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.