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 k
1 = (1.0 ± 0.1) × 109 L
mol–1 s–1 at pH 4–5 and
9, k
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 k
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: k
4 = (1.5 ± 0.2) × 108 L mol–1 s–1, k
5 = (2.8 ±
0.5) × 108 L mol–1 s–1, and k
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.