1. Workshop on Air Quality Data Analysis and Interpretation Role of Hydrocarbon Oxidation in Ozone Formation

2. Atmospheric Oxidants  OH and O 3 are the major daytime oxidants, while NO 3 may contribute at night. HO 2 reacts rapidly with aldehydes.  Sources of OH O 3 + hv  O( 1 D) + O 2 O( 1 D) + H 2 O  OH + OH HNO 2 + hv  OH + NO H 2 O 2 + hv  OH + OH  These are net sources of radicals.

3. HO 2 to OH Conversion  The largest source of OH is made from HO 2 + NO  OH + NO 2  When the ambient NO concentration is greater than 10 pptv, the HO 2 radical reaction with NO is an important source of OH, but not new radicals.

4. Sources of Other Radicals  Sources of NO 3 radicals NO 2 + O 3  NO 3 + O 2  This is the only significant source. Because NO 3 photolysis is so rapid, the chemistry of NO 3 is only significant at night.  Sources of HO 2 H + O 2 + M  HO 2 + M HCO + O 2  HO 2 + CO As we have seen earlier.

5. Tropospheric lifetimes of organics  We will characterize the speed of the oxidation reactions with organics by examining the tropospheric lifetimes in the presence of various oxidants. A + Ox  Products k ox τ A = 1/(k ox [Ox])  The lifetime is the time required for A to decay to 1/e (base of natural log e=2.718…) of its initial concentration, at fixed Ox concentration.

6. Oxidation Reactions of Organics

7. OH Reactions with Alkanes Alkanek (in 10 -12 cm 3 molecule -1 s -1 ) methane0.00618 ethane0.254 propane1.12 n-butane2.44 2-methylpropane2.19 n-pentane4.0 2-methylbutane3.7 2,2-dimethylpropane0.85 n-hexane5.45 2-methylpentane5.3 3-methylpentane5.4

8. Discussion of OH – Alkanes  The rate constants for most of these reactions are a few x 10 -12 cm 3 molecule -1 s -1. The notable exceptions are methane about 400 times slower and ethane about 10 times slower than the typical reactions with alkanes. OH + RH  R + H 2 O  The reason we talk about NMHC: Methane is so much less reactive than other HC Methane concentrations are relatively high, about 1.5 ppm

9. Alkyl and Alkylperoxy Radicals Alkyl Radical Reactions R + O 2 (+ M)  RO 2 k  (0.8 - 2) x 10 -11 cm 3 molecule -1 s -1 τ R = 1/(k[O2]) = 1/{(1 x 10 -11 )(5 x 10 18 )}  2 x 10 -8 s Alkylperoxy Radical Reactions  Alkylperoxy radicals do not react with O 2. RO 2 + NO  RO + NO 2 k  8 x 10 -12 cm 3 molecule -1 s -1  If the NO concentration is about 1 ppb, then τ RO2  5 s

10. Alkoxy Radical Reactions  Small alkoxy radicals react with O 2, RCH 2 O + O 2  RCHO + HO 2  The rate constant for primary alkoxy radicals is about 9.5 x 10 -15 and for the secondary alkoxy radical is about 8 x 10 -15.  Larger alkoxy radicals either decompose or isomerize.

11. OH Reactions with Alkenes Alkenek (in 10 -12 cm 3 molecule -1 s -1 ) ethene8.52 propene26.3 1-butene31.4 cis-2-butene56.4 trans-2-butene64.0 2-methylpropene51.4 1-pentene31.4 cis-2-pentene65 trans-2-pentene67 cyclopentene67 3-methyl-1-butene31.8

12. Discussion of OH - Alkene  The rate constants for most of these reactions are a few x 10 -11 cm 3 molecule -1 s -1. These are about 10 times faster than the OH reactions with alkanes. OH + H 2 C=CH 2  H 2 C-CH 2 OH  The OH radical adds to one end of the double bond, forming a hydroxy substituted alkyl radical, that will react similarly to the other alkyl radical reactions discussed above.

13. O 3 Reactions with Alkenes Alkenek (in 10 -18 cm 3 molecule -1 s -1 ) ethene1.6 propene10.1 1-butene9.64 2-methylpropene11.3 cis-2-butene125 trans-2-butene190 1-pentene10.0 cyclopentene570 2-methyl-2-butene403 1-hexene11.0 cyclohexene81.4

14. Discussion of O 3 – Alkene Reactions  O 3 can also react with alkenes at a significant rate.  There are no significant reactions between O 3 and alkanes.  Lifetimes of alkenes in the atmosphere can vary from a few hours for the reactive ones to several days for ethene.  The mechanism of the reactions are complicated, but many lead to net OH formation and smaller organic molecular fragments.

15. OH Yields

16. OH Reactions with Aromatics Aromaticsk (in 10 -12 cm 3 molecule -1 s -1 ) benzene1.2 toluene6.0 ethylbenzene7.1 o-xylene13.7 m-xylene23.6 p-xylene14.3 n-propylbenzene6.0 i-propylbenzene6.5 o-ethyltoluene12.3 m-ethyltoluene19.2 p-ethyltoluene12.1

17. Discussion of OH - Aromatics  The mechanism of OH reactions with aromatics is quite complicated, and still not fully understood.  At normal ambient temperatures it is believed that OH reacts by addition to one of the aromatic double bonds in the molecule.  Many of the oxidation products are the result of breaking the ring structure.

18. Atmospheric Oxidation of Hydrocarbons  The atmospheric oxidation of hydrocarbons, whether initiated by OH, O 3, NO 3, HO 2, etc. lead to significant radical formation.  These radicals all participate in the chain hydrocarbon oxidation processes, and most lead to increased ozone production.  The chain of hydrocarbon oxidation is slowed by “chain-terminating” reactions, radical recombination reactions, such as OH + NO 2 → HNO 3

19. Biogenic Hydrocarbons

20. Importance of Biogenics in Air Pollution Chemistry  It is expected that there is a significant contribution of the oxidation of biogenics to the formation of ozone and secondary organic aerosols.  Our quantitative understanding of the significance is limited. The role of biogenics has not been adequately quantified.