Analysis of photosynthetically active radiation and applied parameterization model for estimating of PAR in the North China Plain

As photosynthetically active radiation (PAR) variability and PAR estimating methods play an important role in climate change and ecological process research, PAR variation trends and broadband global solar radiation (Rs) ratios (PAR/Rs) in the North China Plain (NCP) are examined using in situ PAR and Rs observed data for 2005 to 2011. The annual average PAR value found in the NCP is 22.9 mol m−2 d−1. The highest and lowest values were recorded at Changwu and Luancheng sites, respectively. The highest PAR/Rs value was found in Jiaozhouwan due to large water vapor volumes present in this area. PAR/Rs levels have increased in the NCP due to a decrease in fine aerosols and increase in water vapor concentration. From these analysis results, a parameterization model that can be applied to all sky conditions was checked. Empirical estimation model comparisons for obtaining PAR values indicate that model was least accurate when Rs was used independently. When the model included Rs, the clearness index (Ks) and the solar zenith angle, the model estimated PAR values with acceptable accuracy. A parameterization model was constructed by considering Ks and attenuation factors of PAR under clear weather conditions (ρclear). The improved parameterization model more accurately predicts values for local sites and for various observation sites.