Alcide di Sarra

Lidar observations of the Pinatubo aerosol layer at Thule, Greenland

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.

Observations of correlated behavior of stratospheric ozone and aerosol at Thule during winter 1991-1992

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.

Empirical correction of multifilter rotating shadowband radiometer (MFRSR) aerosol optical depths for the aerosol forward scattering and development of a long-term integrated MFRSR-Cimel dataset at Lampedusa.

Aerosol optical properties have been measured on the island of Lampedusa (35.5°N, 12.6°E) with seven-band 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 long-term 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 Ångström 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.

Determination of ultraviolet cosine-corrected irradiances and aerosol optical thickness by combined measurements with a Brewer spectrophotometer and a multifilter rotating shadowband radiometer

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.

The Evolution of the Pinatubo Stratospheric Aerosol Layer Observed by Lidar at South Pole, Rome, Thule: a Summary of Results

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.

On the Importance of Spectral Responsivity of Robertson-Berger–type Ultraviolet Radiometers for Long-term Observations¶

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.

The GLAM Airborne Campaign across the Mediterranean Basin

AbstractThe Gradient in Longitude of Atmospheric Constituents above the Mediterranean Basin (GLAM) airborne campaign was set up to investigate the summertime variability of gaseous pollutants, greenhouse gases, and aerosols between the western (∼3°E) and eastern (∼35°E) sections of the Mediterranean basin as well as how this connects with the impact of the Asian monsoon anticyclone on the eastern Mediterranean in the mid- to upper troposphere (∼5–10 km). GLAM falls within the framework of the Chemistry–Aerosol Mediterranean Experiment (ChArMEx) program. GLAM used the French Falcon-20 research aircraft to measure aerosols, humidity, and chemical compounds: ozone, carbon monoxide, methane, and carbon dioxide. GLAM took place between 6 and 10 August 2014, following a route from Toulouse (France) to Larnaca (Cyprus) and back again via Minorca (Spain), Lampedusa (Italy), and Heraklion (Crete, Greece). The aircraft flew at an altitude of 5 km on its outbound journey and 10 km on the return leg. GLAM also collec...

Determination of global and diffuse photosynthetically active radiation from a multifilter shadowband radiometer

A method to estimate the photosynthetically active radiation from multifilter shadowband radiometer (MFRSR) measurements was developed and tested from observations carried out on the island of Lampedusa in the central Mediterranean. Calibrated irradiances in the four MFRSR bands within the PAR spectral range were combined linearly to estimate PAR. The coefficients of the linear combination were derived with the least squared method for different sky conditions. The analysis shows that global PAR irradiance may be estimated with an overall uncertainty of 4%-6%. The applicability of the method was tested by using radiative transfer simulations of the diffuse irradiance spectrum for different aerosol and cloud conditions. The diffuse PAR irradiance can be estimated with an overall accuracy of less than 9%. The application of this method allows us to obtain continuous and long-term calibrated measurements of global and diffuse PAR; in addition, information on the spectral dependency of PAR can be derived from the signals in the four bands.

Sea-Salt Aerosol Forecasts Over the Mediterranean Sea Evaluated by Daily Measurements in Lampedusa from 2006 to 2010

Detailed knowledge of sea-salt aerosol (SSA) space-time variations is essential for a deeper understanding of the process of SSA loading in the atmospheric boundary layer. In order to reproduce variability of SSA concentrations over the Mediterranean Sea, the regional DREAM-Salt model has been running daily at Tel-Aviv University since February 2006 (http://wind.tau.ac.il/salt-ina/salt.html). The model performance in producing accurate SSA forecasts over the Mediterranean Sea was evaluated using a 5-year record (2006–2010) of daily SSA mass concentration measurements at the island of Lampedusa. Model-vs.-measurement comparisons showed a distinct dependence of model performance on wind direction. On average, for wind direction from 30° to 300°, the model performance was quite acceptable. It was characterized by a relatively high correlation of over 0.65 and a rather small mean bias. For north winds (0°–30°, and 300°–360°), some discrepancy between modeled and measured SSA concentrations was observed. This was characterized by the model underestimation of SSA measurements and a rather low correlation between model data and measurements. Probably, for north winds, SSA production in the surf zone, located in the vicinity of the monitoring site, contributed to observed SSA concentrations.