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The time-patterns of ground-level solar irradiance during the solar eclipse of 29 March, 2006, were observed at three Italian stations (Lampedusa, Mt. Cimone and Bologna) using different radiometric techniques. The global irradiance measured at the sites was found to reach the minimum at times not coinciding with those predicted by radiative transfer model evaluations, with ahead or lag times depending on the optical characteristics of the surface–atmosphere system in the areas surrounding of the stations. This different behaviour has been mainly attributed to the different influence of the environmental conditions on the diffuse radiance component measured at the observation sites. The present results indicate that the incoming diffuse radiance recorded at the three stations was appreciably affected by contributions arising from extended regions of about 30–100 km range from the stations. Such an explanation agrees very well with the theoretical evaluations obtained in earlier studies. The surrounding environmental areas of impact at ultraviolet wavelengths have been found to be wider than those in the visible and near-infrared spectral ranges.

Using a recently installed lidar, a series of measurements of aerosol concentrations have been carried out between 12-1991 and 3-1992. This work was in conjunction with the European Arctic Stratospheric Ozone Experiment. Scattering ratios were very high because of the presence of aerosols from the Mt Pinatubo volcanic eruption. Ozone observations were made in this period by sondes. This paper reports on correlations between these observations. The correlations observed were not always positive. Both observations saw distinct layered structures representing density variations with height.

[1] Ground-based measurements of aerosol optical depth and surface shortwave irradiance carried out at the Mediterranean island of Lampedusa during 2004–2007 are used to estimate the surface aerosol direct radiative forcing for desert dust (DD), urban/industrial-biomass burning (UI-BB), and mixed aerosols (MA). The aerosol single scattering albedo, ω, at 415.6 and 868.7 nm is derived at 60° solar zenith angle, θ, from measurements of global and diffuse radiation using radiative transfer model calculations. The shortwave forcing efficiency (FES) is derived, for θ between 20° and 75°, for the three identified classes of aerosol and for all the observed data (AD). The absolute value of FES decreases for increasing θ for all the aerosol types. FES varies between −185 and −81.7 W m−2 for DD, −168 and −84 W m−2 for UI-BB, −251 and −120.2 W m−2 for MA, and −208 and −106.5 W m−2 for AD. The daily average forcing efficiency (FEd) at the equinox is −67.2 W m−2 for DD, −59.0 W m−2 for UI-BB, and −93.2 W m−2 for MA. The forcing efficiency of DD, UI-BB, and MA at θ = 60° was calculated for three intervals of single scattering albedo (0.7 ≤ ω < 0.8, 0.8 ≤ ω < 0.9, 0.9 ≤ ω ≤ 1) at 415.6 and 868.7 nm. The absolute value of FES decreases with increasing ω at 868.7 nm for all aerosol types, while it decreases with increasing ω at 415.6 nm for UI-BB and MA and increases for DD. A 0.1 increment in the single scattering albedo at 868.7 nm produces a reduction in FES by 25–30 W m−2, and a reduction by 10–15 W m−2 in FEd.

[1] 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.

Aerosol optical properties have been measured on the island of Lampedusa (35.5°N, 12.6°E) with sevenband multifilter rotating shadowband radiometers (MFRSRs) and a CE 318 Cimel sunphotometer (part of the AERONET network) since 1999. Four different MFRSRs have operated since 1999. The Cimel sunphotometer has been operational for a short period in 2000 and in 2003-2006 and 2010-present. Simultaneous determinations of the aerosol optical depth (AOD) from the two instruments were compared over a period of almost 4 years at several wavelengths between 415 and 870 nm. This is the first longterm comparison at a site strongly influenced by desert dust and marine aerosols and characterized by frequent cases of elevated AOD. The datasets show a good agreement, with MFRSR underestimating the Cimel AOD in cases with low Angström exponent; the underestimate decreases for increasing wavelength and increases with AOD. This underestimate is attributed to the effect of aerosol forward scattering on the relatively wide field of view of the MFRSR. An empirical correction of the MFRSR data was implemented. After correction, the mean bias (MB) between MFRSR and Cimel simultaneous AOD determinations is always smaller than 0.004, and the root mean square difference is ≤0.031 at all wavelengths. The MB between MFRSR and Cimel monthly averages (for months with at least 20 days with AOD determinations) is 0.0052. Thus, by combining the MFRSR and Cimel observations, an integrated long-term series is obtained, covering the period 1999-present, with almost continuous measurements since early 2002. The long-term data show a small (nonstatistically significant) decreasing trend over the period 2002-2013, in agreement with independent observations in the Mediterranean. The integrated Lampedusa dataset will be used for aerosol climatological studies and for verification of satellite observations and model analyses. © 2015 Optical Society of America.

