A Novel Method for Measuring Photokinetic Parameters Under Variable Natural and Artificial Light Regimes

Photolysis is an important process involved in both the transformation of anthropogenic contaminants as well as biogeochemical reactions such as the degradation of dissolved organic matter. While many environmental photochemistry studies are conducted using simulated solar light where the irradiance is relatively constant, experiments conducted in natural sunlight pose many challenges. Changing light regimes due to seasonal variations, times of day, cloud cover, and as a function of latitude constitute a considerable challenge in the evaluation of photokinetics between separate experiments. These changes in photo-irradiance as a function of time and space precludes the direct comparison for kinetic data measured in simulated relative to natural sunlight. Traditionally, chemical actinometers are used to determine the quantum yields of the reaction (i.e. moles of transformed compound normalized to total photons absorbed by the system), allowing the comparison between experiments conducted under varying light regimes. Yet this method is of limited utility in the evaluation of reaction kinetics and cannot account for changes in light conditions throughout a single experiment, i.e. cloud cover and sun angle. Further, the use of a large range of different actinometers poses additional challenges when undertaking inter-laboratory comparisons of photokinetics. We have developed a novel method using a PMA2100 data-logging radiometer (Solar Light Company), which allows us to track the accumulated UVA and UVB dose throughout each experiment. The accumulated dose at each time point is then normalized to the target UV irradiance, which accounts for changes in light intensity. Using the p-nitroanisole/pyridine and p-nitroacetophenone/pyridine actinometers as probes, we demonstrate that this method is robust and can accommodate for changes in cloud cover and sun angle. Our approach can be used to directly compare experiments conducted under both natural and simulated sunlight. After initial establishment of proper radiometer calibration using previously established chemical actinometers, our method may be used for the direct comparison of photolytic kinetics measured between different laboratories. We believe that this simple and elegant method for the normalization of photodegradation kinetics serves as an important bridge between laboratory and field experimentation and supports wider applicability of experimental results to a variety of environments.