Evaluation of the aerosol forcing efficiency in the UV erythemal range at Granada, Spain

Atmospheric aerosols represent one of the most important components that attenuate solar radiation reaching the Earth's surface. The aerosol radiative forcing (ARF) at the surface is usually determined in the visible range of the solar spectrum. In contrast, there are few experimental works in the literature about the ARF in the ultraviolet (UV) region. Therefore, this paper focuses on quantifying the aerosol forcing efficiency in the UV erythemal range (AFEery), ARF per unit of aerosol optical depth (AOD). Simultaneous UV erythemal irradiance (UVER) and AOD measurements recorded between January 2006 and December 2008 in Granada (Spain) were used. In addition, an empirical model is utilized to estimate the UVER values for an atmosphere with very low aerosol loads (clean conditions). The AFEery varies from −62 to −26 mW/m2 per unit of AOD at 380 nm when the solar zenith angle (SZA) changes from 20° to 55°, showing a strong influence of the SZA on AFEery. The variations of the aerosol size and absorption properties also cause significant changes of this variable. Thus, 1 μm aerosols (related to desert dust particles) produce significantly higher AFEery (in absolute values) than submicrometer particles (associated with urban or industrial aerosols). For instance, AFEery varies from −52 mW/m2 per unit of AOD for Angström exponents smaller than 0.5 to −29 mW/m2 per unit of AOD for Angström exponents higher than 1.5. In addition, the AFEery values are −59 mW/m2 per AOD unit for single‐scattering albedos (SSAs) smaller than 0.85 and −28 mW/m2 per AOD unit for SSAs larger than 0.85, showing that stronger aerosol absorption (low SSA) leads to a larger surface forcing efficiency (in absolute values). All these results highlight the outstanding role that atmospheric aerosol plays in the modifying levels of UV radiation reaching the surface. Key Points Aerosol forcing efficiency in the UV range presents a strong dependence on solar Changes in aerosol size produce a large effect over the UV aerosol forcing Large mineral dust load can lead to notably reduced UV erythemal irradiance at surface