Long-Term Trends in Photosynthetically Active Radiation in Beijing

A long-term dataset of photosynthetically active radiation (Qp) is reconstructed from a broadband global solar radiation (Rs) dataset through an all-weather reconstruction model. This method is based on four years' worth of data collected in Beijing. Observation data of Rs and Qp from 2005--2008 are used to investigate the temporal variability of Qp and its dependence on the clearness index and solar zenith angle. A simple and efficient all-weather empirically derived reconstruction model is proposed to reconstruct Qp from Rs. This reconstruction method is found to estimate instantaneous Qp with high accuracy. The annual mean of the daily values of Qp during the period 1958--2005 period is 25.06 mol m-2 d-1. The magnitude of the long-term trend for the annual averaged Qp is presented (-0.19 mol m-2 yr-1 from 1958--1997 and -0.12 mol m-2 yr-1 from 1958--2005). The trend in Qp exhibits sharp decreases in the spring and summer and more gentle decreases in the autumn and winter.