Saharan Dust Transport Events Research Articles

Lidar Profiling of Aerosol Vertical Distribution in the Urbanized French Alpine Valley of Annecy and Impact of a Saharan Dust Transport Event

The vertical aerosol layering of the troposphere is poorly documented in mountainous regions, particularly in the Alpine valleys, which are influenced by valley and mountain winds. To improve our knowledge of particulate matter trapped in the Annecy valley, synergetic measurements performed by a ground-based meteorological Raman lidar and a Rayleigh-Mie lidar aboard an ultralight aircraft were implemented as part of the Lacustrine-Water vApor Isotope inVentory Experiment (L-WAIVE) over Lake Annecy. These observations were complemented by satellite observations and Lagrangian modeling. The vertical profiles of aerosol optical properties (e.g., aerosol extinction coefficient (AEC), lidar ratio (LR), particle linear depolarization ratio (PDR)) are derived from lidar measurements at 355 nm during the period between 13 and 22 June 2019. The background aerosol content with an aerosol optical thickness (AOT) of 0.10 ± 0.05, corresponding to local–regional conditions influenced by anthropogenic pollution, has been characterized over the entirety of Lake Annecy thanks to the mobile ultralight payload. The aerosol optical properties are shown to be particularly variable over time in the atmospheric column, with mean LRs (PDRs) varying between 40 ± 8 and 115 ± 15 sr (2 ± 1 and 35 ± 2%). Those conditions can be disturbed by air masses that have recirculated over the valley, as well as by contributions from neighboring valleys. We have observed an important disruption in the atmospheric aerosol profiles by the arrival of an exceptionally dry air mass (RH ~ 30%), containing aerosols identified as coming from the Great Western Erg (AOT ~ 0.5, LR = 65 ± 10 sr, PDR = 20–35%) in the Sahara. These desert dust particles are shown to influence the entire atmospheric column in the Annecy valley. Such an experimental approach, coupling upward and downward lidar and spaceborne observation/Lagrangian modelling, was shown to be of significant interest for the long-term monitoring of the evolution of aerosol loads over deep valleys. It allows a better understanding of the influence of dust storms in the presence of severe convective weather processes.

Aerosol Characterization with Long-Term AERONET Sun-Photometer Measurements in the Naples Mediterranean Area

We report on the characterization of columnar aerosol optical and microphysical properties in the Naples Mediterranean area over a period of five years by the ground-based sun–sky–lunar photometer operating at our observational station in the frame of the AERONET network. Statistical and climatological analyses of daily mean values of aerosol optical depth at 440 nm (AOD440) and Ångström exponent at 440/870 nm (α440/870) allowed for highlighting their typical seasonal behavior. In particular, we observe higher mean values of the AOD440 during summer or spring, which are consistent with an increased frequency of both Saharan dust transport events and biomass burning episodes affecting the measurement area in these periods of the year. Conversely, α440/870 does not show any typical seasonal trend. In order to gain information on the different aerosol contributions along the atmospheric column, the frequency distributions of AOD440 and α440/870 were analyzed and fitted by a superposition of Gaussian functions. The most populated modes are centered at α440/870 = 1.26 ± 0.07 and AOD440 = 0.16 ± 0.01. These values are associated with continental polluted aerosol mixed with sea salt aerosol and correspond to the background conditions typically observed in clear atmospheric conditions. Daily size distributions averaged over each month highlight that the fine aerosol component always prevails over the coarse fraction, except for the few months in which Saharan dust events are particularly frequent. Finally, the mean value of the SSA at 440 nm resulted as 0.94 ± 0.05, while the refractive index real and imaginary part were 1.47 ± 0.07 and (6.5 ± 0.2) × 10−3, respectively. These values are in agreement with those observed in other Mediterranean sites located in Southern Italy, evidencing a rather characteristic feature of the geographical region.

