ALBERT

Description: 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.

Description: [1]  Observations of gas-phase iodine species were made during a field campaign in the Eastern Pacific marine boundary layer (MBL). The Climate and HAlogen Reactivity tropicaL EXperiment (CHARLEX) in the Galápagos Islands, running from September 2010 to present, is the first long-term ground-based study of trace gases in this region. Observations of gas-phase iodine species were made using Long Path Differential Optical Absorption Spectroscopy (LP-DOAS), Multi-axis DOAS (MAX-DOAS) and Resonance and Off-resonance Fluorescence by Lamp Excitation (ROFLEX). These measurements were supported by ancillary measurements of ozone, nitrogen oxides and meteorological variables. Selective halocarbon and ultrafine aerosol concentration measurements were also made. [2]  MAX-DOAS observations of iodine monoxide (IO) display a weak seasonal variation. The maximum differential slant column density was 3.8 × 10 13 molecule cm -2 (detection limit ~7 × 10 12 molecule cm -2 , or ~0.8 pptv). The seasonal variation of reactive iodine IO x (= I + IO) is stronger, peaking at 1.6 pptv during the warm season (February-April). This suggests a dependence of the iodine sources on the annual cycle in sea surface temperature, although perturbations by changes in ocean surface iodide concentration and solar radiation are also possible. An observed negative correlation of IO x with Chlorophyll indicates a predominance of abiotic sources. The low IO mixing ratios measured are not consistent with satellite observations, if IO is confined to the MBL. The IO x loading is consistent with the observed absence of strong ozone depletion and nucleation events, indicating a small impact of iodine chemistry on these climatically relevant factors in the Eastern Pacific MBL.