Gian Paolo Gobbi

Altitude-resolved properties of a Saharan dust event over the Mediterranean

Some insight is provided about the time and altitude evolution of a Saharan dust event observed by lidar during the spring 1999 EU campaign PAUR-II at Crete (353N}233E). The dust episode lasted approximately eight days, reaching maximum optical depth q+0.6, at 532 nm. Clear tropospheric conditions (q+0.1) preceded and followed the event. Maritime aerosols, mixed-phase clouds and cirrus-generating particles were also observed during the campaign. An altitude-resolved description of lidar-derived backscatter and depolarization of observed aerosols and clouds is provided. The nature and phase of the particles is inferred on the basis of these variables. Particles extinction and surface area are estimated on the basis of an aerosol model. The data analysis shows how Saharan dust events can reach and persist in the 10 km altitude region, overturning the vertical structure of the tropospheric aerosol optical depth. The analysis will also show that both aerosols and clouds mainly existed as mixed phases of solid and liquid particles. ( 2000 Elsevier Science Ltd. All rights reserved.

Lidar estimation of tropospheric aerosol extinction, surface area and volume: Maritime and desert‐dust cases

A numerical model, based on a Monte Carlo approach, is presented to determine functional relationships linking backscatter and other important properties as extinction, surface area, and volume of tropospheric aerosols. If existing, such relationships allow for a direct estimate of such properties by means of a single-wavelength lidar measurement. To be employed in a lidar inversion procedure, the extinction to backscatter ratio is also analyzed. Maritime and desert dust aerosol particles are addressed. In the latter case, both spherical and nonspherical shape of particles are considered. Large differences (up to 200%) result from the comparison of extinction computed for spherical and nonspherical particles. On the whole, maximum errors to be associated to the model estimation of the aerosol extinction coefficient and surface area are of the order of 50%. Conversely, errors associated to volume estimates range from 15% to 100%. To validate the model, a first comparison is performed between lidar and Sun-photometer-derived aerosol optical thickness of both maritime aerosols and Saharan dust.

Volcanic aerosol and ozone depletion within the Antarctic polar vortex during the austral spring of 1991

In the spring of 1991 the Antarctic lower stratosphere was characterized by a layer of volcanic aerosol from the Cerro Hudson eruption. This aerosol layer was observed from McMurdo Station, Antarctica, with both lidar and balloonborne particle counters beginning around 10 September. After 20 September the aerosol was observed daily between 9 and 13 km. In this layer homogeneous nucleation of new aerosol was observed with concentrations greater than 6000/cu cm. Comparisons of scattering ratio calculated from measured particle size distributions agree best with the lidar measurements when a real index of refraction near 1.5 is used. In the past 5 years of measurements, ozone below 13 km has been relatively unchanged during the annual Antartic ozone depletion; however, in 1991 ozone below 13 km decreased at a rate of 4 - 8 ppb/day over 30 days. This change began shortly after the appearance of the volcanic aerosol, providing direct measurements correlating volcanic aerosol and ozone depletion. 16 refs.

Evidence for denitrification in the 1990 Antarctic spring stratosphere: I, Lidar and temperature measurements

Lidar soundings of the lower stratosphere were made between August 30 and October 11, 1990, at McMurdo Station, Antarctica (78S–167E). Polar stratospheric clouds (PSCs) were observed in only two periods: between September 5 and 10, and on October 7. During these days McMurdo was well within the polar vortex, and temperatures in the lower stratosphere reached seasonal minima. Temperature soundings and two water vapor measurements were also made in the same period. The water vapor content between 11 and 20 km was found to be between 2 and 3 ppmv. Using these values for water vapor, condensation temperatures for water and nitric acid trihydrate were calculated and compared with the temperature measurements. Analysis of these comparisons together with the lidar observations indicated that PSCs appeared only when temperatures were below the threshold condensation point for an air mass containing approximately 1 ppbv nitric acid. This was observed from the beginning of measurements on August 30, and suggests that the Antarctic lower stratosphere was highly denitrified both at the end of winter and during the early spring of 1990.

Polar stratospheric clouds and volcanic aerosol during spring 1992 over McMurdo Station, Antarctica : lidar and particle counter comparisons

Coordinated observations with lidar and balloon-borne particle counters were used to characterize polar stratospheric clouds and to estimate a particle index of refraction. The index of refraction was estimated from comparisons of calculated and measured scattering ratios at a wavelength of 532 nm. The clouds, measured from McMurdo Station, Antarctica (78°S), were observed above 11 km at temperatures below 198 K and were divided into three classes based on their scattering properties and particle size. Clouds with a low scattering ratio, high depolarization, and significant fraction of particles with radii of >2.0 μm had a mean index of refraction of 1.42±0.04 and a mode of 1.43. Clouds with a moderate scattering ratio, low depolarization, and fewer particles of >2.0 μm, had a mean index of refraction of 1.39±0.03 and a mode of 1.37. Ice clouds, apparent from measurements of high scattering ratio, high depolarization, and high concentrations of particles of >1.0 μm, had a mean index of refraction of 1.32±0.02 and a mode of 1.31. Measurements in volcanic aerosol indicated a mean index of 1.43±0.04.

