Ultraviolet (0.280–0.400 μm) and broadband solar hourly radiation at Kwangju, South Korea: analysis of their correlation with aerosol optical depth and clearness index

A series of measurements, including ultraviolet (UV)-B (0.280 to 0.320 μm), UV-A (0.320 to 0.400 μm), broadband global (HG), and diffuse (HD) horizontal solar radiation, continuously recorded from June 1998 to August 2001 at a station in Kwangju, South Korea, and stored on a 1-min time interval, has been analyzed in this study. Results indicate that the monthly mean total of HUVT radiation (UV-A+UV-B) constitutes 7.7% of HG. In order to determine the effects of cloud and other atmospheric turbidity, such as clouds, aerosol, biomass burning on the HUVT and HG, a cloud modification factor was considered. The result suggests that aerosol, biomass and cloud cover of ∼8.1/10 attenuates HUVT by 25%, 23% and 52%, respectively, at Kwangju, South Korea. The cloud modification factor for HUVT and HG also presents dependencies with solar elevation. At low and high solar zenith angles (SZAs), dimensionless empirical relationships in the form of HUVT/HD=m(HD/HG)−n were developed, which can allow for the estimation of HUVT radiation from commonly measured global and diffuse solar radiation. Results from this study show that improved correlations and less dependence on location can be achieved by using the HUVT hemispheric transmittance (KUVT), and the total clearness index, Kt. The KUVT index obtained at Kwangju is 71% of the Kt. The difference in the levels of UV-B solar irradiance due to varying amount of the total columnar ozone shows that a 1% decrease in the total ozone leads to ∼0.4% and 2.0% increase in the UV-B solar irradiance at solar zenith angles 30° and 50°, respectively.