Impact of heterogeneous uptake of nitrogen dioxide on the conversion of acetaldehyde on gamma-alumina in the absence and presence of simulated solar irradiation

Abstract

Heterogeneous reactions of various atmospheric gaseous pollutants on particle surfaces are important pathways for the formation of secondary aerosols. However, these heterogeneous reactions are inevitable affected by each other in the real atmosphere. In this study, the impacts of the heterogeneous uptake of NO2 on the conversion of acetaldehyde on the particle surfaces in the absence and presence of simulated solar irradiation were investigated by using diffuse reflectance infrared spectroscopy (DRIFTS) at 298 K. It is found that in the absence of simulated solar irradiation the hydration reaction and subsequent polyoligomeric reaction as well as oligomeric reaction of CH3CHO are promoted by the pre-adsorption of NO2 for a short reaction time, but these reactions are significantly hindered, accompanied with the slightly enhanced formation of crotonaldehyde and adsorbed acetate by the pre-adsorption of NO2 for a longer time. In the presence of simulated solar irradiation, the occurrence of the aldol condensation and the oxidation of CH3CHO are promoted by the combined impacts of illumination, hydrogen bond (H-bond) catalysis, the enhanced surface acidity and the formed nitrite and nitrate, especially, the photolysis of the formed nitrite and nitrate, whereas the hydration reaction and subsequent polyoligomeric reaction as well as oligomeric reaction of CH3CHO molecules are suppressed. Meanwhile, the hydrogen bond catalysis for aldol condensation is significantly enhanced by light irradiation, showing a combined result of hydrogen bond catalysis and illumination. The heterogeneous reaction mechanism is proposed, and atmospheric implications based on these results are discussed in this study.