Physical properties of stratospheric clouds during the Antarctic winter of 1995

Lidar observations collected during winter 1995 at McMurdo Station, Antarctica (78°S-l67°E), are analyzed to determine polar stratospheric cloud (PSC) physical properties. A scheme to infer PSC phase from lidar depolarization and backscatter profiles is presented. Interpretation is supported by collocated temperature soundings and by isentropic back trajectories. The analysis shows that first appearance of PSC is consistent with frozen sulfates, mixing with liquid ternary solutions (H 2 SO 4 -HNO 3 -H 2 O) when temperature lowers. Finally, solids consistent with HNO 3 mixing ratios form as mixed phases first, then followed by full solid phases. Mixed phases (i.e., coexisting solid and liquid aerosols) are detected during the whole winter. While mixed phase PSCs form particularly in the altitude range 15-20 km and are the last to disappear, full solid phases are mainly observed above 20 km and last until the end of August. Mixed phases possess the largest PSC surface areas and, as a result of selective growth, can reach large, fast settling sizes. The considerable denitrification and halogen activation observed in the Antarctic lower stratosphere, where the ozone hole takes place, appears to be well correlated with the action of this kind of PSC.

Modeling the Aerosol Extinction versus Backscatter Relationship for Lidar Applications: Maritime and Continental Conditions

Abstract A model to derive functional relationships linking extinction (α) and backscatter (β) of continental and maritime aerosol at 532 nm is presented and tested. These relationships are needed to solve the single-wavelength lidar equation, where both α and β appear as unknown variables. To obtain the investigated relationships, the extinction and backscatter coefficients of a large number (40 000) of continental and maritime aerosol size distributions and compositions have been computed. Each computation is performed randomly choosing the aerosol microphysical parameters within a variability range fixed according to data available in the literature. An altitude (z) dependence of aerosol microphysical parameters is included in the model so that z-dependent values α = α(z) and β = β(z) are obtained. By fitting the scatterplots of the α versus (β, z) points, three analytical expressions α = f(β, z) are then obtained corresponding, respectively, to maritime and continental aerosol computations and to thei...

Study of atmospheric aerosols and mixing layer by LIDAR

The LIDAR (laser radar) is an active remote sensing technique, which allows for the altitude-resolved observation of several atmospheric constituents. A typical application is the measurement of the vertically resolved aerosol optical properties. By using aerosol particles as a marker, continuous determination of the mixing layer height (MLH) can also be obtained by LIDAR. Some examples of aerosol extinction coefficient profiles and MLH extracted from a 1-year LIDAR data set collected in Milan (Italy) are discussed and validated against in situ data (from a balloon-borne optical particle counter). Finally a comparison of the observation-based MLH with relevant numerical simulations (mesoscale model MM5) is provided.

Polar stratospheric clouds over McMurdo, Antarctica, during the 1991 spring: Lidar and particle counter measurements

Lidar and balloonborne particle counter measurements were performed simultaneously on two days when polar stratospheric clouds were observed in late August 1991 at McMurdo, Antarctica. Both nitric acid trihydrate and ice clouds were observed in the lower stratosphere between 10 and 23 km in different formation stages and with different cooling rate; however in all cases the size distributions were bimodal. Comparison of scattering ratios measured by lidar and calculated from particle size distributions are in good agreement; however, discrepancies were observed when the lower stratosphere was highly perturbed by wave activity. Lee waves generated by air flowing over the Trans Antarctic Mountains induced ice cloud formation at altitudes as high as 20 km. No PSCs were observed after the end of August in 1991.

Early stratospheric effects of the Pinatubo Eruption

The Pinatubo eruptions of June 1991 introduced large plumes into the local stratosphere. On several occasions, volcanic gases and particles reached altitudes of about 30 km, quickly spreading to the west. Twenty days after the first eruption, the volcanic aerosol cloud was detected by lidar 14 km over Frascati, Italy. The upper portion of the cloud was observed for the first time on September 4, 1991, at an altitude of 23 km. Vertical and temporal evolution of the cloud, as observed from Frascati, are in agreement with trajectories estimated by means of northern hemisphere, stratospheric wind maps. Temperature records and characteristics of the cloud during the first 6 months following the eruption are reported. These characteristics are also compared to the ones of El Chichon, whose eruption, in 1982, rated amongst the largest in the century. This first analysis shows that, three and a half months after the eruption, the aerosol perturbation generated by Pinatubo reached and exceeded the maximum loads, recorded 11 months after the El Chichon event. However, by the end of the year, the aerosol columns of the two events tend towards comparable magnitudes.