Estimating photosynthetically active radiation at the earth´s surface from MODIS data

Photosynthetically active radiation (PAR), known as solar radiation available for photosynthesis, regulates the rate of carbon fixed by plant. During PAR propagating from the top of atmosphere to the surface, they undergo Rayleigh scattering, absorption by ozone, Mie scattering, scattering and absorption by aerosols, and surface bidirectional reflection. Due to the difficulty of acquiring extensive ground-based observations, increasing efforts are being devoted to estimate PAR from satellite measurement. A new approach for PAR based on satellite algorithm is proposed in this study, which defines net surface PAR (NSPAR_SFC) as the attenuated downward PAR at the TOA for the effects of atmospheric molecular, cloud, aerosol, and albedo caused by surface BRDF. Taking temperate forest area in northeast China as an example, it is tested as a practical tool in estimating NSPAR_SFC using MODIS data and other auxiliary data. The results accurately show PAR dynamics of temperate forest woodland from day to year, and are successfully applied to simulate net primary productivity of temperate forest in northeast China