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

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.

Aromatic Carbonyl and Nitro Compounds as Photosensitizers and Their Photophysical Properties in the Tropospheric Aqueous Phase.

Secondary organic aerosol formation in the atmospheric aqueous/particulate phase by photosensitized reactions is currently subject to uncertainties. To understand the impact of photosensitized reactions, photophysical and -chemical properties of photosensitizers, kinetic data, and reaction mechanisms of these processes are required. The photophysical properties of acetophenones, benzaldehydes, benzophenones, and naphthalenes were investigated in aqueous solution using laser flash excitation. Quantum yields of excited photosensitizers were determined giving values between 0.06-0.80 at 298 K and pH = 5. Molar absorption coefficients (εmax(3PS*) = (0.8-13) × 104 L mol-1 cm-1), decay rate constants in water (k1st = (9.4 ± 0.5) × 102 to (2.2 ± 0.1) × 105 s-1), and quenching rate constants with oxygen (kq(O2) = (1.7 ± 0.1-4.4 ± 0.4) × 109 L mol-1 s-1) of the excited triplet states were determined at 298 K and pH = 5. Photosensitized reactions of carboxylic acids and alkenes show second-order rate constants in the range of (37 ± 7.0-0.55 ± 0.1) × 104 and (27 ± 5.0-0.04 ± 0.01) × 108 L mol-1 s-1. The results show that different compound classes act differently as a photosensitizer and can be a sink for certain organic compounds in the atmospheric aqueous phase.

Five-Membered Heterocycles as Potential Photosensitizers in the Tropospheric Aqueous Phase: Photophysical Properties of Imidazole-2-carboxaldehyde, 2-Furaldehyde, and 2-Acetylfuran.

Photosensitized reactions of organic compounds in the atmospheric aqueous and particle phase might be potential sources for secondary organic aerosol (SOA) formation, addressed as aqueous SOA. However, data regarding the photophysical properties of photosensitizers, their kinetics, as well as reaction mechanisms of such processes in the aqueous/particle phase are scarce. The present study investigates the determination of the photophysical properties of imidazole-2-carboxaldehyde, 2-furaldehyde, and 2-acetylfuran as potential photosensitizers using laser flash excitation in aqueous solution. Quantum yields of the formation of the excited photosensitizers were obtained by a scavenging method with thiocyanate, resulting in values between 0.86 and 0.96 at 298 K and pH = 5. The time-resolved absorbance spectra of the excited photosensitizers were measured, and their molar attenuation coefficients were determined ranging between (0.30 and 1.4) × 104 L mol-1 cm-1 at their absorbance maxima (λmax = 335-440 nm). Additionally, the excited photosensitizers are quenched by water and molecular oxygen, resulting in quenching rate constants of k1st = (1.0 ± 0.2-1.8 ± 0.2) × 105 s-1 and kq(O2) = (2.1 ± 0.2-2.7 ± 0.2) × 109 L mol-1 s-1, respectively.

OH-Initiated Oxidation of Imidazoles in Tropospheric Aqueous-Phase Chemistry.

Imidazoles formed via the reaction of dicarbonyls with nitrogen containing compounds in the atmospheric particle phase can be expected to initiate secondary organic aerosol (SOA) growth due to their potential to act as photosensitizers. Recent field studies quantified imidazoles for the first time in ambient aerosol samples from Europe and China. However, knowledge about the kinetics and mechanisms of particle-phase reactions involving imidazoles is still very limited. In the present study, the radical-driven aqueous-phase oxidative degradation reactions of the hydroxyl radical (·OH) with imidazoles were investigated. For the imidazoles, the following rate constants at 298 K and acidic conditions are obtained as k(imidazole-2-carboxaldehyde) = (1.8 ± 0.1) × 109 L mol-1 s-1, k(1-methylimidazole) = (2.3 ± 0.1) × 109 L mol-1 s-1, k(2-methylimidazole) = (3.9 ± 0.1) × 109 L mol-1 s-1, k(4-methylimidazole) = (3.2 ± 0.2) × 109 L mol-1 s-1, k(1-ethylimidazole) = (2.6 ± 0.1) × 109 L mol-1 s1, and k(2-ethylimidazole) = (3.9 ± 0.1) × 109 L mol-1 s-1. Temperature and pH dependencies of the reactions as well as the activation parameters have been determined and discussed. The possible implications and restrictions for imidazoles acting as photosensitizers in tropospheric particles have been considered.