Measurements and estimations of photosynthetically active radiation in Beijing

The measurements of the photosynthetic photon flux density (Qp) and other solar components have been in Beijing for 2-year period. The Qp, broadband solar radiation (Rs) and the PAR fraction (Qp / Rs) showed similar seasonal features that peaked in value during the Summer and reached their lowest value during the Winter. The PAR fraction ranged from 1.68 E M J− 1 (Winter) to 1.98 E M J− 1 (Summer) with an annual mean value of 1.83 E M J− 1. The analysis of the hourly values also revealed a diurnal pattern, with higher values of Qp and Rs being observed around noon. The PAR fraction increased from 1.78 to 1.89 μE J− 1 (hourly values), as the sky conditions changed from clear to cloudy. The monthly mean hourly PAR fraction also revealed a diurnal variation, however, with lower values being observed around noon during most months. In November, the diurnal variations showed an opposite feature in comparison with other months. This is mainly attributed to the diurnal variations in the water vapor concentration.Two models were developed to estimate Qp from Rs. The models consisted of atmospheric parameters that were found to cause substantial changes to the PAR fraction, such as sky clearness, brightness and path length. The estimated Qp obtained via different equations was much closer to the observed values, with relative errors below 20% in Beijing. The Qp and Rs data collected at three stations with featuring different climate types from within Beijing were used for verifying the transferability of the models. The correlation coefficients between the measured and estimated Qp values decreased at these stations, and the relative error increased. This indicates that the estimation models need to be modified accordingly for the local climatic conditions.