A Fluence-Based Method for the Direct Comparison of Photolysis Kinetics under Variable Light Regimes

Abstract

Chemical actinometers are traditionally used to account for photochemical experiments conducted under different light regimes (simulated vs natural; also seasonal, daytime, cloud cover, and latitude changes). Their many limitations and the lack of a universally applicable actinometer demand development of a new approach for studying environmentally relevant photochemical processes in sunlight. We suggest the use of fluence-based rate constants (converted to time-based rate constants and half-lives with irradiance normalization), using a datalogging radiometer to track the accumulated dose of UVA and UVB radiation. Our results suggest that this method can effectively account for minor changes in cloud cover and sun angle in the photolysis of p-nitroanisole/pyridine and p-nitroacetophenone/pyridine. The greatest error is caused by factors (e.g., dense cloud cover, extreme sun angles, and changes in ozone) that affect relative UVA and UVB fluence. We believe that this simple and elegant method serves as an important bridge between laboratory and fieldbased environmental photochemistry experiments.

p-Nitroanisole/Pyridine and p-Nitroacetophenone/Pyridine Actinometers Revisited: Quantum Yield in Comparison to Ferrioxalate

“…In practice, the stringency of these requirements depends on how accurately the spectral distribution of the radiation is quantified and the available equipment. For example, identical time scales are unnecessary if radiometric monitoring and correction can be calibrated to actinometer reaction rates . Nonetheless, actinometry is most reliable when it meets the stated conditions.…”

“…For example, identical time scales are unnecessary if radiometric monitoring and correction can be calibrated to actinometer reaction rates. 7 Nonetheless, actinometry is most reliable when it meets the stated conditions. Dulin and Mill introduced two actinometers based on photonucleophilic substitution of excited state nitrobenzene derivatives by pyridine.…”

Self-Accelerated Photodegradation of 2,4-Dihydroxybenzophenone in Water: Formation of Photoactive Products and Implications for the Transformation of Coexisting Organic Contaminants

“…5 The half-life time of 2-hydroxy-4methoxybenzophenone-5-sulfonic acid (BP4) was approximately 15 days under constant irradiation equivalent to noon midsummer sunlight. 6 The bare BP backbone without the adjacent hydroxyl group and the intramolecular hydrogen bond, however, is incapable of releasing the light energy in the form of heat to return to its low-energy state after excitation under UV irradiation. Instead, it tends to transfer the extra energy to other molecules, such as dissolved oxygen and other organic compounds in water.…”

“…It was reported that 2-hydroxy-4-methoxybenzophenone (BP3) was only degraded by 4% after continuous irradiation for 4 weeks in aqueous solution . The half-life time of 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid (BP4) was approximately 15 days under constant irradiation equivalent to noon midsummer sunlight …”