Mechanism development

1. OH yields from O₃ + alkene reactions

The OH radical is the major oxidant of volatile organic compounds (VOCs) in the atmosphere. Ozone alkene reactions are a non-photolytic source of HOx radicals and appear to be a significant source of new HOx radicals in urban and rural (especially forested) air. The dark reactions between O₃ and trans-2-butene and isoprene have been studied in HIRAC. Only results from the O₃+isoprene study will be discussed here but further information about the O₃+trans-2-butene results have been published by Glowacki et al. (2007).

Pressure dependent (100 – 1000 mbar) OH yields from O₃+isoprene have been measured both directly and indirectly in HIRAC.

Indirect measurements

Scavenger technique: Using cyclohexane as an OH scavenger, the yield of cyclohexanone was measured using GC-FID along with the concurrent loss rate of isoprene.

Kinetic technique: Kinetics studies using cyclohexane. A 10 fold excess O₃ over isoprene was used to ensure that the decay of isoprene was pseudo-first order. The consumption of isoprene was monitored using both FTIR and GC-FID.

Direct measurements

OH and HO₂ yields were measured using FAGE for chamber pressures between 300 and 1000 mbar. A simple kinetic model based on a subset of the Master Chemical Mechanism (MCM) was used to model the OH and HO₂ concentrations and hence determine pressure dependent OH and HO₂ yields.

Pressure dependent OH yields determined by indirect and direct techniques.

Direct FAGE OH measurements p = 1000 mbar (red) with modelled data (black) giving YOH = 0.26±0.02.

2. Product formation yields from O₃ + isoprene reactions

Recently, more investigations were performed studying the temperature dependence of the OH and HO₂ yields from the isoprene + O₃ reactions over the range of temperature 0 – 70 °C. The work is in progress and preliminary results were presented at International Symposium on Gas Kinetic, Szeged, 2014 (Bejan et al., 2014, poster).

Product formation yields of methacrolein and methylvinylketone as the main stable products formed in the ozonolysis of isoprene have been recently investigated using the facilities offered by HIRAC chamber (wide range of temperature, FTIR, GC).

Preliminary results are presented below for one of the products investigated.

3. OH Production in the Reaction of CH₃C(O)O₂ + HO₂

One of the ‘hot topics’ in atmospheric chemistry is the observation of much higher than expected levels of the OH radical in tropical forested environments. As the Tropics are the region where most of the greenhouse gas methane is removed via reaction with OH, it is vital to have accurate measurements of OH concentrations in these regions. Interferences with the measurements could explain some of the observations and using HIRAC to calibrate and characterise FAGE (OH and HO₂ measurements) is important part of the work we do.

Unknown chemistry could also be contributing to OH production and one postulated mechanism is the reaction of certain RO₂ with HO₂ which had previously been considered as a radical sink. Indirect studies in 2004 by Hassoon et al. suggested a substantial OH yield from the title reaction, but this remained controversial. However, more recently Dillon and Crowley positively identified OH. This study is however, the first time in which products from all three channels of the reaction have been positively identified.

HO₂ and OH are followed using FAGE, peracetic and acetic acid via FTIR and O₃ via the calibrated O₃ box. Below are some product profiles. Our best estimates of the yields are:  α_R3a = 0.35 ± 0.03, α_R3b = 0.12 ± 0.02 and α_R3c = 0.54 ± 0.09. This work has been published in Atmospheric Chemistry and Physics in 2016. For more information see the poster section.

CH₃C(O)OOH as a function of Δ[CH₃CHO] for [CH₃OH]0:[CH₃CHO]0 ≈ 3.8 in air at 1000 mbar and 293 K. Good agreement was observed between experimental data and the chemical model for all datasets with the above yields. All uncertainties quoted to ± 1σ

CH₃C(O)OH as a function of Δ[CH₃CHO] for [CH₃OH]0:[CH₃CHO]0 ≈ 3.8 in air at 1000 mbar and 293 K. Good agreement was observed between experimental data and the chemical model for all datasets with the above yields. All uncertainties quoted to ± 1σ

O₃ as a function of Δ[CH₃CHO] for [CH₃OH]0:[CH₃CHO]0 ≈ 3.8 in air at 1000 mbar and 293 K. Good agreement was observed between experimental data and the chemical model for all datasets with the above yields. All uncertainties quoted to ± 1σ

4. HO₂ + Carbonyls and Subsequent Reactions

An important secondary process in the CH₃C(O)OO + HO₂ is the reaction of HO₂ with formaldehyde. This process is reversible with complex formation increasing with lower temperatures:

HO₂  +  HCHO  ↔  HOCH₂O₂

Reaction of this peroxy radical with HO₂ can also lead to OH production.

HO₂  +  HCHO  ↔  HOCH₂O₂

Preliminary studies have observed direct OH production and the figure below shows the dramatic increase in secondary HCOOH (formic acid) production as the temperature is lowered. The temperature control available to HIRAC is critical to these studies.