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.