Chlorine-driven photochemical mechanisms in secondary organic aerosol formation /growth: Insights from trans-2-pentenal oxidation

Abstract

Understanding the formation and growth of secondary organic aerosols (SOA) from chemical reactions requires detailed knowledge of reaction products and their formation pathways. In this study, we investigated the reaction mechanisms and kinetics of chlorine-initiated trans-2-pentenal (T2P) reactions, including the decomposition of key intermediates and complexes, using the M06-2X/6-311+G(2df,2p) and CBS-QB3 computational methods. Thermodynamic and kinetic parameters were computed at the M06-2X level of theory, enabling the prediction of thermochemical properties for T2P, chlorine atoms, and the reaction products. For selected pathways, rate constants were determined over a temperature range of 300–3000 K at atmospheric pressure using conventional transition-state theory (CTST) and Rice–Ramsperger–Kassel–Marcus (RRKM) theory. At room temperature, the P2, P3, and P6 adducts emerged as the dominant kinetic products, with calculated rate constants aligning well with experimental data. At room temperature (300 K), the main kinetic products identified were P2 (CH₃CH₂CH(Cl)CHCHO), P3 (CH₃CHCHCHCHO + HCl), and P6 (CH₂CH₂CHCHCHO + HCl). The calculated rate constants for these products were 2.91 × 10⁻¹⁰, 1.71 × 10⁻¹⁰, and 2.78 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ for P2, P3, and P6, respectively, which agree well with experimental observations. Dynamic modeling showed that these adducts contribute significantly to SOA formation, with particle growth up to 50 μg m⁻³ within 1 h at 300 K. Additionally, volatility trends of the products were analyzed to understand their atmospheric persistence and contribution to aerosol mass. Finally, leveraging insights from dynamic studies, we examined SOA formation from the relevant adducts, observing growth within 1 h at 300 K. We also discuss SOA formation in the context of volatility and its broader atmospheric implications.