Dust transport and advection measurement with spaceborne lidars ALADIN and CALIOP and model reanalysis data

Abstract. In this paper, a long-term large-scale Saharan dust transport event which occurred between 14 and 27 June 2020 is tracked with the spaceborne lidars ALADIN (Atmospheric Laser Doppler Instrument) and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) together with ECMWF (European Centre for Medium-Range Forecasts) and HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory model) analysis. We evaluate the performance of ALADIN and CALIOP on the observations of dust optical properties and wind fields and explore the possibility of tracking the dust events and calculating the dust mass advection with the combination of satellite and model data. The dust plumes are identified with the AIRS/Aqua Dust Score Index and with the vertical feature mask product from CALIOP. The emission, dispersion, transport and deposition of the dust event are monitored using the data from AIRS/Aqua, CALIOP and HYSPLIT. With the quasi-synchronized observations by ALADIN and CALIOP, combined with the wind field and relative humidity, the dust advection values are calculated. From this study, it is found that the dust event generated on 14 and 15 June 2020 from the Sahara in North Africa dispersed and moved westward over the Atlantic Ocean, finally being deposited in the western Atlantic Ocean, the Americas and the Caribbean Sea. During the transport and deposition processes, the dust plumes are trapped in the northeasterly trade-wind zone between latitudes of 5∘ and 30∘ N and altitudes of 0 and 6 km. Aeolus provided the observations of the dynamics of this dust transport event in the Saharan Air Layer (SAL). From the measurement results on 19 June 2020, the dust plumes are captured quasi-simultaneously over the emission region (Western Sahara), the transport region (middle Atlantic) and the deposition region (western Atlantic) individually, which indicates that the dust plume area over the Atlantic on the morning of this day is quite enormous and that this dust transport event is massive and extensive. The quasi-synchronization observation results of 15, 16, 19, 24 and 27 June by ALADIN and CALIOP during the entire transport process show good agreement with the Dust Score Index data and the HYSPLIT trajectories, which indicates that the transport process of the same dust event is tracked by ALADIN and CALIOP, verifies that the dust transport spent around 2 weeks from the emission to the deposition and achieved the respective observations of this dust event's emission phase, development phase, transport phase, descent phase and deposition phase. Finally, the advection values for different dust parts and heights on 19 June and on the entire transport routine during transportation are computed. On 19 June, the mean dust advection values are about 1.91±1.21 mg m−2 s−1 over the emission region, 1.38±1.28 mg m−2 s−1 over the transport region and 0.75±0.68mgm-2s-1 over the deposition region. In the whole lifetime of the dust event, the mean dust advection values were about 1.51±1.03mgm-2s-1 on 15 June 2020, 2.19±1.72mgm-2s-1 on 16 June 2020, 1.38±1.28mgm-2s-1 on 19 June 2020, 1.60±1.08mgm-2s-1 on 24 June 2020 and 1.03±0.60mgm-2s-1 on 27 June 2020. During the dust development stage, the mean advection values gradually increased and reached their maximum on 16 June with the enhancement of the dust event. Then, the mean advection values decreased during the transport and the deposition of the dust over the Atlantic Ocean, the Americas and the Caribbean Sea.

Characterization of In Situ Aerosol Optical Properties at Three Observatories in the Central Mediterranean

In this work, results of scattering and backscattering coefficients, scattering Ångström exponent (SAE), single scattering albedo (SSA), and asymmetry parameter (g) of atmospheric aerosols are presented. All these parameters were measured during the month of April 2016 in Southern Italy on three different Global Atmosphere Watch observatories in the Central Mediterranean. This is the first time, to our knowledge, that optical aerosol properties were studied at the same time, even if in a brief intensive measurement campaign, at three sites in the South of Italy. In order to obtain a characteristic value for aerosol optical properties, different kinds of particle sources (i.e., dust, marine, and anthropic) have been identified and studied. In the measurement period, one event of a long-range transport of Saharan dust from Northern Africa was observed at all observatories. During the Saharan dust transport event, a minimum value of the SAE (0.69 ± 0.34) and a relatively higher values of SSA were observed. During the dust event, g increased up to 0.69. Marine aerosol contribution and anthropic/urban aerosol intrusion were analysed. From this analysis, SAE average values were 0.70, 0.84, and 1.22, respectively, for dust, marine, and anthropic particles. On the other hand, the SSA minimum value was 0.86 for anthropic particles, and it increased for dust (0.88) and marine (0.93) aerosols. The asymmetry parameter had a limited variability for the three types of aerosol from 0.62 to 0.58, as reported also in literature.