The impact of extreme energy cosmic rays on the Earth's atmosphere is manifested by the generation of fluorescence and Cherenkov radiation: these effects constitute source fields that propagate and, once detected, provide an indirect way to deduce energy, trajectory, composition of the primary particle. Because of the relatively infrequent probability of detection of such effects at the ground, proposals have been advanced to observe them from space: one of the proposed experiments is EUSO (Extreme Universe Space Observatory). Both the generation and propagation of these fields are affected by the high variability of the atmospheric conditions: to limit the deriving uncertainties, the observations would greatly benefit by the support of adequate instrumentation to provide the status of the atmosphere at the time of detection. Thus the use of an optical radar––lidar––has been suggested to complement the EUSO capability. This paper discusses: (a) the distortion induced by different atmospheric conditions on the fluorescence and Cherenkov profiles acquired from space; (b) the lidar retrieval of the transmission and scattering characteristics of the atmosphere and the use of these data to correct the detected signals to reach a realistic assessment of the atmospheric sources and of the primary particle characteristics. The simulations are based on experimental results with regard to aerosol and cloud backscatter data, on a relatively simple transfer model and on realistic assumptions regarding the extinction to backscatter ratio.

Observations of Polar Stratospheric Clouds (PSCs) were carried out with an airborne lidar on the stratospheric M55 Geophysica aircraft during a .ight from Rovaniemi, Finland, on 9 January, 1997. The clouds were observed at the zenith, downwind from the Norwegian Alps: three PSCs, of somewhat di=erent characteristics, were detected at heights between 23 and 28 km. In two of the clouds, di=erent types of particles seem to coexist: echoes attributable to types I and II PSCs are found in di=erent portions of the clouds. The formation of the PSCs is related to an orographic lee-wave, whose development was forecast by a mesoscale dynamical model used to plan the .ight path. The largest observed PSC displays a complex structure, that appears to be in.uenced by waves of di=erent wavelengths. In particular, lidar and in situ data suggest the presence of a wave having a relatively short length (about 18 km) that overlaps on the main lee-wave. The short wavelength oscillation is thought to play a major role in the cloud development, determining the rapid formation and evaporation of particles and therefore the non-stationary character of the PSC. ? 2003 Elsevier Science Ltd. All rights reserved.

Observations of polar stratospheric clouds (PSCs) carried out at the Amundsen Scott South Pole Station by lidar in the winter of 1990 are reported. Echoes attributable to PSCs began to appear in late May, were a persistent feature till early September, and from that time until the end of October were sporadically observed. Values of the backscatter ratio as high as 16 were recorded in late August at 13 km. Analyses based on the sensitivity of the backscattering to temperature confirm that the attribution of the echoes to PSC type I or II is possible in some cases. Those that could be ascribed to type II were observed mostly during July and August, while those attributable to type I appeared only at the beginning of the winter. The comparison of water vapor mixing ratios derived from PSC type II occurrence temperatures and frost point hygrometer measurements indicates a fast dehydration in the beginning of the winter.

Observations of polar stratospheric clouds (PSCs), carried out at the Amundsen Scott South Pole Station by lidar from May 1992 through October 1993, are reported and compared with previously obtained results. At that time the Antarctic stratosphere was loaded with sulfuric acid aerosol due to the eruptions of Mount Pinatubo, primarily, and of Mount Hudson. The seasonal evolution of the backscatter profiles has been investigated in relation to the presence of the volcanic aerosol and to the processes of PSC formation, particle sedimentation, and dehydration. During the first winter after the eruptions the PSC activity was more intense than in the following year, particularly above 12.5 km, where the amount of volcanic aerosol was larger in 1992 than in 1993. At lower altitudes the volcanic aerosol loading as well as the PSC phenomenon were comparable during the 2 years. No substantial changes in the signal due to the volcanic aerosol has been observed comparing the backscattering profiles before and after the PSC periods (June-September), except for a downward shift, attributed to the subsidence of the air inside the polar vortex. It is concluded that only a small fraction of the aerosol particles, probably those with the largest radii, were involved in the nucleation of PSC particles.

