Abstract
Abstract. Gas-phase rate coefficients are fundamental to understanding atmospheric chemistry, yet experimental data are not available for the oxidation reactions of many of the thousands of volatile organic compounds (VOCs) observed in the troposphere. Here, a new experimental method is reported for the simultaneous study of reactions between multiple different VOCs and OH, the most important daytime atmospheric radical oxidant. This technique is based upon established relative rate concepts but has the advantage of a much higher throughput of target VOCs. By evaluating multiple VOCs in each experiment, and through measurement of the depletion in each VOC after reaction with OH, the OH + VOC reaction rate coefficients can be derived. Results from experiments conducted under controlled laboratory conditions were in good agreement with the available literature for the reaction of 19 VOCs, prepared in synthetic gas mixtures, with OH. This approach was used to determine a rate coefficient for the reaction of OH with 2,3-dimethylpent-1-ene for the first time; k = 5.7 (±0.3) × 10−11 cm3 molecule−1 s−1. In addition, a further seven VOCs had only two, or fewer, individual OH rate coefficient measurements available in the literature. The results from this work were in good agreement with those measurements. A similar dataset, at an elevated temperature of 323 (±10) K, was used to determine new OH rate coefficients for 12 aromatic, 5 alkane, 5 alkene and 3 monoterpene VOC + OH reactions. In OH relative reactivity experiments that used ambient air at the University of York, a large number of different VOCs were observed, of which 23 were positively identified. Due to difficulties with detection limits and fully resolving peaks, only 19 OH rate coefficients were derived from these ambient air samples, including 10 reactions for which data were previously unavailable at the elevated reaction temperature of T = 323 (±10) K.
Highlights
The atmosphere is an extremely complex and reactive mixture containing large numbers of inorganic and organic chemicals (Lewis et al, 2000; Goldstein and Galbally, 2007)
We have demonstrated here a simple and versatile method for measuring relative rate coefficients for reactions of OH with multiple volatile organic compounds (VOCs) simultaneously, based upon the wellestablished relative rate kinetic technique
Three synthetic gas mixtures have been used with this technique, covering a broad range in VOC functionality and in rates of reaction with OH radicals
Summary
The atmosphere is an extremely complex and reactive mixture containing large numbers of inorganic and organic chemicals (Lewis et al, 2000; Goldstein and Galbally, 2007). The relative rate method requires neither an accurate knowledge of VOC concentration, nor direct monitoring of OH radicals (Atkinson, 1986) This technique instead relies on the simultaneous measurement of two species: a target VOC that is the focus of the investigation, and a well-characterised reference compound, which ideally possess similar rate coefficients for reaction with OH. The ratio of their depletion allows for the calculation of the target rate coefficient, providing the OH + reference VOC rate coefficient is accurately known and the losses of both compounds in the reactive system are solely governed by reactions with OH Both absolute and relative rate methods have been used to obtain rate coefficients for one OH + VOC reaction at a time; as a result, these methods are time consuming and few laboratories are capable of sustaining this type of fundamental science. Viously unmeasured OH + VOC reactions determined at two reaction temperatures
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