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 k1 = (1.0 ± 0.1) × 109 L mol–1 s–1 at pH 4–5 and 9, k2 = (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 k3 = (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: k4 = (1.5 ± 0.2) × 108 L mol–1 s–1, k5 = (2.8 ± 0.5) × 108 L mol–1 s–1, and k6 = (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.