Heterogeneous Reactions of C₃H₆/O₃/NH₃ on α-Fe₂O₃ Particles: The Effects of Relative Humidity on Secondary Organic Aerosol Formation

Heterogeneous reactions of volatile organic compounds (VOCs) with mineral dust can influence secondary aerosol formation in the atmosphere. This study investigated the heterogeneous reactions of C₃H₆/O₃ in the presence of NH₃ on α-Fe₂O₃ particles using a gas-flow system combined with Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) under simulated different relative humidity (RH) conditions. The results indicate that a secondary organic aerosol (SOA) is formed from the appearance of η²(N,O) bands at 1341 cm^-1 and R-COOH bands at 1476 and 1608 cm^-1. C₃H₆ reacts with O₃ to form the Criegee intermediates (CIs) such as CH₃HCOO and CH₂OO. Some of these CIs are neutralized with NH₃ to form amides, while others react with aldehydes to form intermediates that finally decompose into carboxylic acids. Water molecules play negative potential roles in the formation of SOA on α-Fe₂O₃ particles. Under continuous RH changes, the formation rate of organic amides increases from 1.07 × 10¹⁶ to 2.73 × 10¹⁶ ions·g^-1·s^-1, while the formation rate of carboxylic acids increases from 9.39 × 10¹⁵ to 2.11 × 10¹⁶ ions·g^-1·s^-1 with decreasing RH in the range of 80-0%. Furthermore, the reactive uptake coefficient for NH₃ in the initial stage increases from (4.85 ± 0.35) × 10^-10 to (2.08 ± 0.03) × 10^-9 with decreasing RH from 77.4% to 0% during the heterogeneous reaction of C₃H₆/O₃/NH₃ on α-Fe₂O₃ particles under constant RH conditions. The inhibiting effect under high RHs indicates that water molecules are unfavorable for SOA formation due to the consumption of reactant gases and the competitive effect on active sites of α-Fe₂O₃ particles. These results not only significantly enhance the understanding of the reaction kinetics of C₃H₆/O₃/NH₃ on the surface of α-Fe₂O₃ particles under different RHs but also provide theoretical insights into SOA formation mechanisms under atmospheric pollution conditions.