A model to predict the kinetics of direct (endogenous) virus inactivation by sunlight at different latitudes and seasons, based on the equivalent monochromatic wavelength approach

Wavelength-dependent direct and indirect photochemical transformations of organic pollutants

Sunlight-induced photochemical transformations greatly affect the persistence of organic pollutants in natural environment. Whereas sunlight intensity is well-known to affect pollutant phototransformation rates, the reliance of pollutant phototransformation kinetics on sunlight spectrum remains poorly understood, which may greatly vary under different spatial-temporal, water matrix, and climatic conditions. Here, we systematically assessed the wavelength-dependent direct and indirect phototransformations of 12 organic pollutants. Their phototransformation rates dramatically decreased with light wavelength increasing from 375 to 632 nm, with direct photolysis displaying higher wavelength-dependence than indirect photolysis. Remarkably, UV light dominated both direct (90.4-99.5 %) and indirect (64.6-98.7 %) photochemical transformations of all investigated organic pollutants, despite its minor portion in sunlight spectrum (e.g., 6.5 % on March 20 at the equator). Based on wavelength-dependent rate constant spectrum, the predicted phototransformation rate of chloramphenicol (4.5 ± 0.7 × 10-4 s-1) agreed well with the observed rate under outdoor sunlight irradiation (4.3 ± 0.0 × 10-4 s-1), and there is no significant difference between the predicted rate and the observed rate (p-value = 0.132). Moreover, rate constant and quantum yield coefficient (QYC) spectrum could be applied for facilely investigate the influence of spectral changes on the phototransformation of pollutants under varying spatial-temporal (e.g., season, latitude) and climatic conditions (e.g., cloud cover). Our study highlights the wavelength-dependence of both direct and indirect phototransformation of pollutants, and the UV part of natural sunlight plays a decisive role in the phototransformation of pollutants.

Global modeling of lake-water indirect photochemistry based on the equivalent monochromatic wavelength approximation: The case of the triplet states of chromophoric dissolved organic matter

Chromophoric dissolved organic matter (CDOM) plays key role as photosensitizer in sunlit surface-water environments, and it is deeply involved in the photodegradation of contaminants. It has recently been shown that sunlight absorption by CDOM can be conveniently approximated based on its monochromatic absorption at 560 nm. Here we show that such an approximation allows for the assessment of CDOM photoreactions on a wide global scale and, particularly, in the latitude belt between 60°S and 60°N. Global lake databases are currently incomplete as far as water chemistry is concerned, but estimates of the content of organic matter are available. With such data it is possible to assess global steady-state concentrations of CDOM triplet states (³CDOM*), which are predicted to reach particularly high values at Nordic latitudes during summer, due to a combination of high sunlight irradiance and elevated content of organic matter. For the first time to our knowledge, we are able to model an indirect photochemistry process in inland waters around the globe. Implications are discussed for the phototransformation of a contaminant that is mainly degraded by reaction with ³CDOM* (clofibric acid, lipid regulator metabolite), and for the formation of known products on a wide geographic scale.

Direct photolysis of contaminants in surface freshwaters, within the equivalent monochromatic wavelength (EMW) approximation

Abiotic photochemical reactions are usually very important degradation pathways for biorecalcitrant pollutants in surface freshwaters. Therefore, the assessment of photolytic lifetimes of contaminants helps estimate their impact on aquatic systems. This is commonly carried out by combining irradiation experiments and modelling, where the latter considers mathematical functions with polychromatic parameters, such as sunlight spectra, photolysis quantum yields (when Kasha's rule does not hold), and absorption coefficients. With the polychromatic approach, the photolytic lifetime is calculated by solving several integrals, which requires quite demanding modelling resources. In this work, we applied a recently developed approach, which is based on the equivalent monochromatic wavelength (EMW) approximation, to compute the direct-photolysis lifetimes of a range of >40 pollutants in inland waters. The EMW approximation allowed for easier modelling procedure, at the same time providing very good agreement with the polychromatic system. To further show EMW potentialities, lifetimes of three contaminants were mapped over the Piedmont region (NW Italy), as an example of how easy it becomes to geographically EMW-assess the potential of watercourses, to get photochemically decontaminated from pollutants.