Development and validation of a new experimental set-up to study reactions between peroxy RO2 and HOx radicals

 Kravtchenkoa, L. Pilliera, S. Batuta, B. Calimetb and C. Fittschena

a) Laboratoire de Physico-Chimie des Processus de Combustion et de l'Atmosphère, PC2A, Université de Lille

b) Centre d’Etudes et de Recherches Lasers et Applications, CERLA, Université de Lille

In the atmosphere, organic pollutants such as Volatile Organic Compounds (VOCs) from biogenic or anthropogenic sources are photochemically oxidized and lead to the formation of peroxy radicals such as hydroperoxy HO2 and alkylperoxy RO2, which play a major role in tropospheric chemistry. The reactivity of these radicals controls the oxidative capacity of the atmosphere and the formation of tropospheric ozone and secondary pollutants. However, it is still poorly known and subject to controversy in the literature, especially in clean environments containing low NOx concentrations (remote regions: marine boundary layer or tropical forest).

The reaction between RO2 and HO2 radicals can lead to termination (R.1a: radicals sink) or propagation (R.1c: formation of new OH radicals) pathways [1]:

RO2 + HO2 → ROOH + O2             (R.1a)

RO2 + HO2 → ROH + O3 (R.1b)

RO2 + HO2 → RO + OH + O2       (R.1c)

If reactions with simple alkylperoxy radicals (R= CH3, C2H5) are well known, significant disparities appear in the literature for more complex RO2 radicals. Moreover, very recently, a new reaction pathway has been suggested as possible fate of RO2 radicals in clean environments: the reaction with OH radicals. Only few studies exist on this class of reactions and to date, they are not included in atmospheric chemistry models. Then, there is clearly a need for new experimental studies of RO2 + HOx (OH, HO2) reactions to improve our knowledge of this class of reactions and their atmospheric implication.

The aim of the present work is the development of a new experimental device to study reactions between RO2 and HOx (rate constants and branching ratios measurements). The new setup consists of a fast flow tube (10-30 m.s-1) coupled to three complementary techniques:

  • Laser Induced Fluorescence (LIF) for in-situ OH radicals measurement,
  • continuous wave Cavity Ring-Down Spectroscopy (cw-CRDS) for HO2 radicals measurement,
  • Mass Spectrometry with Molecular Beam sampling (MB/MS) for measurement of stable reaction products and radical species.

We will present the validation of each individual technique and of the overall system through the study of different reactions between alkanes (ethane, propane) + OH, alcohol (methanol) + OH and CO + OH.

[1] Hasson, A. S., Tyndall, G. S., Orlando, J. J, Singh, S., Hernandez, S.Q, Campbell, S. & Ibarra, Y . Branching Ratios for the Reaction of Selected Carbonyl-Containing Peroxy Radicals with Hydroperoxy Radicals. The Journal of  Physical Chemistry A 116, 6264–6281 (2012)

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