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A new approach is introduced to characterize the dependence on altitude of the aerosol fine mode radius (Rf) and of the fine mode contribution (η) to the aerosol optical thickness (AOT) by three-wavelength lidar measurements. The introduced approach is based on the graphical method of Gobbi et al. (2007), which was applied to AERONET spectral extinction observations and relies on the combined analysis of the Ångstrom exponent (å) and its spectral curvature Δå. Lidar measurements at 355, 532 and 1064 nm were used in this study to retrieve the vertical profiles of å and Δå and to determine the dependence on altitude of Rf and η (532 nm) from the å-Δå combined analysis. Lidar measurements were performed at the Mathematics and Physics Department of Universita' del Salento, in south eastern Italy. Aerosol from continental Europe, the Atlantic, northern Africa, and the Mediterranean Sea are often advected over south eastern Italy and as a consequence, mixed advection patterns leading to aerosol properties varying with altitude are dominant. The proposed approach was applied to eleven measurement days to demonstrate its feasibility in different aerosol load conditions. The selected-days were characterized by AOTs spanning the 0.23-0.67, 0.15-0.41, and 0.04-0.25 range at 355, 532, and 1064 nm, respectively. Lidar ratios varied within the 28-80, 30-70, and 30-55 sr range at 355, 532, and 1064 nm, respectively, for the high variability of the aerosol optical and microphysical properties. å(355 nm, 1064 nm) values retrieved from lidar measurements ranged between 0.12 and 2.5 with mean value ±1 standard deviation equal to 1.4 ± 0.5. Δå varied within the -0.10-0.87 range with mean value equal to 0.1 ± 0.4. Rf and η (532 nm) values spanning the 0.02-0.30 μm and the 0.30-0.99 range, respectively were associated to the å-Δå data points. Rf and η values showed no dependence on the altitude. 72% of the data points were in the Δå-å space delimited by the η and Rf curves varying within 0.70-0.95 and 0.15-0.05 μm, respectively for the dominance of fine mode particles in driving the AOT over south eastern Italy. Volume depolarization vertical profiles retrieved from lidar measurements, aerosol products from AERONET sunphotometer measurements collocated in space and time, the BSC-DREAM model, analytical back trajectories, and satellite images were used to demonstrate the robustness of the proposed method. [ABSTRACT FROM AUTHOR]

The eruption of the Icelandic volcano Eyjafjallajökull in April/May 2010 represents a "natural experiment" to study the impact of volcanic emissions on a continental scale. For the first time, quantitative data about the presence, altitude, and layering of the volcanic cloud, in conjunction with optical information, are available for most parts of 5 Europe derived from the observations by the European Aerosol Research Lidar NETwork (EARLINET). Based on multi-wavelength Raman lidar systems EARLINET is the only instrument worldwide that is able to provide dense time series of high-quality optical data to be used for aerosol typing and for the retrieval of particle microphysical properties as a function of altitude. In this work we show the four-dimensional (4-D) distribution of the Eyjafjallajökull volcanic cloud over Europe as observed by EARLINET during the entire volcanic event (15 April-26 May 2010). All optical properties directly measured (backscatter, extinction, and particle linear depolarization ratio) are stored in the EARLINET database available at www.earlinet.org. A specific relational database providing the volcanic mask over 15 Europe, realized ad hoc for this specific event, has been developed and is available on request at www.earlinet.org. During the first days after the eruption, volcanic particles were detected over Central Europe within a wide range of altitudes, from the lower stratosphere down to the local Planetary Boundary Layer (PBL). After 19 April 2010, volcanic particles were detected 20 over South and South Eastern Europe. During the first half of May (5-15 May), material emitted by the Eyjafjallajökull volcano was detected over Spain and Portugal and then over the Mediterranean and the Balkans. Last observations of the event were recorded until 25 May in Central Europe and in the Eastern Mediterranean area. For the first time, volcanic aerosol layering and optical properties are presented and 25 discussed for the entire volcanic event on a continental scale providing an unprecedented data set for evaluating satellite data and aerosol dispersion models for these kind of volcanic events. [ABSTRACT FROM AUTHOR]

