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.