3 + OH CH3CH2CH2CH()CH3+ H2O OK, ON TO STEP #3 !!! 2 + O2 3 CH3CH2CH2CH(OO)CH3 + NO CH3CH2CH2CH(O)CH3+ NO2 4 + O2 CH3CH2CH2C(=O)CH3+ HO2

  38. 3 PEROXY RADICAL CHEMISTRY MAIN REACTION IS WITH NO, CONVERTS PEROXY TO ALKOXY RADICAL NO Reaction (MAIN PATHWAY): RO2 + NO  RO + NO2 CH3O2 + NO  CH3O + NO2 This reaction propogates radicals.

  39. 3 PEROXY RADICAL CHEMISTRY MAIN REACTION IS WITH NO, CONVERTS PEROXY TO ALKOXY RADICAL NO Reaction (MAIN PATHWAY): RO2 + NO  RO + NO2 CH3O2 + NO  CH3O + NO2 This reaction propogates radicals. BUT, ALSO ANOTHER MINOR CHANNEL THAT COMPETES: RO2 + NO  RONO2 CH3O2 + NO  CH3ONO2 CH3CH2CH2CH(OO)CH3 + NO CH3CH2CH2CH(ONO2)CH3 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!

  40. Rate coefficient independent of structure, all k  10-11 cm3 molecule-1 s-1 So what are typical lifetimes for an RO2 (peroxy) radical in the real world (Earth’s surface)? [NO] (pptv) LOCATION Approx. RO2 LIFETIME 5 Very remote regions 800 sec. 1000 Rural conditions 4 sec. 100000 Mexico City (e.g.) 0.04 sec.

  41. 3 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 HO2, main channel RO2 + HO2 ROOH + O2 Radical termination.

  42. 1 CH3CH2CH2CH2CH3 + OH CH3CH2CH2CH()CH3+ H2O 2 + O2 3 Ea = 13 kcal CH3CH2CH2CH(OO)CH3 Ea = 8 kcal + NO CH3CH2CH2 + CH3CHO CH3CH2CH2CH(O)CH3 + NO2 4 CH2CH2CH2CH(OH)CH3 + HO2 + NO + O2 3b CH3CH2CH2C(=O)CH3 + HO2 CH3CH2CH2CH(OOH)CH3 CH3CH2CH2CH(ONO2)CH3

  43. RATE CONSTANTS FOR REACTION OF PEROXY RADICALS WITH HO2 (Boyd et al., JPCA, 107, 818, 2003) Similar values to RO2 + NO reactions.

  44. 1 CH3CH2CH2CH2CH3 + OH CH3CH2CH2CH()CH3+ H2O OK, ON TO STEP #4, WE CAN DO IT !!! 2 + O2 3 CH3CH2CH2CH(OO)CH3 + NO CH3CH2CH2CH(O)CH3+ NO2 4 + O2 CH3CH2CH2C(=O)CH3+ HO2

  45. 4 ALKOXY RADICAL CHEMISTRY MAIN REACTION IS WITH O2, CONVERTS ALKOXY RADICAL TO A CARBONYL COMPOUND, ALSO GET HO2 (a peroxy radical) formed. PROPOGATION!! CH3O + O2 CH2O + HO2 CH3CH2CH2CH(O)CH3 + O2 CH3CH2CH2C(=O)CH3 + HO2 Rate coefficient typically about 10-14 cm3 molecule-1 s-1 So lifetime is about 20 ms For larger alkoxy radicals, like 2-pentoxy, can have competing reactions: Decomposition

  46. 4 H CH3CH2CH2C O CH3CH2CH2C(=O)CH3 + H CH3 CH3CH2CH2CHO + CH3 CH3CHO + CH3CH2CH2 (Baldwin et al., 1977; Choo and Benson, 1981; Atkinson, 1999) Energyk = 5e13 * exp (-Ea/RT) sec-1

  47. CH3CH2CH2CH2CH3 + OH CH3CH2CH2CH()CH3 + H2O + O2 Ea > 20 kcal Ea = 17 kcal H + CH3CH2CH2C(=O)CH3 CH3 +CH3CH2CH2CHO CH3CH2CH2CH(OO)CH3 Ea = 13 kcal + NO CH3CH2CH2 + CH3CHO CH3CH2CH2CH(O)CH3 + NO2 + O2 CH3CH2CH2C(=O)CH3 + HO2

  48. H CH3CH2CH2C(=O)CH3 + H CH3CH2CH2C O CH3 CH3CH2CH2CHO + CH3 CH3CHO + CH3CH2CH2 •CH2CH2CH2CH(OH)CH3 (Isomerization via 6-Member Transition State)

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