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 ((3)2-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 ((DMB)-D-3*) 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 (3)2-IC* were determined to be k(1) = (1.0 +/- 0.1) x 10(9) L mol(-1) s(-1) at pH 4-5 and 9, k(2) = (1.4 +/- 0.4) x 10(9) L mol(-1) s(-1) and (1.5 +/- 0.1) x 10(9) L mol(-1) s(-1) at pH 4-5 and 9, and k(3) = (1.4 +/- 0.4) x 10(9) L mol(-1) s(-1) and (1.1 +/- 0.4) x 10(8) L mol(-1) s(-1) at pH 4-5 and 9, respectively. The main products of the [2 + 2] photocycloaddition reactions of (3)2-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 ((DMB)-D-3*) with MVK was observed. The second order rate constants for the reactions of (DMB)-D-3* were determined: k(4) = (1.5 +/- 0.2) x 10(8) L mol(-1) s(-1), k(5) = (2.8 +/- 0.5) x 10(8) L mol(-1) s(-1), and k(6) = (5.2 +/- 1.2) x 10(6) 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 (DMB)-D-3* 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.