Atmospheric Ice Nucleating Particle measurements at the high mountain observatory Mt. Cimone (2165 m a.s.l., Italy)

Measurement campaigns at the high mountain Observatory Mt. Cimone (CMN; 2165 m a.s.l.) were performed during May 2014 and October 2015. The concentration of Ice Nucleating Particles (INPs) were measured offline with a dynamic filter processing chamber (DFPC), in the deposition and condensation mode, after collecting PM1 and PM10 aerosol samples. Presented INP data are the first ever published for a high mountain site in the Mediterranean basin. During the May campaign, parallel INP measurements were also carried out at San Pietro Capofiume (SPC), a low altitude rural background area within the Po Valley basin, by the same offline technique.The average INPPM10 concentration at CMN was 86 m−3 (saturation ratio Sw = 1.01; T = −18 °C) during the May 2014 campaign, while it was 171 m−3 at SPC, in the same period. A lower average INPPM10 concentration was observed at CMN during October 2015 (43 m−3). A significantly higher activated fraction (AF) characterized the October 2015 campaign, suggesting that the seasonal changes in the aerosol sources have an impact on the INP efficiency of the aerosol at CMN. Super-micrometer INP contributed for ∼30% of total INP in the May 2014 campaign, at both sites, and for ∼70% in the October 2015 campaign, showing the importance of coarse particles in the INP population.The analysis of meteorological parameters, gaseous tracers concentrations and backwards trajectories suggests that the INP population at CMN is contributed by transport processes occurring at diverse spatial scales, from the local to the synoptic scale. During the Saharan Dust transport Event observed in May 2014, a reduction of the AF was observed, suggesting, for this case, a limited ice nucleating activity for Saharan dust particles. This may be due to physico-chemical aging of the Saharan dust particles during transport or to the relatively high activation temperature at which the experiments were performed in this study.

Long-term (2002–2012) investigation of Saharan dust transport events at Mt. Cimone GAW global station, Italy (2165 m a.s.l.)

Mineral dust transport from North Africa towards the Mediterranean basin and Europe was monitored over an 11-y period (2002–2012) using the continuous observations made at Mt. Cimone WMO/GAW global station (CMN). CMN is in a strategic position for investigating the impact of mineral dust transported from northern Africa on the atmospheric composition of the Mediterranean basin and southern Europe. The identification of “dusty days” is based on coupling the measured in situ coarse aerosol particle number concentration with an analysis of modeled back trajectories tracing the origin of air masses from North Africa. More than 400 episodes of mineral dust transport were identified, accounting for 15.7% of the investigated period. Our analysis points to a clear seasonal cycle, with the highest frequency from spring to autumn, and a dust-induced variation of the coarse particle number concentration larger than 123% on a seasonal basis. In addition, FLEXTRA 10-d back trajectories showed that northwestern and central Africa are the major mineral dust source regions. Significant inter-annual variability of dust outbreak frequency and related mineral dust loading were detected and during spring the NAO index was positively correlated (R2 = 0.32) with dust outbreak frequency. Lastly, the impact of transported mineral dust on the surface O3 mixing ratio was quantified over the 11-y investigation period. Evidence of a non-linear and negative correlation between mineral dust and ozone concentrations was found, resulting in an average spring and summer decrease of the O3 mixing ratio down to 7%.

Chemical Composition of Fine and Coarse Aerosol Particles in the Central Mediterranean Area during Dust and Non-Dust Conditions

A two-month field campaign was carried out from May to June 2010 at a remote site (Trisaia ENEA Research Centre) in the Southern Italy aiming to identify and quantify the changes of aerosol chemical composition in the presence of Saharan dust. The 24-hr PM_(10) and PM_(2.5) filter samples were analyzed by mass, carbonaceous species, inorganic ions and elemental composition. Saharan dust transport events were identified with two approaches: one recommended by EC (2011) and one based on indicators derived from measurements. Three indicators were used: PM_(2.5)/PM_(10) mass concentrations ratio, Ca/Al ratio and Al concentration. Based on these criteria, four Saharan dust transport events were identified, but only one had elevated dust concentration and leaded to an exceedance of the European short-term (24 hour) limit value of 50 μg/m^3 for PM_(10) (June 16^(th)). The comparison of chemical composition of fine and coarse aerosol fractions during dust and non-dust conditions shows that the presence of dust increases NH_4 and nssSO_4 concentrations in the fine fraction and NO_3 and nssSO_4 concentrations in the coarse fraction. OC and EC concentrations also increase in the fine fraction during dust transport. The uptake of primary and secondary species, inorganic and organic, by dust particles changes their composition and, thus, their properties and this may have implications for human health and climate change.

