Tropospheric Photochemistry Problems:

1. OH is a key reactant to the photochemical production of ozone. (13 pts)

a) Give the two reactions by which it forms in the troposphere including photolysis.

O₃ + hn → O₂ + O(1D) [or + O*]

O(1D) [or + O*] + H₂O → 2OH

b) In its oxidation reactions (e.g. with CO or hydrocarbons), it will typically give rise to another (relatively stable) radical species. Give an example reaction – list a reactant that reacts with OH and the first product that forms which is O₂ stable (will not react with O₂). Note that there are usually two reactions before the O₂ stable radical forms.

reactants:

1) CO + OH → HOCO

HOCO + O₂ → HO₂ + CO₂ [only need to give CO and HO₂]

2) CH₄ + OH → CH₃ + H₂O

CH₃ + O₂ → CH₃O₂[only need to give CH₄ + CH₃O₂]

3) CH₃CH₂CH₃ + OH → CH₃CHCH₃ + H₂O

CH₃CHCH₃ + O₂ → CH₃CO₂CH₃[only need to give CH₃CH₂CH₃ + CH₃O₂]

c) Using words or equations, explain how the O₂ stable radical contributes to the formation of ozone.

We can use RO₂ to represent any of the peroxy radicals above. The reactions leading to ozone formation are:

1) RO₂ + NO → RO + NO₂

2) NO₂ + hn → NO + O

3) O + O₂ + M →O₃ + M

or in words, peroxy radicals oxidize nitric oxide to nitrogen dioxide, which photolyzes to produce oxygen atoms that can form ozone.

2. A Colorado air pollution control scientist is worried that butane emissions are increasing because of their use in hash oil production (it can be used as a solvent for marijuana extraction). If we assume that the butane emissions are well quantified and that vehicle exhaust is the other big hydrocarbon source (and also well quantified), describe how can we determine if butane emissions are significantly contributing to ozone production. You can further assume that an “average” vehicle exhaust molecule is 1-hexene and that the reaction of butane and 1-hexene with OH is a rate limiting step toward reaction initiation. What other information might be needed to make this assessment? (7 pts)

The scientist would first need to know if the pollution problem that he is looking at is NO_x or VOC limited. If it is NO_x limited, the additional butane emissions could be insignificant for ozone production even if they significantly increase the VOC load. Secondly, if conditions (somewhere) are VOC limiting, one would want to determine if butane would produce a significant increase in ozone. To do this the scientist should look at both emissions (moles or mass emitted per unit time) and their reaction rates. Because butane is an alkane, it will react more slowly than 1-hexene (the surrogate for uncombusted gasoline). He also would want to know the OH reaction rates for both butane and 1-hexene to decide if the butane is significant relative to vehicle emissions.