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ABSTRACT: The measurements of UV spectral irradiance, ozone and aerosol load obtained with a double monochromator Brewer at the Station for Climate Observations of the National Agency for New Technology, Energy and Environment (ENEA) in Lampedusa, Italy, are presented. To derive the aerosol optical depth, the direct-sun measurements at the Brewer operational wavelengths (302.1, 306.3, 310.1, 313.5, 316.8 and 320.1 nm) were calibrated using the Langley method. A radiative transfer model was used to investigate the role of ozone and aerosols in modulating UV irradiance and to reproduce the measured UV spectra. The optical scattering and absorption properties of aerosols input to the model have been derived from measured size-distributions. The modelled and measured UV spectra are in agreement for different atmospheric conditions and allowed us to estimate the radiative impact of the aerosols for two case studies related to the PAUR II campaign held in 1999.
Annals of Geophysics. 01/2009;
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ABSTRACT: Aerosol optical depth and Ångström exponent were obtained from multi filter rotating shadowband radiometer (MFRSR) observations carried out at the island of Lampedusa, in the Central Mediterranean, in the period July 2001–September 2003. The average aerosol optical depth at 495.7 nm, τ, is 0.24±0.14; the average Ångström exponent, α, is 0.86±0.63. The observed values of τ range from 0.03 to 1.13, and the values of α vary from −0.32 to 2.05, indicating a large variability in aerosol content and size. In cloud-free conditions, 36% of the airmasses come from Africa, 25% from Central-Eastern Europe, and 19% from Western France, Spain and the North Atlantic. In summer, 42% of the airmasses is of African origin. In almost all cases African aerosols display high values of τ and low values of α, typical of Saharan dust (average values of τ and α are 0.36 and 0.42, respectively). Particles originating from Central-Eastern Europe show relatively large average values of τ and α (0.23 and 1.5, respectively), while particles from Western France, Spain and the North Atlantic show the lowest average values of τ (0.15), and relatively small values of α (0.92). Intermediate values of α are often connected with relatively fast changes of the airmass originating sector, suggesting the contemporary presence of different types of particles in the air column. Clean marine conditions are rare at Lampedusa, and are generally associated with subsidence of the airmasses reaching the island. Average values of τ and α for clean marine conditions are 0.11 and 0.86, respectively. The largest values of α (about 2) were observed in August 2003, when large scale forest fires in Southern Europe produced consistent amounts of fine combustion particles, that were transported to the Central Mediterranean by a persistent high pressure system over Central Europe. Smoke particles in some cases mix with desert dust, producing intermediate values of α. The seasonal distribution of the meteorological patterns over the Mediterranean, the efficiency of the aerosol production mechanisms, and the variability of the particles' residence time produce a distinct seasonal cycle of aerosol optical depths and Ångström exponent values. Particles originating from all sectors show a summer maximum in aerosol optical depth. The summer increase in optical depth for European aerosols is linked with an increment in the values of α, that indicates an enhancement in the number of fine particles. The summer maximum of τ for African particles is associated with a weak reduction in the Ångström exponent, suggesting an increase in the total number of particles and a relatively more intense transport of large particles. The observations were classified according to the aerosol optical properties, and two main classes have been identified: desert dust and biomass burning/urban-industrial aerosols. Values of τ and α averaged over the whole observing period are 0.37 and 0.15 for desert dust, and 0.27 and 1.77 for urban-industrial/biomass burning aerosols.
Atmospheric Chemistry and Physics. 01/2006;
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ABSTRACT: Aerosol optical properties were retrieved from direct and diffuse spectral irradiance measurements made by a multi-filter rotating shadowband radiometer (MFRSR) at the island of Lampedusa (35.5° N, 12.6° E), in the Central Mediterranean, in the period July 2001–September 2003. In a companion paper (Pace et al., 2006) the aerosol optical depth (AOD) and Ångström exponent were used together with airmass backward trajectories to identify and classify different aerosol types. The MFRSR diffuse-to-direct ratio (DDR) at 415.6 nm and 868.7 nm for aerosol classified as 'biomass burning-urban/industrial', originating primarily from the European continent, and desert dust, originating from the Sahara, is used in this study to estimate the aerosol single scattering albedo (SSA). A detailed radiative transfer model is initialised with the measured aerosol optical depth; calculations are performed at the two wavelengths varying the SSA values until the modelled DDR matches the MFRSR observations. Sensitivity studies are performed to estimate how uncertainties on AOD, DDR, asymmetry factor ( g ), and surface albedo influence the retrieved SSA values. The results show that a 3% variation of AOD or DDR produce a change of about 0.02 in the retrieved SSA value at 415.6 and 868.7 nm; a ±0.06 variation of the asymmetry factor g produces a change of the estimated SSA of <0.04 at 415.6 nm, and <0.06 at 868.7 nm; finally, an increase of the assumed surface albedo of 0.05 causes very small changes (0.01–0.02) in the retrieved SSA. The calculations show that the SSA of desert dust (DD) increases with wavelength, from 0.81±0.05 at 415.6 nm to 0.94±0.05 at 868.7 nm; on the contrary, the SSA of urban/industrial (UN) aerosols decreases from 0.96±0.02 at 415.6 nm to 0.87±0.07 at 868.7 nm; the SSA of biomass burning (BB) particles is 0.82±0.04 at 415.6 nm and 0.80±0.05 at 868.7 nm. Episodes of UN aerosols occur usually in June and July; long lasting BB aerosol episodes with large AOD are observed mainly in July and August, the driest months of the year, when the development of fires is frequent.
Atmospheric Chemistry and Physics. 01/2006;