Thermal perception of teenagers in a cool outdoor environment: A case study

Subjective thermal sensation of late teenagers was examined aiming to reveal potential discrepancies in its estimation compared to adults. Since teenagers have different clothing habits and preferences from adults, it is important to know whether the conclusions reached by studies on thermal sensation, usually involving adults, can be also applied to teenagers. A group of late teenagers was interviewed, based on a structured questionnaire, in an outdoor environment during two winter days and under an unexpected Saharan dust transport event during the second day, while meteorological measurements were obtained by the closest to the interview site weather station. Moreover, the performance of the bioclimatic index Cooling Power in simulating subjects' thermal sensation was evaluated. Although differences in clothing thermal insulation of late teenagers compared to that suggested by similar studies were recognized, generally the results of this study were in agreement with the findings of similar field surveys focusing on individual thermal sensation and with no evidence of effects of the dust transport event on thermal sensation. Cooling Power based on the Mediterranean thermal sensation scale predicted thermal sensation vote fairly well.

Analysis of meteorology–chemistry interactions during air pollution episodes using online coupled models within AQMEII phase-2

This study reviews the top ranked meteorology and chemistry interactions in online coupled models recommended by an experts' survey conducted in COST Action EuMetChem and examines the sensitivity of those interactions during two pollution episodes: the Russian forest fires 25 Jul–15 Aug 2010 and a Saharan dust transport event from 1 Oct to 31 Oct 2010 as a part of the AQMEII phase-2 exercise. Three WRF-Chem model simulations were performed for the forest fire case for a baseline without any aerosol feedback on meteorology, a simulation with aerosol direct effects only and a simulation including both direct and indirect effects. For the dust case study, eight WRF-Chem and one WRF-CMAQ simulations were selected from the set of simulations conducted in the framework of AQMEII. Of these two simulations considered no feedbacks, two included direct effects only and five simulations included both direct and indirect effects. The results from both episodes demonstrate that it is important to include the meteorology and chemistry interactions in online-coupled models. Model evaluations using routine observations collected in AQMEII phase-2 and observations from a station in Moscow show that for the fire case the simulation including only aerosol direct effects has better performance than the simulations with no aerosol feedbacks or including both direct and indirect effects. The normalized mean biases are significantly reduced by 10–20% for PM10 when including aerosol direct effects. The analysis for the dust case confirms that models perform better when including aerosol direct effects, but worse when including both aerosol direct and indirect effects, which suggests that the representation of aerosol indirect effects needs to be improved in the model.

Composition and Mass Closure of PM2.5 in Urban Environment (Athens, Greece)

Daily PM2.5 concentrations and composition were monitored at an urban site at 14 m above ground level, at the National Technical University of Athens campus from February to December 2010. Total sulfur, crustal origin elements and elements of a major crustal component (Al, Si, Fe, Ca, K, Mg, Ti) trace elements (Mn, V, Pb, Cu, Zn, Ni) and water soluble ions (Cl – , NO3 – , SO4 2– , Na + , K + , NH4 + , Ca 2+ , Mg 2+ ) were determined. Carbonaceous compounds were determined for a period of one month. Sulfur (sulfates) and carbonaceous material were the most abundant constituents (35% and 30% respectively). Ionic balance calculations revealed a cation deficit in PM2.5 attributed to H + associated with sulfates. The concentrations of the elements of crustal origin presented abrupt increases during Saharan dust transport events. Mass closure was attempted considering Secondary Inorganic Aerosol (SIA), Particulate Organic Matter (POM), Elemental Carbon (EC), Dust, Mineral Anthropogenic Component (MIN) and Sea Salt (SS). The sum of these components accounted for about 75% of the measured PM2.5 concentrations. SIA and carbonaceous material (OM + EC) contributed almost equally for about 30% in the PM2.5 mass, and while the dust contribution was significant during dust transport events, it was only about 5% in absence of such events. The contribution of sea salt was calculated to be about 4%.

Influence of Saharan Dust Transport Events on PM2.5 Concentrations and Composition over Athens

