Open questions on rate constants for the reactions of OH with simple alkanes and aromatics at tropospheric temperatures

Cornelius Zetzscha,b)

a)University of Bayreuth, Atmos. Chem. Unit in BayCEER, Dr.-Hans-Frisch-Str. 1-3, 95448 Bayreuth, Germany

b)Max Planck Institute for Chemistry, Dep. of Multiphase Chem., Hahn-Meitner-Weg 1, 55128 Mainz, Germany

 Rate constants of simple alkanes and aromatics are an important data base for structure-reactivity relations for the understanding and prediction of rate constants, product distributions and tropospheric lifetimes of more complex molecules. Recommendations by Atkinson [1] cover a wide temperature range and mainly employ the modified Arrhenius equation with an additional T2-factor (accounting for the obvious curvature of the Arrhenius plots for the majority of the alkanes), whereas the updated review by Calvert et al. [2] and the subsequent application and compilation for the understanding of the formation of tropospheric ozone by Calvert et al. [3] employ the simple Arrhenius equation mainly, separately for high temperatures and for the atmospheric temperature range. The simple equation enables us to extrapolate data existing at room temperature and above to the lower range of tropospheric temperatures and delivers more trustworthy absolute reference values for relative rate constant measurements. The simple equation is also sufficient for the low-temperature range of the aromatics, where addition of OH prevails and the scatter is large. 

 A recent study on the simultaneous consumption of 17 hydrocarbons (the n-alkanes from n-butane to nonane, the 2,2-dimethylalkanes from 2,2-dimethylbutane to 2,2-dimethylhexane, the multibranched  alkanes 2,2,4-trimethylpentane and 2,2,3,3-tetramethylbutane, cyclooctane and the aromatics benzene, toluene, ethylbenzene and o- and p-xylene) in a smog chamber at Bayreuth by OH at 288 and 248 K [4] gives the opportunity to determine relative rate constants against a large number of selected reference compounds, where absolute rate constants can be taken from individual studies or from recommendations of Atkinson (2003) or Calvert et al. (2008 and 2015).

 Further to minor limitations of the experiments (three runs at 288K, 2 runs at 248 K), the evaluation demonstrates huge gaps of existing data in this temperature range, leading to a large uncertainty of the absolute rate constants and activation energies derived by taking either lower alkanes (n-butane or n-pentane), higher alkanes (n-heptane or n-octane) or the aromatics benzene or toluene or several compounds as simultaneous reference. The expected linear increase of rate constants of the n-alkanes and the 2,2-dimethylalkanes with chain length and a comparison with existing data with 25 compounds at room temperature present additional test criteria for the final results.

[1] Atkinson, R. Atmos. Chem. Phys, 3, 2233–2307 (2003).

[2] Calvert, J. G.; Derwent, R. G.; Orlando, J. J.; Tyndall, G. S.; Wallington, T. J. Mechanisms of Atmospheric Oxidation of the Alkanes; Oxford University Press (2008).

[3] Calvert, J.G., Orlando, J.J., Stockwell, W.R., Wallington, T.J. The Mechanisms of Reactions Influencing Atmospheric Ozone; Oxford University Press (2015).

[4] Han, L, Siekmann, F., Zetzsch, C., Atmosphere 9, 320 (2018).