Abstract
A clear–sky method to estimate the photosynthetically active radiation (PAR) at the surface level in cloudless atmospheres is presented and validated. It uses a fast and accurate approximation adopted in several radiative transfer models, known as the k-distribution method and the correlated-k approximation, which gives a set of fluxes accumulated over 32 established wavelength intervals. A resampling technique, followed by a summation, are applied over the wavelength range [0.4, 0.7] µm in order to retrieve the PAR fluxes. The method uses as inputs the total column contents of ozone and water vapor, and optical properties of aerosols provided by the Copernicus Atmosphere Monitoring Service. To validate the method, its outcomes were compared to instantaneous global photosynthetic photon flux density (PPFD) measurements acquired at seven experimental sites of the Surface Radiation Budget Network (SURFRAD) located in various climates in the USA. The bias lies in the interval [−12, 61] µmol m−2 s−1 ([−1, 5] % in values relative to the means of the measurements at each station). The root mean square error ranges between 37 µmol m−2 s−1 (3%) and 82 µmol m−2 s−1 (6%). The squared correlation coefficient fluctuates from 0.97 to 0.99. This comparison demonstrates the high level of accuracy of the presented method, which offers an accurate estimate of PAR fluxes in cloudless atmospheres at high spatial and temporal resolutions useful for several bio geophysical models.
Highlights
Active radiation (PAR) is the part of solar radiation which lies in the wavelength range of [0.4, 0.7] μm
The squared correlation coefficient fluctuates from 0.97 to 0.99. This comparison demonstrates the high level of accuracy of the presented method, which offers an accurate estimate of Photosynthetically active radiation (PAR) fluxes in cloudless atmospheres at high spatial and temporal resolutions useful for several bio geophysical models
When the PAR estimates from the technique were compared against the values obtained by the detailed spectral calculations of libRadtran, Wandji Nyamsi et al [30] found no systematic bias as a function of any inputs
Summary
Active radiation (PAR) is the part of solar radiation which lies in the wavelength range of [0.4, 0.7] μm. PAR is the incident power per unit surface area; its unit is W m−2. PAR is a measure of the amount of photons per time unit per surface unit, called the photosynthetic photon flux density (PPFD), whose unit is μmol m−2 s−1. The widely used approximation of McCree (1972) [4] relates the PAR and the PPFD: 1 W m−2 ≈ 4.57 μmol m−2 s−1. Researchers and other specialists in ecophysiological, agricultural, and bio-geophysical domains demand high quality estimates of PAR and of its direct and diffuse components. Both components summed together give the global PAR. The diffuse and direct components have diverse effects on the Atmosphere 2019, 10, 219; doi:10.3390/atmos10040219 www.mdpi.com/journal/atmosphere
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