Volcanic aerosols resulting from the Eyjafjallajökull eruption have been detected in Southeastern Italy from 20 to 22 April 2010, at a distance of approximately 4000km from the volcano site, and have been characterized by lidar, sun/sky photometer, and in-situ measurements. Numerical simulations by the FLEXPART dispersion model, meteorological synoptic maps, and analytical backtrajectories confirm the advection of volcanic aerosols to the monitoring site. However, both the peak concentrations as well as the total column loadings of volcanic ash simulated by FLEXPART were about one order of magnitude lower than corresponding values simulated over Central Europe on 16 April. This suggests that the volcanic ash over Southeastern Italy was strongly diluted. Nevertheless, volcanic particles added to the pre-existing aerosol load and the integrated use of FLEXPART simulations and experimental measurements has allowed to clearly identifying the impact of volcanic particles on the aerosol vertical distribution, the aerosol size distribution, and the ground-level particulate-matter concentrations. Lidar measurements performed at the Physics Department of the University of Salento (40.4° N; 18.1° E) within EARLINET (European Aerosol Research LIdar NETwork EARLINET) have revealed the first arrival of volcanic aerosols on the afternoon of 20 April. In particular, lidar measurements have shown that at 18:30UTC of 20 April, lidar ratios (LRs) at 355nm varied from 65 to 71 sr inside the volcanic aerosol layer located between 2.5-3.5 km from the ground level and were characterized by smaller values (≃=45 sr) in the underlying layer. The LR dependence on altitude has decreased with time as volcanic particles also reached ground level. Then, LRs varied between 41 and 60 sr all over the aerosol column at 02:30UTC of 21 April. The time evolution of the aerosol optical depth from lidar measurements was similar to that of the ash-total column mass concentration from FLEXPART simulations after midday of 21 April, for the larger contribution of volcanic particles to the whole aerosol load. Sun/sky photometer measurements performed within AERONET, have revealed that the mass size distribution of volcanic particles retrieved from measurements performed on 21 April was in reasonable accordance with the volcanic-ash mass size distribution from FLEXPART simulations. Volcanic particles with radius r >0.5 μm have mainly been advected over Southeastern Italy and the contribution of coarse volcanic particles has increased from 20 to 22 April. The aerosol fine mode fraction from sun-sky photometer measurements varied between 0.85 and 0.94 on 20 April, but decreased to values between 0.25 and 0.82 on 22 April. Surface measurements of particle size distributions have also supported the advection of coarse volcanic particles. More specifically, mass concentrations of daily PM1 and PM2.5 samples revealed that the PM1/PM2.5 mass ratios were 0.69, 0.66, and 0.60 on 20, 21, and 22 April, respectively, indicating an increasing fraction of super-micron particles. Finally, measurements from the Regional Air Quality Agency have revealed enhanced PM10 and SO2 mass concentrations on 20, 21 and/or 22 April, 2010 all over the ~400 km long Apulia Region. The estimated enhancement of PM10 from volcanic particles was ~6 μgm-3 on 21 April at the monitoring site of this study, in satisfactory accordance with FLEXPART simulations. [ABSTRACT FROM AUTHOR]

The regional climate model RegCM3 coupled with a radiatively active aerosol model with online feedback is used to investigate direct and semi-direct radiative aerosol effects over the Sahara and Europe in a test case of July 2003. The aerosol model includes dust particles in addition to sulfates, hydrophobic and hydrophilic black carbon and organic carbon. The role of the aerosol online feedback on the radiation budget and the direct radiative forcing (short-wave and long-wave) by dust particles are investigated by intercomparing results from three experiments: REF, including all interactive aerosol components, Exp1, not accounting for the aerosol radiative feedback, and Exp2 not accounting for desert dust particles. The comparison of results in the REF experiment with satellite observations, sun/sky radiometer measurements, and lidar profiles at selected Central Mediterranean sites reveals that the spatio-temporal evolution of the aerosol optical depth is reasonably well reproduced by the model during the entire month of July. Results for the dust outbreaks of 17 and 24 July, averaged over the simulation domain, show that the daily-mean SW direct radiative forcing by all particles is -24 W/m² and -3.4 W/m² on 17 July and -25 W/m² and -3.5 W/m² on 24 July at the surface and top of the atmosphere, respectively. This is partially offset by a LW direct radiative forcing of ∼30% at the surface and of ∼50% at the ToA. It is also shown that atmospheric dynamics and hence dust production and advection processes are dependent on the simulation assumptions and may significantly change within few tens of kilometers. The comparison of REF and Exp1 shows that the aerosol online feedback on the radiation budget decreases the domain-average daily-mean value of the 2 m-temperature, aerosol column burden (CB), and short-wave (SW) atmospheric forcing by 0.52°C, 14%, and 0.9%, respectively on 17 July and by 0.39°C, 12% and 12%, respectively on 24 July. The comparison of REF and Exp2 reveals that on 17 July, radiatively-active dust particles decrease the daily-mean 2-m temperature averaged over the whole simulation domain by 0.12°C even if are responsible for 99.8% and 97% of the daily-mean aerosol column burden and SW atmospheric forcing, respectively. [ABSTRACT FROM AUTHOR]

