Estimation of hourly and daily photosynthetically active radiation in Inner Mongolia, China, from 1990 to 2012

Measurements of photosynthetically active radiation ( PAR ) in Inner Mongolia, China, from 2005 to 2012 were used to investigate its relationship with global solar radiation ( G ) under various sky conditions. Monthly mean daily PAR generally increased from 13.98 mol m −2 d −1 in December to 40.48 mol m −2 d −1 in June with annual mean value being 29.02 mol m −2 d −1 . Monthly daily F p (ratio of PAR to G) increased from 1.68 mol MJ ‐1 in December to 1.88 mol MJ ‐1 in August with annual average of 1.80 mol MJ ‐1 . Clearness index ( K t ) was used for characterizing sky conditions and it was discovered that clear skies (49.4%) were the dominated sky conditions. By studying the dependence of PAR on optical air mass, K t and cosine of solar zenith angle, two different models have been introduced and tested at four other sites in China. The model performances have been analysed and the daily PAR values during 1990–2012 at Inner Mongolia, China, have been reconstructed. It was found that PAR increased at the rate of 0.54 mol m −2 d −1 decade ‐1 since 1990 and the most evident increases were observed in summer (1.96 mol m −2 d −1 decade ‐1 ). However, PAR decreased at the rate of ‐0.37 mol m −2 d −1 decade ‐1 in spring during 1990–2012.