On the Cl-initiated degradation of selected ƒÑ,ƒÒ unsaturated compounds in the troposphere

MARIANA ACHAD, MARÍA B. BLANCO AND MARIANO A. TERUEL

Florianopolis , Brasil

Conferencia; 10th Latin American Conference on Physical Organic Chemistry (CLAFQO); 2009

£\,£]-unsaturated and  oxygenated volatile organic compounds (VOCs) are olefins that can be formed by the photooxidation of isoprene, terpenes and other alkenes from vegetation1,2 as well as from conjugated dienes emitted from the traffic road.3 Once in the atmosphere, the atmospheric fate of these oxygenated and unsaturated compounds includes photolysis and gas-phase reactions initiated by oxidants like OH and NO3 radicals, O3 molecules or Cl atoms in marine environments to produce ozone and other oxidants that can affect the oxidation capacity of the atmosphere.4  While there are many studies of OH radicals and Cl atom reactions with a variety of oxygenated volatile organic compounds (OVOCs), only limited information is available for £\,£]-unsaturated and oxygenated VOCs.5 The aim of this study was to extend the existing scant data base of the rate constants for the reactions of chlorine atoms with £\,£]-unsaturated and  oxygenated VOCs. In the present study we reported a kinetic study of the reaction between Cl atoms and ethyl vinyl ketone (k1), vinyl propionate (k2) and ethyl acrylate (k3):                  Cl + CH2=CHC(O)CH2CH3                           Products               (k1)                                                                   Cl + CH2=CHOC(O)CH2CH3                        Products               (k2)                                                                                       Cl + CH2=CHC(O)OCH2CH3                        Products               (k3)     Kinetic experiments at room temperature and atmospheric pressure of N2 were performed using a relative-rate technique with GC-FID analysis. Cl atoms were produced by the photolysis of Cl2.    The following values in units of cm3 molecule-1 s-1 of k1= (2.91¡Ó1.10) ¡Ñ 10-10, k2= (2.06¡Ó0.36) ¡Ñ 10-10 and k3= (2.53¡Ó0.46) ¡Ñ 10-10 were obtained. To the best of our knowledge, this work provides the first kinetic study for these reactions. On the other hand, our work aims to better define the reactivity of the oxygenated and unsaturated VOCs towards Cl atoms as an extension of previous work involving the Cl-initiated degradation of acrylic acid, acrylonitrile and acetates.6,7 Structure activity relationships (SARs) were developed for the reactions of Cl wide a wide range of unsaturated compounds. Kinetic data are used to estimate the atmospheric lifetimes for the studied oxygenated unsaturated VOCs. Lifetimes of few days were obtained for the Cl-initiated degradation of the unsaturated VOCs indicating their importance in the photochemical smog production in marine boundary layer.