The all-sky direct radiative effect by anthropogenic aerosol (DREa) is calculated in the solar (0.3-4 μm) and infrared (4-200 μm) spectral ranges for six Mediterranean sites. The sites are differently affected by pollution and together reflect typical aerosol impacts that are expected over land sites of the central Mediterranean basin. Central to the simulations are aerosol optical properties from AERONET sun-/sky-photometer statistics for the year 2003. A discussion on the variability of the overall (natural+anthropogenic) aerosol properties with site location is provided. Supplementary data include MODIS satellite sensor based solar surface albedos, ISCCP products for high- mid- and low cloud cover and estimates for the anthropogenic aerosol fraction from global modelling. Since anthropogenic aerosol particles are considered to be smaller than 1 μm in size, mainly the solar radiation transfer is affected with impacts only during sun-light hours. At all sites the (daily average) solar DREa is negative all year round at the top of the atmosphere (ToA). Hence, anthropogenic particles produce over land sites of the central Mediterranean a significant cooling effect. Monthly DREa values vary from site to site and are seasonal dependent as a consequence of the seasonal dependence of available sun-light and microphysical aerosol properties. At the ToA the monthly average DREa is -(4±1)Wm-2 during spring-summer (SS, April-September) and -(2±1)Wm-2 during autumn-winter (AW, October-March) at the polluted sites. In contrast, it varies between -(3±1)Wm-2 and -(1±1)Wm-2 on SS and AW, respectively at the less polluted site. Due to atmospheric absorption the DREa at the surface is larger than at the ToA. At the surface the monthly average DREa varies between the most and the least polluted site between -(7±1)Wm-2 and -(4±1)Wm-2 during SS, and between -(4±3)Wm-2 and -(1±1)Wm-2 during AW. The DREa at infrared wavelengths is positive but negligible, especially at the ToA (<0.3Wm-2). DREa monthly-means referring to all sites have been averaged to evaluate the yearly-mean value of the DREa. The ToA- and sfc-DREa yearly-mean value is -(3±2) and -(5±3)Wm-2, respectively at solar wavelengths. Last data further more reveal that the radiative energy-balance of the Central Mediterranean land sites is quite affected by anthropogenic particles. [ABSTRACT FROM AUTHOR]

Aerosol products by AERONET sun-sky radiometer measurements combined with airmass backtrajectories were analyzed to identify source regions and pathways of air masses carrying aerosols to south-east Italy, and to determine the dependence of aerosol mean optical properties on advection patterns. Aerosol optical depth (AOD), fine mode fraction (η), single scattering albedo (SSA), asymmetry factor (g), and lidar ratio (Lr) at 440nm were used to characterize aerosol properties. The analysis of 5-day-backtrajectories ending in Lecce on south-east Italy and referring to 240 measurement days of the 2003-2004 years revealed that 32% of the measurement days were characterized by air masses coming from Sector A, which includes all continental European sources with the exception of Spain. 3% of the measurement days were characterized by air masses coming from both the Sector B, which includes the Southern Mediterranean Sea and the Africa continent, and the Sector C, which includes the Western Mediterranean, the Iberian Peninsula, and the Atlantic Ocean. 62% of the measurement days were characterized by mixed advection patterns (Sector M). We found that AOD, SSA and g average values were not significantly dependent on air mass source Sector. In contrast, η and Lr average values were quite affected by the air mass source Sector. AOD, η, SSA, g, and Lr average values, which were equal to 0.29±0.15, 0.93±0.03, 0.93±0.03, 0.67±0.03, and 72±20 sr, respectively indicated that the Sector A aerosol could be considered representative of "continental average aerosol", mostly made of water soluble and a small amount of soot and insoluble components. Polluted-desert dust particles characterized by AOD=0.29±0.05, η=0.72±0.05, SSA=0.94±0.03, g=0.69±0.02, Lr=56±13 sr, were advected over south-east Italy from Sector B. Sector C was instead responsible for the advection of maritime-polluted particles, which were characterized by AOD=0.3±0.2, η=0.8±0.1, SSA=0.94±0.03, g=0.67±0.03, Lr=60±20 sr. Hence, we found that the aerosol load over south-east Italy was dominated by moderately-absorbing, fine-mode particles even if it was also affected by the minor contribution of desert and maritime type aerosol. The application of an aerosol mask to the Sector M data points supported last comment [ABSTRACT FROM AUTHOR]

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.

