Effect of aerosol optical depth on diffuse photosynthetically active solar irradiance

Photosynthetically active solar irradiance (PAR, 400-700 nm) data measured with a Rotating Shadowband Radiometer (RSR) at Yonsei University, Seoul (37.57°N, 126.97°E) from 1996 to 2000 have been analyzed to investigate the dependence of solar irradiance on aerosol optical depth (AOD). It is found that diffuse solar irradiance is largely affected by solar zenith angle (SZA) and AOD. Diffuse solar irradiance increases with increasing AOD up to a certain critical value due to the increasing effect of aerosol scattering, but then decreases after the critical AOD value due to the excessive extinction by aerosol. The diffuse solar irradiance at a given SZA can be expressed as an empirical function of AOD, by fitting the long-term diffuse measurements. This empirical model is validated with its sensitivity tests for AOD. The empirical model was compared with the CLIRAD model over the globe, which showed reasonably good agreements.