Analysis of photosynthetic photon flux density and its parameterization in Northern China

The relationship between broadband global solar radiation (RS) and photosynthetic photon flux density (QP) is investigated using 2 year's worth of radiation data collected at a site in northern China. These data are used to determine the temporal and monthly variability of the ratio QP/RS and its dependence on aerosol optical depth (AOD) and the column-integrated water vapor content. A simple and efficient all-weather empirically derived model is proposed to estimate QP from RS. Results reveal that the monthly variation of the ratio QP/RS ranges from 1.87 E MJ−1 in January to 2.08 E MJ−1 in July with an annual mean value of 1.96 E MJ−1. Large day-to-day variations in aerosol loading resulted in large variations in QP/RS. Under cloudless conditions, aerosols lead to a reduction of about 0.14 E MJ−1 in QP/RS per unit increase of AOD at 500 nm. The ratio QP/RS increases from about 1.82 E MJ−1 to about 1.97 E MJ−1 when the water vapor content increases from 2 to 10 mm, with the effect diminishing for higher values of water vapor content. The simple all-weather empirically derived model estimates instantaneous QP with high accuracy at the site where the model is developed. The mean bias error is close to zero and root mean square error is 3.8%, respectively. Application of the model to data collected from different locations also results in reasonable estimates of QP.