Abstract
Abstract. The multivariate relative rate method was applied to a range of volatile organic compound (VOC) reactions with OH. This previously published method (Shaw et al., 2018) was improved to increase the sensitivity towards more slowly reacting VOCs, broadening the range of compounds which can be examined. A total of 35 room temperature relative rate coefficients were determined, eight of which have not previously been reported. Five of the new reaction rate coefficients were for large alkyl substituted mono-aromatic species recently identified in urban air masses, likely with large ozone production potentials. The new results (with kOH (296 K) values in units of 10−12 cm3 molec.−1 s−1) were n-butylbenzene, 11 (±4); n-pentylbenzene, 7 (±2); 1,2-diethylbenzene, 14 (±4); 1,3-diethylbenzene, 22 (±4); and 1,4-diethylbenzene, 16 (±4). Interestingly, whilst results for smaller VOCs agreed well with available structure–activity relationship (SAR) calculations, the larger alkyl benzenes were found to be less reactive than the SAR prediction, indicating that our understanding of the oxidation chemistry of these compounds is still limited. kOH (296 K) rate coefficients (in units of 10−12 cm3 molec.−1 s−1) for reactions of three large alkanes with OH were also determined for the first time: 2-methylheptane, 9.1 (±0.3); 2-methylnonane, 11.0 (±0.3); and ethylcyclohexane, 14.4 (±0.3), all in reasonable agreement with SAR predictions. Rate coefficients for the 27 previously studied OH + VOC reactions agreed well with available literature values, lending confidence to the application of this method for the rapid and efficient simultaneous study of gas-phase reaction kinetics.
Highlights
The troposphere contains many thousands of organic compounds (Lewis et al, 2000; Goldstein and Galbally, 2007)
Volatile organic compounds (VOCs) are organic chemicals which take part in photochemical degradation cycles leading to the formation of ground-level ozone and photochemical smog and the production of secondary organic aerosols – significantly impacting upon air quality and climate
Aromatic VOCs can account for a large proportion of the non-methane hydrocarbon mass in the atmosphere
Summary
The troposphere contains many thousands of organic compounds (Lewis et al, 2000; Goldstein and Galbally, 2007). These compounds differ substantially in their chemical and physical properties. Aromatic VOCs can account for a large proportion of the non-methane hydrocarbon mass in the atmosphere. Summed concentrations of 11 aromatic compounds were measured to be 30 ppbv in Yokohama City (Tiwari et al, 2010). In Mumbai, the total concentration of five aromatic compounds was measured to be 27 (±8) ppbv in Mumbai (Pandit et al, 2011). Peak concentrations of aromatic VOCs can be much greater, ; Dunmore et al (2015) measured daily wintertime peaks for mono-aromatics containing four carbon atoms as part of alkyl substituents
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