Impact of aerosols on surface solar radiation and gross primary productivity over cropland and grassland ecosystems in North China
The increasing atmospheric aerosols from anthropogenic activities in North China are anticipated to change the proportion of direct and diffuse solar radiation in global solar radiation. These solar radiation variations can further influence the photosynthesis of the sunlit and shaded leaves, thereby affecting the terrestrial productivity. The goal of this research was to investigate the impact of aerosols on surface solar radiation and gross primary productivity (GPP) over cropland and grassland ecosystems in North China. Results suggest a significant (p < 0.05) increasing proportion of small-size anthropogenic aerosols from 2000 to 2016 and a significant (p < 0.01) increasing tendency of diffuse solar radiation but a significant (p < 0.01) decreasing tendency of direct solar radiation over the period of 1959-2016 in North China. Aerosols have contributed around 63% to the variations of diffuse solar radiation during 2000-2016. Besides, new empirical methods perform well in estimating direct solar radiation at monthly and daily level. By considering aerosols using the remote sensing-based aerosol optical depth (AOD), the estimation accuracy of diffuse solar radiation is significantly improved. The comparison of GPP under two aerosol scenarios suggests that the presence of aerosols has reduced GPP of the sunlit leaves due to the declined direct solar radiation but enhanced that of the shaded leaves on account of the increased diffuse solar radiation above and below the canopy over both ecosystems. The aerosol-effect on GPP is more significant (p < 0.05) over cropland ecosystem given the more complex canopy structure during the peak period of the growing season. Besides, an AOD value of 0.3-0.6 with a diffuse fraction (the fraction of diffuse solar radiation in global solar radiation) around 30-40% can increase total GPP over cropland ecosystem. This study highlights the significant impact of aerosols on surface solar radiation and gross primary productivity of terrestrial ecosystem over the highly-polluted region. The new empirical methods have improved the estimation accuracy of diffuse solar radiation at monthly and daily level. Meanwhile, the BEPS improved by the new empirical method can better model the terrestrial GPP over polluted regions.