Analysis of relationships between ultraviolet radiation (295-385 nm) and aerosols as well as shortwave radiation in North China Plain

The fraction of ultraviolet (UV) radiation to broadband shortwave (SW) radiation (F-UV=UV/SW) and the influences of aerosol, precipitable water vapor content (PWV) and snow on F-UV were examined using two year's worth of ground-based measurements of relevant variables in northern China. The annual mean F-UV was 3.85%. Larger monthly values occurred in summer and minimum appeared in winter. Under cloudless condition, F-UV decreased linearly with tau(500 nm) and the resulting regression indicated a reduction of about 26% in daily F-UV per unit tau(500 nm), implying that aerosol is an efficient agent in lowering the ground-level UV radiation, especially when the sun is high. Given that the annual mean tau(500 nm) is 0.82, aerosol induced reduction in surface UV radiation was from 24% to 74% when the solar zenith angle (theta) changed from 20 degrees to 80 degrees. One cm of PWV led to an increase of about 17% in daily F-UV. One case study of snow influence on surface irradiance showed that UV and SW radiation increased simultaneously when the ground was covered with snow, but SW radiation increased much less than UV radiation. Accordingly, F-UV increased by 20% for this case. Models were developed to describe the dependence of instantaneous UV radiation on aerosol optical depth, the cosine of the solar zenith angle (mu), and clearness index (Kt) under both clear and all-weather conditions.