Spatiotemporal characteristics of photosynthetically active radiation in China

Abstract

Photosynthetically active radiation (PAR) plays an important part in climate change and ecological processes. Few PAR measurements are usually available, especially in China. Thus it is important and significant to investigate the spatiotemporal characteristics of PAR in China for radiation budget and ecosystem studies. In order to study the spatiotemporal characteristics of PAR, photosynthetic photon flux density (Qp) and broadband solar radiation (Rs) measurement data were analyzed for the period of January 2005 to June 2006. Qp increases gradually from spring and reaches its maximum and minimum in summer and winter, respectively. The highest value of annual mean daily Qp (40.9 ± 4.1 mol m−2 d−1) appears in the Qinghai‐Tibet Plateau region, along with higher atmospheric transmission; and the lowest value (17.4 ± 9.03 mol m−2 d−1) is found in the northern subtropics, along with the largest aerosol optical depth (AOD) and a lower water vapor content. PAR fraction shows a similar seasonal trend as that of Qp at all sites except for some near to lakes and sea. The annual mean daily value of PAR fraction varies from 1.75 ± 0.12 to 2.3 ± 0.15 mol MJ−1 over China. The largest value appears in tropical regions because of higher relative humidity (RH). The lowest value is observed at the Luancheng site, which features low humidity and an abundance of fine aerosols, instead of sites in China's driest northern desert region. The variability of PAR fraction is mainly controlled by the selective scattering of aerosol particles and absorption of water vapor. Two different diurnal trends of PAR fraction are observed in China. In most sites, PAR fraction tends to peak during sunrise or sunset and reaches its lowest value around noon. However, it exhibits an opposite trend in the northern desert area because of the distinctive diurnal variation of water vapor in this region. Further analysis of annual mean hourly PAR fraction shows that the national average is 1.82 ± 0.11 and 2.00 ± 0.08 μmol J−1 for clear and cloudy days, respectively. The cloudy day's ratio is therefore 10% higher than that for clear days. The altitude dependency of PAR fraction is very weak below 1500 m because of uneven distributions of water vapor, clouds, and aerosols. Above 1500 m, PAR fraction increases gradually with altitude on both cloudy and clear days, attributed to the weaker extinction of Qp at higher‐altitude sites. The distribution pattern of annual mean daily PAR fraction is similar to that of the hourly value. Its magnitude is medial to the hourly values on cloudy and clear days. These results are helpful for understanding the climatic, agricultural, and ecological processes over China and useful for primary productivity estimation and ecosystem–atmosphere CO2 exchange study in China.