Abstract
Abstract. The fraction of ultraviolet (UV) radiation to broadband shortwave (SW) radiation (FUV=UV/SW) and the influences of aerosol, precipitable water vapor content (PWV) and snow on FUV were examined using two year's worth of ground-based measurements of relevant variables in northern China. The annual mean FUV was 3.85%. Larger monthly values occurred in summer and minimum appeared in winter. Under cloudless condition, FUV decreased linearly with τ500 nm and the resulting regression indicated a reduction of about 26% in daily FUV per unit τ500 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 τ500 nm is 0.82, aerosol induced reduction in surface UV radiation was from 24% to 74% when the solar zenith angle (θ) changed from 20° to 80°. One cm of PWV led to an increase of about 17% in daily FUV. 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, FUV 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 (μ), and clearness index (Kt) under both clear and all-weather conditions.
Highlights
Ultraviolet (UV) radiation, defined as electromagnetic radiation having wavelengths within the range 100–400 nm, composes 8.73% of the solar spectrum at the top of the atmosphere and represents a smaller part of the spectrum at the Earth’s surface (Foyo-Moreno et al, 1998; Canada et al, 2000; Ogunjobi and Kim, 2004)
6 Conclusions aerosol radiative effects on surface UV radiation are of significance to UV radiation estimations, air quality studies, as
The monthly variation of Surface UV radiation under all-weather conditions are this fraction and the dependence on τ and precipitable water vapor content (PWV) were ana- linked with two independent quantities, i.e. μ and SW radilyzed
Summary
Ultraviolet (UV) radiation, defined as electromagnetic radiation having wavelengths within the range 100–400 nm, composes 8.73% of the solar spectrum at the top of the atmosphere and represents a smaller part of the spectrum at the Earth’s surface (Foyo-Moreno et al, 1998; Canada et al, 2000; Ogunjobi and Kim, 2004). UV radiation is very important because of its biological and photochemical effects, understanding the amount of UV near the Earth’s surface and its spatial and temporal availability is of significance to a wide range of disciplines, for example, forestry, agriculture and oceanography (Zerefos, 1997; He et al, 2002), as well as air pollution and human health (Diffey, 1991; Luet al., 1997). Major atmospheric constituents influencing surface UV radiation variability include cloudiness, aerosols and ozone (Wang et al, 1999; Calboet al., 2005). Our understanding of UV radiative transfer in the atmosphere, spatial-temporal variations and influential factors (aerosol, cloud, snow cover, etc.) have improved substantially thanks to advances in radiative transfer modeling, ground-based and space-borne observation techniques and remote sensing
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