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Photosynthetically active radiation (PAR; wavelength 400–700 nm) is the spectral range of solar radiation used by plants for photosynthesis. It is necessary to use PAR data in various fields, such as the surface vegetation growth, light energy–chemical energy conversion process, and terrestrial carbon cycle. In this study, a direct normal irradiance (DNI) spectroradiometer installed at the Korea Institute of Energy Research was used to observe direct insolation over the whole wavelength ranges including PAR. The variabilities in solar radiation and wavelength sensitivity according to the solar zenith angle and the clearness index (an indicator of atmospheric turbidity), were analyzed using data collected during June–October, 2020. As the optical path increased, PAR DNI—having a short wavelength—showed greater attenuation than the near-infrared radiation (NIR). Additionally, an increase in the clearness index under the same optical path increased the PAR DNI relatively more. These results mean that the attenuation effect of atmospheric constituents under optical path can be different depending on the wavelength range. Based on the empirical model presented in previous studies and the observation data in Daejeon, the PAR DNI estimation model using DNI in the entire wavelength range were developed and evaluated. Although it was a short analysis period, each model showed an R of 90% or more, and the model with the clearness index, an indicator of atmospheric conditions, presented higher estimation accuracy. We expect that further data collection will enhance the accuracy of the model despite seasonal variability. Thus, this research will encourage broader applications of the model in other regions.

Development of an Empirical Model for Photosynthetically Active Radiation Direct Normal Irradiance using Spectral Direct Normal Irradiance in Daejeon