Stratospheric aerosol observations have been carried out with three lidars, in the period preceeding and following the eruption of Mt. Pinatubo. The lidars were located at South Pole, Rome and Thule. The detailed analysis of the results is still under way: their general features and highlights are summarised in this paper. The aerosol backscattering data show the global evolution of the volcanic aerosol cloud in relation to the general circulation of the atmosphere and to microphysical processes. Other inferred parameters are the mass, the center of mass of the cloud. and the size distribution of the aerosol. Correlations between the aerosol and the ozone contents. found after all main eruptions since 1962, have been confirmed. Large effects on polar stratospheric cloud activity have been recorded.

Lidar measurements of the stratospheric aerosol content have been carried out in Thule (Greenland), in connection with the European Arctic Stratospheric Ozone Experiment (EASOE). Aerosols attributable to the Pinatubo eruption were detected in the stratosphere late in September; during autumn and early winter the stratospheric aerosol load slowly increased. In the region above approximately 18 km the aerosol load depended on the location of the station relative to the polar vortex. A temporary increase of the aerosol concentration was detected up to about 26 km at the beginning of February, when Thule was located for a few days outside the vortex. A final increase of the aerosol load was related with the final breakup of the vortex. The aerosol integrated backscatter values ranged between 0.001 and 0.005 sr−1 during most of the winter, and increased up to 0.007 sr−1 in early spring; in this period the aerosol columnar mass was estimated to be as high as 0.1 g m−2.

During 1991 the terrestrial stratosphere went through a substantially increased aerosol load, due mainly to the eruptions of Mt. Pinatubo in the Philippines, and, to a lesser extent, to those of Mt. Hudson in Chile. This paper reports lidar observations of the stratospheric aerosols at South Pole. Two layers were present at different altitudes during the austral summer, but only the higher one persisted in the stratosphere until the onset of the polar stratospheric cloud (PSC) phenomenon. Data have been analyzed in terms of the integrated backscattering coefficient and the aerosol mass content has been estimated.

Combined measurements of diffuse-to-global radiation ratio and global spectral irradiances in the UV are used to derive cosine-corrected UV irradiances and aerosol optical depth (AOD). The diffuse-to-global radiation ratio is used first in the cosine correction of the global irradiance, then to calculate absolutely calibrated direct irradiances. The Beer-Lambert law is applied to derive the UV AOD using independent measurements of the extraterrestrial solar flux. The AOD can be derived with an uncertainty of about 0.03 at 60 degrees solar zenith angle. The method was applied to measurements obtained with two UV multifilter rotating shadowband radiometers (UV-MFRSRs) and a MK III Brewer spectrophotometer on the Island of Lampedusa in the Central Mediterranean during two periods of 2002 and 2004. The derived AOD at 318 and 332 nm was compared with UV AOD measured at 318, 320, and 368 nm with different techniques. The retrieved AOD, combining MFRSR and Brewer measurements, is in good agreement with the optical depth derived with the other methods.

Measurements of the ozone absorption cross section in the 590- to 610-nm region of the Chappuis bands have been carried out at the temperatures T = 230 K and T = 299 K. Ozone is produced with an electrical discharge and stored cryogenically. Differential absorption measurements are obtained in a slowly evolving mixture of ozone and molecular oxygen. Previous results by other authors indicate a certain disagreement on the temperature dependence of the absorption cross section. Our data at each wavelength show no significant dependence of the absorption coefficient on the temperature, within experimental errors. The statistical consistency of the results is discussed.

With a Brewer spectrophotometer, an estimation of total ozone is made from relative measurements of direct-sun ultraviolet radiation at six wavelengths from 300 to 320 nm. During normal operations, one of six neutral-density filters is selected automatically to maintain the detector in its linear response range. On the basis of these standard direct-sun observations, estimates of aerosol optical depth can be derived, provided that a calibration of the relative measurements is available for each neutral-density filter. To obtain the calibration, we implemented a routine to measure direct-sun signals with a fixed neutral-density filter and applied the Langley method to the measured photon counts. Results show that if a sufficiently large number of cloud-free mornings or afternoons is available, a reliable calibration can be achieved even at sea-level sites that are characterized by large aerosol variability. The derived aerosol optical depths appear consistent with those measured independently by a multifilter rotating shadow-band radiometer. Existing relatively long-term series of direct-sun ozone measurements by Brewer instruments may be used for retrieval of aerosol optical depth.

