Heterogeneous reactions of methacrolein and methyl vinyl ketone: Kinetics and mechanisms of uptake and ozonolysis on silicon dioxide

Abstract

Methacrolein (MAC) and methyl vinyl ketone (MVK), two major first‐generation products in the oxidation of isoprene, play important roles in tropospheric chemistry. However, little is known about their heterogeneous fate. Here we investigated the heterogeneous reactions of MAC and MVK on particles of silicon dioxide (SiO2), the major constituent of mineral dust in the troposphere, under simulated tropospheric conditions. We first investigated the adsorption and desorption processes. It was found that MAC and MVK molecules were adsorbed onto the surface of SiO2 particles by van der Waals forces and hydrogen bonding forces in a non‐reactive state, and the presence of water vapor did not result in the formation of new substances but could decrease the adsorption ability by consuming isolated hydroxyl groups on the surface of SiO2 particles. The initial adsorption and desorption rates, initial uptake coefficients, and adsorption concentrations at equilibrium were determined at different relative humidities. Notably, in the desorption process, a considerable amount of MAC or MVK molecules remained on SiO2 particles in dry air but were almost completely desorbed in high‐humid air. We also investigated the heterogeneous ozonolysis of MAC and MVK adsorbed onto SiO2 particles, determining product yields at different relative humidities. The heterogeneous ozonolysis of MAC and MVK adsorbed onto SiO2 particles yielded formaldehyde and methylglyoxal as the major secondary carbonyl products and formic acid and acetic acid as the major organic acid products, as in their gas‐phase ozonolysis. However, the yield of two major organic peroxides, methyl hydroperoxide and hydroxymethyl hydroperoxide, was much greater in their heterogeneous ozonolysis than in their gas‐phase ozonolysis. The mechanisms of heterogeneous ozonolysis of MAC and MVK onto the SiO2 surface are deduced.