Study on the characteristics of actinic radiation and direct aerosol radiation effects in the Pearl River Delta region

Combined air pollution prevails in the Pearl River Delta (PRD) region in recent years; large amounts of aerosols have caused a significant attenuation of actinic radiation. Solar radiation is the driving energy for photochemical reactions and plays an important role in the ozone formation. In this study, characteristics of actinic radiation and atmospheric composition in the PRD region have been analyzed based on long-term observation. The radiation transfer model (SBDART) is used to quantitatively estimate the direct radiation effect and actinic radiation effect of aerosol. Results indicate that PM2.5 concentration and atmospheric extinction coefficient have shown a downward trend. Meanwhile, ozone and Single Scattering Albedo (SSA) have increased. The average value of SSA is 0.914 ± 0.041, attaining the highest level in spring, followed by autumn and summer, and the lowest is found in winter. A linear relationship between UVA/UVB of actinic radiation flux and solar shortwave radiation has been revealed. Through the conversion formula proposed in the article, conventional radiation data can be converted into actinic radiation flux and species photolysis rate. The actinic radiation and radiation attenuation caused by aerosols shows an overall downward trend at the surface. In the ultraviolet to visible wavelength range (280–670 nm), the annual average attenuation of aerosol direct radiation and actinic radiation is 61.6 ± 31.8 W/m2 and 120.1 ± 59.7 W/m2, respectively. The radiation attenuation caused by aerosols is the largest in spring, with little difference in summer, autumn and winter, which are 92.9 ± 42.1 W/m2, 53.7 ± 21.3 W/m2, 48.0 ± 13.7 W/m2 and 52.1 ± 15.8 W/m2, respectively. The attenuation of actinic radiation caused by aerosol is the largest in spring, followed by winter, and there is little difference between summer and autumn, which are 174.8 ± 78.4 W/m2, 111.1 ± 33.1 W/m2, 98.3 ± 38.4 W/m2 and 96.7 ± 26.9 W/m2, respectively. The direct radiation effect of aerosol and the actinic radiation effect have opposite trends with increasing height. Below the aerosol scale height, the larger SSA is corresponding with greater radiant flux, and it is opposite above the aerosol scale height. SSA has the same effect on the actinic radiant flux of the entire layer: The larger SSA is in accordance with greater actinic radiant flux.