Abstract
Irradiance reference spectra are used to calculate environmentally relevant photodegradation half-lives, but the currently used spectra were originally published in the 1980s with limited validation. The goal of this work is to provide updated irradiance reference spectra using the Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS). The SMARTS irradiance spectra were validated against measurements from several high-resolution spectroradiometers, and the updated irradiance reference spectra use current measurements for atmospheric species that can affect the irradiance that reaches the Earth's surface. These updated irradiance spectra are provided in 1 nm increments from 280 to 800 nm for 0° to 70° latitude at 10° increments in both the northern and southern hemisphere. Lastly, the influence of the input parameters on the modeled irradiance spectra was investigated. This work will allow users to calculate more accurate photodegradation half-lives using the updated irradiance reference spectra, and it also provides insight for users to calculate their own location- and time-specific irradiance spectra using SMARTS.
Highlights
In contrast to laboratory conditions, the irradiance that reaches the surface of the Earth can vary substantially based on the geographic location, altitude, time of day, time of year, and the concentration of atmospheric gases and particles. Because of this variability in irradiance in the environment, reference solar irradiance spectra are typically used to calculate environmentally relevant photochemical half-lives for chemicals in surface waters. Both the EPA guideline (OPPTS 835.2210)[9] and the OECD guideline (316)[10] recommend using the day-averaged solar irradiance values for a clear sky day originally published in The Kinetics of Environmental Aquatic Photochemistry by Leifer (1988).[11]
The total irradiance values derived from Ll values were 17–23% higher in the UVB region, 22–29% higher in the UVA region, and 7–12% higher in the PAR region compared to the SMARTS irradiance values
Some of this difference may be due to accounting for the increased pathlength in water that is done with Ll values, but this should be at least partially offset by accounting for the fraction of light that is re ected off the water surface
Summary
Pollutants can be degraded or transformed by photochemical reactions in the aquatic environment.[1,2,3,4,5] These reactions can either proceed through direct photodegradation, where the pollutant absorbs light and undergoes reaction, or through indirect photoreaction, where other chemical species absorb light and subsequently undergo reaction with the pollutant.[6,7,8] In both cases, the rate of pollutant degradation or transformation is dependent on the ux of photons that reach Earth's surface, which is called the solar irradiance or the photon uence rate.In laboratory studies, the photodegradation of chemicals is usually investigated using a well-characterized light source. The relative irradiance can be measured using an Because of this variability in irradiance in the environment, reference solar irradiance spectra are typically used to calculate environmentally relevant photochemical half-lives for chemicals in surface waters. Both the EPA guideline (OPPTS 835.2210)[9] and the OECD guideline (316)[10] recommend using the day-averaged solar irradiance values for a clear sky (i.e., cloudless) day originally published in The Kinetics of Environmental Aquatic Photochemistry by Leifer (1988).[11] These values, denoted as Ll, represent the irradiance averaged over a 24 hour day and were an extension of the irradiance values previously published by Zepp and Cline (1977), which were for solar noon and denoted as either Wl or Zl.[12]
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