Kinetics and Mechanisms of Aqueous-Phase Reactions of Triplet-State Imidazole-2-carboxaldehyde and 3,4-Dimethoxybenzaldehyde with α,β-Unsaturated Carbonyl Compounds

Abstract

Reactions in the atmospheric aqueous phase are an important source of secondary organic aerosols (SOA). Within the present study, the reactions of triplet-state imidazole-2-carboxaldehyde (³2-IC*) with methyl vinyl ketone (MVK, R1), methacrolein (MACR, R2), and methacrylic acid (MAA, R3), as well as the reaction of triplet-state 3,4-dimethoxybenzaldehyde (³DMB*) with the unsaturated compounds (MVK, R4), (MACR, R5), and (MAA, R6), in the aqueous phase were investigated using laser flash excitation-laser long path absorption and ultraperformance liquid chromatography coupled with high definition electrospray ionization spectrometry. The second-order reaction constants for ³2-IC* were determined to be <i>k</i>₁ = (1.0 ± 0.1) × 10⁹ L mol^-1 s^-1 at pH 4-5 and 9, <i>k</i>₂ = (1.4 ± 0.4) × 10⁹ L mol^-1 s^-1 and (1.5 ± 0.1) × 10⁹ L mol^-1 s^-1 at pH 4-5 and 9, and <i>k</i>₃ = (1.4 ± 0.4) × 10⁹ L mol^-1 s^-1 and (1.1 ± 0.4) × 10⁸ L mol^-1 s^-1 at pH 4-5 and 9, respectively. The main products of the [2 + 2] photocycloaddition reactions of ³2-IC* with both monomer and dimer of MVK as well as MACR were characterized. Similarly, the [2 + 2] photocycloaddition of the carbonyl of the excited triplet state of 3,4-dimethoxybenzaldehyde (³DMB*) with MVK was observed. The second order rate constants for the reactions of ³DMB* were determined: <i>k</i>₄ = (1.5 ± 0.2) × 10⁸ L mol^-1 s^-1, <i>k</i>₅ = (2.8 ± 0.5) × 10⁸ L mol^-1 s^-1, and <i>k</i>₆ = (5.2 ± 1.2) × 10⁶ L mol^-1 s^-1 at pH 9. The studied reactions show that different triplet photosensitizers react with strongly varying rate constants. Advanced CAPRAM process model studies show that active photosensitizers such as ³DMB* can quickly react with unsaturated organic compounds under deliquesced aerosol conditions modifying SOA, while the quenching with oxygen dominates the excited photosensitizer loss under cloud conditions.