Francesca Barnaba

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.

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.

Forecast errors in dust vertical distributions over Rome (Italy): Multiple particle size representation and cloud contributions

[1] In this study, forecast errors in dust vertical distributions were analyzed. This was carried out by using quantitative comparisons between dust vertical profiles retrieved from lidar measurements over Rome, Italy, performed from 2001 to 2003, and those predicted by models. Three models were used: the four-particle-size Dust Regional Atmospheric Model (DREAM), the older one-particle-size version of the SKIRON model from the University of Athens (UOA), and the pre-2006 one-particle-size Tel Aviv University (TAU) model. SKIRON and DREAM are initialized on a daily basis using the dust concentration from the previous forecast cycle, while the TAU model initialization is based on the Total Ozone Mapping Spectrometer aerosol index (TOMS AI). The quantitative comparison shows that (1) the use of four-particle-size bins in the dust modeling instead of only one-particle-size bins improves dust forecasts; (2) cloud presence could contribute to noticeable dust forecast errors in SKIRON and DREAM; and (3) as far as the TAU model is concerned, its forecast errors were mainly caused by technical problems with TOMS measurements from the Earth Probe satellite. As a result, dust forecast errors in the TAU model could be significant even under cloudless conditions. The DREAM versus lidar quantitative comparisons at different altitudes show that the model predictions are more accurate in the middle part of dust layers than in the top and bottom parts of dust layers.

Imaginary refractive-index effects on desert-aerosol extinction versus backscatter relationships at 351 nm: numerical computations and comparison with Raman lidar measurements

A numerical model is used to investigate the dependence at 351 nm of desert-aerosol extinction and backscatter coefficients on particle imaginary refractive index (mi). Three ranges (-0.005 < or = mi < or = -0.001, -0.01 < or = mi < or = -0.001, and -0.02 < or = mi < or = -0.001) are considered, showing that backscatter coefficients are reduced as /mi/ increases, whereas extinction coefficients are weakly dependent on mi. Numerical results are compared with extinction and backscatter coefficients retrieved by elastic Raman lidar measurements performed during Saharan dust storms over the Mediterranean Sea. The comparison indicates that a range of -0.01 to -0.001 can be representative of Saharan dust aerosols and that the nonsphericity of mineral particles must be considered.

Transport of Po Valley aerosol pollution to the northwestern Alps. Part 1: phenomenology

Mountainous regions are often considered pristine environments; however they can be affected by pollutants emitted in more populated and industrialised areas, transported by regional winds. Based on experimental evidence, further supported by modelling tools, here we demonstrate and quantify the impact of air masses transported from the Po Valley, a European atmospheric pollution hotspot, to the northwestern Alps. This is achieved through a detailed investigation of the phenomenology of near-range (a few hundred kilometres), trans-regional transport, exploiting synergies of multi-sensor observations mainly focussed on particulate matter. The explored dataset includes vertically resolved data from atmospheric profiling techniques (automated lidar ceilometers, ALCs), vertically integrated aerosol properties from ground (sun photometer) and space, and in situ measurements (PM10 and PM2.5, relevant chemical analyses, and aerosol size distribution). During the frequent advection episodes from the Po basin, all the physical quantities observed by the instrumental setup are found to significantly increase: the scattering ratio from ALC reaches values > 30, aerosol optical depth (AOD) triples, surface PM10 reaches concentrations > 100 μgm−3 even in rural areas, and contributions to PM10 by secondary inorganic compounds such as nitrate, ammonium, and sulfate increase up to 28 %, 8 %, and 17 %, respectively. Results also indicate that the aerosol advected from the Po Valley is hygroscopic, smaller in size, and less light-absorbing compared to the aerosol type locally emitted in the northwestern Italian Alps. In this work, the phenomenon is exemplified through detailed analysis and discussion of three case studies, selected for their clarity and relevance within the wider dataset, the latter being fully exploited in a companion paper quantifying the impact of this phenomenology over the long-term (Diémoz et al., 2019). For the three case studies investigated, a high-resolution numerical weather prediction model (COSMO) and a Lagrangian tool (LAGRANTO) are employed to understand the meteorological mechanisms favouring transport and to demonstrate the Po Valley origin of the air masses. In addition, a chemical transport model (FARM) is used to further support the observations and to partition the contributions of local and non-local sources. Results show that the simulations are important to the understanding of the phenomenon under investigation. However, in quantitative terms, modelled PM10 concentrations are 4–5 times lower than the ones retrieved from the ALC and maxima are anticipated in time by 6–7 h. Underestimated concentrations are likely mainly due to deficiencies in the emission inventory and to water uptake of the advected particles not fully reproduced by FARM, while timing mismatches are likely an effect of suboptimal simulation of up-valley and down-valley winds by COSMO. The advected aerosol is shown to remarkably degrade the air quality of the Alpine region, with potential negative effects Published by Copernicus Publications on behalf of the European Geosciences Union. 3066 H. Diémoz et al.: Transport of Po Valley aerosol pollution to the northwestern Alps on human health, climate, and ecosystems, as well as on the touristic development of the investigated area. The findings of the present study could also help design mitigation strategies at the trans-regional scale in the Po basin and suggest an observation-based approach to evaluate the outcome of their implementation.

On the Interplay Between Upper and Ground Levels Dynamics and Chemistry in Determining the Surface Aerosol Budget

We use the WRF/Chem model to interpret observations of the aerosol concentration and its chemical composition both at surface level and along vertical profiles performed during an intensive campaign in July 2007 in Milan urban area. The model is added with a new diagnostic for aerosol budget analysis, building on that available for gas species, in order to study the contribution of upper levels processes on the aerosol formation at ground level. The analysis illustrates a quite variegated evolution of budget terms, which we found to depend strongly on the hour of the day, the vertical level, the aerosol compound, and the aerosol size. Primary components are generally emitted near the ground and rapidly transported by turbulent motions to the upper levels, where they gradually disperse and age. For some secondary components, such as nitrate, we calculate a net chemical destruction in the bottom layers, as opposed to a net chemical production higher in the boundary layer, which supply new material to ground level aerosol through turbulent mixing.

Chapter 1.5 Assessment of dust forecast errors by using lidar measurements over Rome

Abstract In this study, forecast errors in dust vertical distributions were analyzed. This was carried out by using quantitative comparisons between dust vertical profiles retrieved from lidar measurements over Rome, Italy, and those predicted by models. Three models were used: the four-particle-size Dust Regional Atmospheric Model (DREAM), the older one-particle-size version of the SKIRON model from the University of Athens (UOA), and the pre-2006 one-particle-size Tel Aviv University (TAU) model. SKIRON and DREAM are initialized on a daily basis using the dust concentration from the previous forecast cycle, while the TAU model initialization is based on the Total Ozone Mapping Spectrometer aerosol index (TOMS AI). The quantitative comparison shows that (1) the use of four-particle-size bins in the dust modeling instead of only one-size bin improves dust forecasts, (2) cloud presence could contribute to additional dust forecast errors in SKIRON and DREAM, (3) as far as the TAU model is concerned, its forecast errors were mainly caused by technical problems with TOMS measurements from the Earth Probe satellite. As a result, dust forecast errors in the TAU model could be significant even under cloudless conditions.