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In the upper region, large aerosol concentrations last for a few days; during these events aerosol is often detected up to 7 or 8 kin. Large amounts were detected in mid-May and were very often observed in June. By using meteorological analyses and isentropic backward trajectories, the aerosol behavior above Lampedusa has been related to the large-scale transport patterns and to the source regions. Large aerosol loads are clearly due to dust transport from Africa, occurring through two main paths: from central Sahara, when a high-pressure system was centered over northern Libya, and following the northwestern African coast, often along the Atlas Mountains, when the anticyclone is over Algeria or Libya, at latitudes lower than 30øN. Large aerosol loads are observed even when the air mass trajectories marginally overpass Africa, often up to 5-6 kin. According to the isentropic trajectories, large vertical motions occur when the air masses travel over Africa. Significant differences in the aerosol

[1] The radiative effects of tropospheric aerosols at the island of Lampedusa, in the Mediterranean, have been investigated by comparing measurements and results from a radiative transfer model. The model was used to reproduce the measured ultraviolet irradiance spectra (286.5-363 nm) in three cases of cloud-free conditions in May 1999, allowing the estimation of the aerosol properties and of the direct radiative forcing. Observations show very different aerosol loading and distribution, connected to the different origins of the air masses: low aerosol optical depths are associated with air masses from North Atlantic/Europe (25 and 27 May), and larger particles up to 7 km altitude and larger optical depths are observed for mineral dust coming from the Saharan region (18 May). The detailed description of the atmospheric structure and composition was used to initialize the radiative transfer model. The estimated single-scattering albedo and asymmetry parameter at 500 nm for the desert dust are in the range 0.73-0.84 and in the range 0.75-0.79, respectively. Radiative transfer calculations show that differences of the surface ultraviolet irradiance larger than 10% may arise from the lack of a detailed knowledge of the aerosol size distribution. Aerosol may also increase or reduce the absorption effectiveness of tropospheric ozone, depending on the characteristics of the particles. Estimates of the direct aerosol radiative forcing in the spectral range 300-800 nm at the surface and at the top of the atmosphere (TOA) were also derived. At the surface, aerosols produce a decrease of the instantaneous downward irradiance with respect to an aerosol-free atmosphere by 70.8, 37.2, and 39.1 W m À2 , for 18 May (the aerosol optical depth is 0.511 at 415 nm), 25 (0.165) and 27 (0.224), respectively. The radiative forcing per unit optical depth at the surface is largest for aerosol of continental/ marine origin, transported from North. The forcing at the TOA is negative, thus producing a cooling, in the desert dust case, and close to zero or positive for aerosol originating from North.

Ground‐based measurements of stratospheric constituents were carried out from Thule Air Base, Greenland (76.5°N, 68.7°W), during the winters of 2001–2002 and 2002–2003, involving operation of a millimeter‐wave spectrometer (GBMS) and a lidar system. This work focuses on the GBMS retrievals of stratospheric O3, CO, N2O, and HNO3, and on lidar stratospheric temperature data obtained during the first of the two winter campaigns, from mid‐January to early March 2002. For the Arctic lower stratosphere, the winter 2001–2002 is one of the warmest winters on record. During a large fraction of the winter, the vortex was weakened by the influence of the Aleutian high, with low ozone concentrations and high temperatures observed by GBMS and lidar above ∼27 km during the second half of February and in early March. At 900 K (∼32 km altitude), the low ozone concentrations observed by GBMS in the Aleutian high are shown to be well correlated to low solar exposure. Throughout the winter, PSCs were rarely observed by POAM III, and the last detection was recorded on 17 January. During the lidar and GBMS observing period that followed, stratospheric temperatures remained above the threshold for PSCs formation throughout the vortex. Nonetheless, using correlations between GBMS O3 and N2O mixing ratios, in early February a large ozone deficiency owing to local ozone loss is noted inside the vortex. GBMS O3‐N2O correlations suggest that isentropic transport brought a O3 deficit also to regions near the vortex edge, where transport most likely mimicked local ozone loss.

[1] Measurements of the aerosol properties were carried out at the island of Lampedusa, in the Mediterranean, in May 1999, as part of the Photochemical Activity and Ultraviolet Radiation modulating factors II campaign. Data from ground-based lidar and Sun photometer, and particle counters aboard an instrumented ultralight aircraft, are used in this study. Three different cases, when all the measurements were available in cloud-free conditions, were identified to derive the aerosol microphysical and optical properties. In one of these cases (18 May) the airmasses originated from Africa, and were loaded with a large amount of desert dust. In the other two cases (25 May and 27 May) the airmasses passed over Europe before reaching Lampedusa from North. The microphysical and optical properties of the aerosol strongly depend on the origin of the airmasses. The amount of particles in the 1-6 mm range of radii and the average aerosol surface area per unit volume are larger in the desert dust case than on 25 May and 27 May. The real part of the refractive index of the desert dust at 532 nm is between 1.52 and 1.58; its imaginary part is 5-7 Â 10 À3 and the single scattering albedo is about 0.7-0.75. The aerosol layer of 18 May closest to the surface, that probably contains a mixture of desert dust and marine aerosol, displays a smaller imaginary part (1.2 Â 10 À3 ) and a larger single scattering albedo (0.91). The aerosols originating from the North Atlantic and Europe have a real part of the refractive index between 1.35 and 1.49, and an imaginary part ranging from 8 Â 10 À4 to 1.8 Â 10 À2 ; the single scattering albedo at 532 nm (0.78-0.95) is larger than for desert dust values. The smallest value of the single scattering albedo (0.69) corresponds to an airmass originating from North, characterized by a large imaginary part of the refractive index. The asymmetry factor of the desert dust appears consistently larger for the desert dust (0.75-0.8) than for the other cases (0.61-0.72). The extinction-to-backscattering ratio, also derived from the measurements, is about 40 sr for the desert dust, and between 60 and 81 sr for the aerosol of northern origin. Simple estimates of the aerosol average direct shortwave radiative forcing at the top of the atmosphere indicate that all considered aerosol types induce a cooling. The radiative forcing per unit optical depth of the aerosol originating from North is about À37 Wm À2 over ocean and À(12-17) Wm À2 over land, while is À29 Wm À2 over ocean and À8 Wm À2 over land for desert dust. The largest forcing is however produced by the desert aerosols that generally display a considerably larger optical depth.