An intercomparison of the algorithms used to retrieve aerosol extinction and backscatter starting from Raman lidar signals has been performed by 11 groups of lidar scientists involved in the European Aerosol Research Lidar Network (EARLINET). This intercomparison is part of an extended quality assurance program performed on aerosol lidars in the EARLINET. Lidar instruments and aerosol backscatter algorithms were tested separately. The Raman lidar algorithms were tested by use of synthetic lidar data, simulated at 355, 532, 386, and 607 nm, with realistic experimental and atmospheric conditions taken into account. The intercomparison demonstrates that the data-handling procedures used by all the lidar groups provide satisfactory results. Extinction profiles show mean deviations from the correct solution within 10% in the planetary boundary layer (PBL), and backscatter profiles, retrieved by use of algorithms based on the combined Raman elastic-backscatter lidar technique, show mean deviations from solutions within 20% up to 2 km. The intercomparison was also carried out for the lidar ratio and produced profiles that show a mean deviation from the solution within 20% in the PBL. The mean value of this parameter was also calculated within a lofted aerosol layer at higher altitudes that is representative of typical layers related to special events such as Saharan dust outbreaks, forest fires, and volcanic eruptions. Here deviations were within 15%.

In the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET), 19 aerosol lidar systems from 11 European countries were compared. Aerosol extinction or backscatter coefficient profiles were measured by at least two systems for each comparison. Aerosol extinction coefficients were derived from Raman lidar measurements in the UV (351 or 355 nm), and aerosol backscatter profiles were calculated from pure elastic backscatter measurements at 351 or 355, 532, or 1064 nm. The results were compared for height ranges with high and low aerosol content. Some systems were additionally compared with sunphotometers and starphotometers. Predefined maximum deviations were used for quality control of the results. Lidar systems with results outside those limits could not meet the quality assurance criterion. The algorithms for deriving aerosol backscatter profiles from elastic lidar measurements were tested separately, and the results are described in Part 2 of this series of papers [Appl. Opt. 43, 977-989 (2004)]. In the end, all systems were quality assured, although some had to be modified to improve their performance. Typical deviations between aerosol backscatter profiles were 10% in the planetary boundary layer and 0.1 x 10(-6) m(-1) sr(-1) in the free troposphere.

An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET). The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals. The exercise consisted of processing synthetic lidar signals of various degrees of difficulty. One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms. Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar. The intercomparison was performed in three stages with increasing knowledge on the input parameters. First, only the lidar signals were distributed; this is the most realistic stage. Afterward the lidar ratio profiles and the reference values at calibration height were provided. The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance. These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio. The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well.

Results on the monitoring of strong African dust outbreaks at Lecce in the southeastern corner of Italy (40 degrees 20' N, 18 degrees 6' E) during May 2001 are presented. This activity has been performed in the framework of the European Aerosol Research Lidar Network (EARLINET). The lidar station of Lecce is located on a flat rural area that is approximately 800 km from the northern Africa coast. So it is closer to Africa than most of all other EARLINET stations and allow monitoring African dust transport early in its life cycle, at all levels in the plume. An elastic-backscatter Raman lidar based on a XeF excimer laser (351 nm) has been used to monitor the time evolution and vertical structure of the dust layers and get independent measurements of the aerosol extinction and backscatter coefficients. The findings are presented in terms of vertical profiles of the extinction and backscatter coefficients and of the lidar ratio. A quite deep dust layer extending between 2 and 6 km and characterized by a backscatter coefficient of approximately 0.0016 (km sr)(-1), a lidar ratio of approximately 50 sr, and an aerosol optical depth of 0.26 was observed on 17 May 2001 between 18:55 and 20:07 UT. The layer persisted for approximately five days. Dust layers of lower optical thickness and shorter persistence time have generally been monitored at the lidar site during African dust outbreaks. Results on the chemical and morphological characterization of the dust collected at the lidar station are also given to further support the origin of the monitored aerosol layers.

The benefits of retrieving ozone concentration profiles by a use of a single Raman signal rather than the Raman differential absorption lidar (DIAL) technique are investigated by numerical simulations applied either to KrF- (248 nm) or to quadrupled Nd:YAG- (266 nm) based Raman lidars, which are used for both daytime and nighttime monitoring of the tropospheric water-vapor mixing ratio. It is demonstrated that ozone concentration profiles of adequate accuracy and spatial and temporal resolution can be retrieved under low aerosol loading by a single Raman lidar because of the large value of the ozone absorption cross section both at 248 nm and at 266 nm. Then experimental measurements of Raman signals provided by the KrF-based lidar operating at the University of Lecce (40 degrees 20'N, 18 degrees 6'E) are used to retrieve ozone concentration profiles by use of the Raman DIAL technique and the nitrogen Raman signal.