A system to determine the spectral responsivity of ultraviolet (UV) radiometers has been developed and is routinely operated at the Central Ultraviolet Calibration Facility, at the National Oceanic and Atmospheric Administration. The instrument and the measurement methodologies are described. Results of measurements from thermally controlled broadband UV radiometers of the Robertson-Berger (R-B)-type are described. Systematic differences in the spectral response curves for these instruments have been detected. The effect of these differences on the field operation of UV-B radiometers has been studied by calculating the instrumental response from modeled UV spectra. The differences of the weighted spectral UV irradiances, measured by two radiometers with different spectral response functions, caused by the daily variation in the position of the sun were estimated for fixed values of total ozone, altitude and albedo, and for cloud-free conditions. These differences increase with the solar zenith angle and are as large as 8%. Larger differences in the instrumental response may be produced by ozone variations. Thus, care must be taken when analyzing data from R-B radiometers and comparing results from different instruments. Routine cycling of UV-B radiometers in operative networks without a careful determination of the spectral responsivity, or small drifts of the spectral responsivity, may strongly affect the accuracy of UV radiation measurements and produce an erroneous trend. Because of the possible differences among radiometers, it would not be practical to derive the long-term behavior of UV radiation without routine and thorough characterization of the spectral responsivities of the instruments.

We present the results of the aerosol measurements carried out over the Aegean Sea during the Photochemical Activity and Solar Ultraviolet Radiation campaign held in Greece during June 1996. Simultaneous observations performed with a lidar and a double-monochromator spectrophotometer allowed us to retrieve the optical depth, the Angström coefficient, and the backscatter-to-extinction ratio. The Sun photometric data can be used to improve quantitative aerosol measurements by lidar in the Planetary Boundary Layer. Systematic errors could arise otherwise, because the value of the backscatter-to-extinction ratio has to be supplied. Instead this ratio can be retrieved experimentally by use of an iterative solution of the lidar equation.

Absolute measurements of the ozone absorption coefficient in the Huggins bands at different temperatures have been carried out. Ozone is produced by an electrical discharge and stored cryogenically; differential absorption measurements are subsequently obtained in a slowly evolving mixture of ozone and molecular oxygen. High resolution (to 0.012 nm) measurements cover a spectral range (339–355 nm) where the ozone absorption shows a strong dependance on temperature. Results at 293 and 220 K are reported; they are particularly interesting in view of the utilization of this spectral region as a low-absorption reference channel for the observation of atmospheric ozone profiles by active probing techniques. Coherent radiation at two wavelengths, around 355 and 353 nm, respectively, can be obtained as third harmonic of the fundamental output of a Nd:YAG laser and by H2 Raman shifting of a XeCl excimer laser output.

[1] In February–March 1999, nocturnal observations with a ground-based backscatter/depolarization lidar were conducted during the Airborne Platform for Earth Observation–Third European Stratospheric Experiment on Ozone (APE-THESEO) campaign at Mahe, Seychelles (4.4°S, 55.3°E). Upper tropospheric cirrus clouds were a common feature in the lidar echoes; they were detected for about 67 hours out of a total measurement time of 125 hours. Optical and geometrical characteristics of the clouds detected at altitudes above 9 km throughout the campaign are derived and discussed. Cirri above 14.5 km present a layered structure and appear more persistent and thinner than cirri at lower altitudes. The behavior of average optical and geometrical depths, integrated depolarization, backscatter-to-extinction ratio, integrated backscatter, and extinction coefficient is studied as a function of midcloud temperature and is compared with results from previous measurements and models. The estimated cloud backscatter-to-extinction ratio ranges from 0.01 to 0.2 sr−1, with an average value of 0.051 ± 0.030 sr−1. In the temperature range between 200 and 240 K the integrated depolarization ranges between 0.27 and 0.19 and appears lower than that for midlatitude cirri. In a single night, very high values of the integrated depolarization for a layered cirrus at 16 km could be related to outflow from an active convective cell. The two highest and coldest (temperature around 188 K) layers show values of optical and geometrical depth and integrated depolarization lower than for cirri at temperatures between 190 and 200 K. The optical characteristics of the clouds decrease for temperatures from 190 to 200 K, possibly indicating different evolution phases of the clouds.