Homework |
Photochemical Ozone Creation Potential |
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Reactions of hydrocarbons
The principal precursors of photochemical smog are the volatile organic compounds that arise from automobile use and, to a lesser extent, solvent evaporation from industry.
Hydrocarbons vary in terms of their reactivity in the atmosphere.
The rate of reaction with the hydroxyl radical is an important guide to the likely impact of a hydrocarbon in the atmosphere.
However, more recently hydrocarbons have been compared in terms of their photochemical ozone creation potential or POCP.
This parameter is far more complex to determine as it needs to include not only the initial reaction rate, but subsequent branching, eg as in the case of the reactions of ethene.
This is often dependent on many other constituents, so POCP determinations may involve numerous separate calculations.
I would like you to explore the following table of data.
The rate constants are rather old, so you might do best to register with the Master Chemical Mechanism in Leeds and extract rate constants for the relevant species.
Is possible to see the differences between ozone creation and OH reaction rates .
Can you explain the anomalies?
Again you may need to look at the MCM database to evaluate extreme deviations.
What about the question of eye irritancy?
What reactions do you think would be the most important indicators here and which would be the most important in the UK?
You may be able to make some estimate using the speciated Non-Methane VOC data from the National Atmospheric Emission Inventory (NAEI) if it is complete.
I have a printed copy if it does not seem to be on the web yet.
Individual Hydrocarbon Reactivity
Table 1: Concentrations (c) typical of Los Angeles, along with ozone produced in a smog chamber (hydrocarbon 3ppm NOx 1ppm) over the stated time and the rate constant (k) for reaction with OH
| Hydrocarbon | Conc.
ppb | O3
ppb | Time
min | k(OH)
10^-12s^-1 | Eye
Irritation |
| Methane | 3220 | 0 | --- | 0.0084 | |
| Butane | | | | 2.55 | 0 |
| Pentane | 35 | 180 | 100 | 4.1 | |
| Hexane | | | | 5.58 | 0 |
| Methylheptane | | | | 8.9? | 0.9 |
| 2-methylpentane | | 500 | 170 | 5.5 | |
| 3-methylpentane | 8 | 220 | 80 | 5.6 | |
| 2,2,4-trimethylpentane | | 260 | 80 | 3.66 | |
| 1-butene | | 580 | 45 | 31.4 | 1.3 |
| cis-2-butene | 1.2 | 550 | 35 | 56.1 | 1.6 |
| trans-2-butene | 1.4 | 730 | 35 | 63.7 | 2.3 |
| 1-propene | | | | 26.3 | 3.9 |
| 2-methyl-2-butene | 4 | 450 | 38 | 86.9 | 1.9 |
| 2-ethyl-1-butene | | 720 | 80 | | |
| 2,3-dimethyl-2-butene | | 640 | 70 | 110 | |
| 1,3-butadiene | 2 | 650 | 45 | 66.8 | 6.9 |
| 1-pentene | 2 | 620 | 45 | | |
| cyclohexene | | 450 | 35 | 67.4 | |
| isoprene | | 800 | 45 | 101 | |
| cyclohexane | | 180 | 80 | 7.38 | |
| benzene | | | | 7.57 | 3.1 |
| toluene | | | | 2.1 | 5.3 |
| ethylbenzene | | | | 7.5 | 4.3 |
| o-xylene | | | | 14.7 | 2.3 |
| m-xylene | | | | 24.5 | 2.9 |
| p-xylene | | | | 15.2 | 2.5 |
| propylbenzene | | | | 5.7 | 5.4 |
| iso-propylbenzene | | | | 6.6 | 1.6 |
REFERENCES:
- Concentrations: Los Angeles County Air Pollution Control District, Laboratory Data (1970-2).
- Smog chamber data: [i] USEPA Air Quality Criteria for Hydrocarbons , AP-64, (1970) [ii] Tuesday, C.S. (ed) Chemical Reactions in Urban Atmospheres, Elsevier, N.Y. (1971).
- Rate constants and eye irritancy: JNPitts BJFindlayson-Pitts Atmospheric Chemistry - Fundamentals and Experimental Techniques, Wiley (1986).