Reactivity of chlorine radical with submicron palmitic acid particles: kinetic measurements and product identification

Abstract. The heterogeneous reaction of Cl^• radicals with submicron palmitic acid (PA) particles was studied in an aerosol flow tube in the presence or in the absence of O₂. Fine particles were generated by homogeneous condensation of PA vapours and introduced into the reactor, where chlorine atoms were produced by photolysis of Cl₂ using UV lamps surrounding the reactor. The effective reactive uptake coefficient (γ) has been determined from the rate loss of PA measured by gas chromatography–mass spectrometer (GC/MS) analysis of reacted particles as a function of the chlorine exposure. In the absence of O₂, γ = 14 ± 5 indicates efficient secondary chemistry involving Cl₂. GC/MS analysis has shown the formation of monochlorinated and polychlorinated compounds in the oxidized particles. Although the PA particles are solid, the complete mass can be consumed. In the presence of oxygen, the reaction is still dominated by secondary chemistry but the propagation chain length is smaller than in the absence of O₂, which leads to an uptake coefficient γ = 3 ± 1. In the particulate phase, oxocarboxylic acids and dicarboxylic acids were identified by GC/MS. The formation of alcohols and monocarboxylic acids is also suspected. A reaction pathway for the main products and more functionalized species is proposed. All these results show that solid organic particles could be efficiently oxidized by gas-phase radicals not only on their surface but also in bulk by mechanisms which are still unclear. They help to understand the aging of primary tropospheric aerosol containing fatty acids.