A new method to estimate aerosol radiative forcing on photosynthetically active radiation

Abstract

A new method to estimate aerosol radiative forcing (ARF) on photosynthetically active radiation (PAR) is proposed using as input only the solar position and global irradiance measurements, available in many radiometric stations worldwide. The main contribution of this work is the proposal of a new and simple tool (max-kt method) based in the parameterization of the envelope of the relationship between clearness index (kt) and the solar position. To this aim, a 1-year database (2020) of cloud-free data acquired in a Southwest Mediterranean site was used for the proposal and two more years (2017 and 2018) were used to extend the results. The ARF values retrieved using the new method were compared with estimates calculated by two physical models widely employed in the literature, such as the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer model) and an improved version of the CPCR2 (Code for Physical Computation of Radiation, 2 bands) model. The new method provided ARF values of the order of those provided by these physical models, especially with the SBDART model, confirming the validity of this new method. An ARF seasonal pattern was found with higher values in summer, (-30,7 ± 9,0) Wm- 2 in August during 2017, (- 40,1 ± 11,8) Wm- 2 in June during 2018 and (- 28,8 ± 7,7) Wm- 2 in July during 2020 and minimum values in winter, (- 8,1 ± 5,1) Wm- 2 in January during 2017, (- 5,7 ± 5,8) Wm- 2 in December during 2018 and (- 7,8 ± 5,4) Wm- 2 in December during 2020. Moreover, a dependence on solar zenith angle (θz) was detected excepting during the year 2018, increasing ARF absolute values at θz from 0◦ to 45◦-60◦ and decreasing to zero for the Sun near the horizon. This technique is very useful due to the difficulty of knowing all the inputs requested by the physical models.