Hydrogen Peroxide Enhances the Oxidation of Oxygenated Volatile Organic Compounds on Mineral Dust Particles: A Case Study of Methacrolein

摘要

Heterogeneous oxidation of oxygenated volatile organic compounds (OVOCs) serves as an important sink of OVOCs as well as a source of secondary organic material. However, the roles of gas phase oxidants in these reactions are poorly understood. In this work, we present the first laboratory study of the heterogeneous reactions of methacrolein (MACR) on various mineral dust particles in the presence of gaseous H_2O_2. It is found that the presence of gaseous H_2O_2 significantly promotes both the uptake and oxidation of MACR on kaolinite, α-Al_2O_3, α-Fe_2O_3, and TiO_2, but not on CaCO_3. The oxidation of MACR produces organic acids as its major low-molecular-weight product, whose yields are enhanced by a factor of 2-6 in the presence of H_2O_2. In addition, organic peroxides such as methyl hydroperoxide, peroxyfonnic acid, and peroxyacetic acid are only formed in the presence of H_2O_2, and the formation of methyl hydroperoxide indicates that MACR oxidation on the surface involves reaction with OH radicals. A probe reaction using salicylic acid verifies the production of OH radicals from H_2O_2 decomposition on kaolinite, α-Al_2O_3, α-Fe_2O_3, and TiO_2, which rationalizes the enhanced MACR oxidation observed on these particles. The uptake coefficients of MACR on kaolinite, α-Fe_2O_3, and TiO_2 in the presence of H_2O_2 are on the order of 10~(-5)-10~(-4). Our results provide new insights into the formation and chemical evolution of organic species in the atmosphere.