Influence of aerosol on photosynthetically active radiation under haze conditions

Photosynthetically active radiation (PAR, 400-700 nm), as a sub-band in solar shortwave radiation (280-2800 nm), has profoundly affected vegetation primary productivity and biogeochemical cycles of carbon. However, few previous studies focused on the quantitative effects of aerosols on PAR, a sub-band of the shortwave spectrum, during the winter haze period. Based on ground-based remote sensing observations, we have quantitatively investigated the role of aerosol in PAR variation under haze period through radiative transfer simulations at Wuhan, China. The results indicate that direct solar PAR irradiance is attenuated more dramatically and diffuse downward irradiance is enhanced weaklier as aerosol optical depth (AOD) increases in the haze period, which leads to a result of more significant surface aerosol radiative effect (ARE) in PAR spectrum (AREPAR). Simultaneously, the percentage of AREPAR in ARESW spectrum decreases by 3.33%, indicating that PAR is attenuated more than other sub-bands in SW as pollution enhances. Meanwhile, the ARE efficiency (REE) in PAR weakens from -122 W/m2 under the clear-air condition to -80 W/m2 during the severe-pollution period. These phenomena are attributed to stronger forward scattering in PAR due to the hygroscopic growth of fine-mode aerosol particles during haze. The results provides support for further studies on the effect of pollutants on PAR, and then vegetation productivity.