The evaluation of the contribution of natural sources to PM10 and PM2.5 concentrations is a priority especially for the countries of European south strongly influenced by Saharan dust transport events. Daily PM2.5 concentrations and composition were monitored at an urban site at 14 m above ground level, at the National Technical University of Athens campus from February to December 2010. The typical dust constituents Si, Al, Fe, K, Ca, Mg, and Ti were determined by wavelength dispersive X-ray fluorescence spectrometry (WDXRF). Sulfur, a tracer of anthropogenic origin and major constituent of PM2.5, was determined by both WDXRF and ionic chromatography. The contribution of dust and sulfates in PM2.5 was calculated from the analytical determinations. An annual mean of 20 μg/m3 was calculated from the mean daily PM2.5 concentrations data. Twenty-two per cent of daily concentrations of PM2.5 reached or exceeded the EU annual target concentration of 25 μg/m3. The exceedances occurred during 13 short periods of 1–4 days. Back-trajectory analysis was performed for these periods in order to identify the air masses origin. From these periods, ten periods were associated to Saharan dust transport events. The most intense dust transport event occurred between February 17th and 20th and was responsible for the highest recorded PM2.5 concentration of 100 μg/m3 where the dust contribution in PM2.5 reached 96 %. The other dust transport events were less intense and corresponded to less pronounced enhancements of PM2.5 concentrations, and their contribution ranged from 15 to 39 % in PM2.5 concentrations. Air masses originated from northwest Africa while the influence of central Sahara was quite smaller.

Optical-microphysical properties of Saharan dust aerosols and composition relationship using a multi-wavelength Raman lidar, in situ sensors and modelling: a case study analysis

Abstract. A strong Saharan dust event that occurred over the city of Athens, Greece (37.9° N, 23.6° E) between 27 March and 3 April 2009 was followed by a synergy of three instruments: a 6-wavelength Raman lidar, a CIMEL sun-sky radiometer and the MODIS sensor. The BSC-DREAM model was used to forecast the dust event and to simulate the vertical profiles of the aerosol concentration. Due to mixture of dust particles with low clouds during most of the reported period, the dust event could be followed by the lidar only during the cloud-free day of 2 April 2009. The lidar data obtained were used to retrieve the vertical profile of the optical (extinction and backscatter coefficients) properties of aerosols in the troposphere. The aerosol optical depth (AOD) values derived from the CIMEL ranged from 0.33–0.91 (355 nm) to 0.18–0.60 (532 nm), while the lidar ratio (LR) values retrieved from the Raman lidar ranged within 75–100 sr (355 nm) and 45–75 sr (532 nm). Inside a selected dust layer region, between 1.8 and 3.5 km height, mean LR values were 83 ± 7 and 54 ± 7 sr, at 355 and 532 nm, respectively, while the Ångström-backscatter-related (ABR355/532) and Ångström-extinction-related (AER355/532) were found larger than 1 (1.17 ± 0.08 and 1.11 ± 0.02, respectively), indicating mixing of dust with other particles. Additionally, a retrieval technique representing dust as a mixture of spheres and spheroids was used to derive the mean aerosol microphysical properties (mean and effective radius, number, surface and volume density, and mean refractive index) inside the selected atmospheric layers. Thus, the mean value of the retrieved refractive index was found to be 1.49( ± 0.10) + 0.007( ± 0.007)i, and that of the effective radiuses was 0.30 ± 0.18 μm. The final data set of the aerosol optical and microphysical properties along with the water vapor profiles obtained by Raman lidar were incorporated into the ISORROPIA II model to provide a possible aerosol composition consistent with the retrieved refractive index values. Thus, the inferred chemical properties showed 12–40% of dust content, sulfate composition of 16–60%, and organic carbon content of 15–64%, indicating a possible mixing of dust with haze and smoke. PM10 concentrations levels, PM10 composition results and SEM-EDX (Scanning Electron Microscope-Energy Dispersive X-ray) analysis results on sizes and mineralogy of particles from samples during the Saharan dust transport event were used to evaluate the retrieval.

PM 10 composition during an intense Saharan dust transport event over Athens (Greece)

The influence of Saharan dust on the air quality of Southern European big cities became a priority during the last decade. The present study reports results on PM 10 monitored at an urban site at 14 m above ground level during an intense Saharan dust transport event. The elemental composition was determined by Energy Dispersive X-ray Fluorescence Spectrometry (EDXRF) for 12 elements: Si, Al, Fe, K, Ca, Mg, Ti, S, Ni, Cu, Zn and Mn. PM 10 concentrations exceeded the EU limit (50 μg/m 3) several times during the sampling period. Simultaneous maxima have been observed for the elements of crustal origin. The concentrations of all the elements presented a common maximum, corresponding to the date where the atmosphere was heavily charged with particulate matter permanently for an interval of about 10 h. Sulfur and heavy metal concentrations were also associated to local emissions. Mineral dust represented the largest fraction of PM 10 reaching 79%. Seven days back trajectories have shown that the air masses arriving over Athens, originated from Western Sahara. Scanning Electron Microscopy coupled with Energy Dispersive X-ray analysis (SEM-EDX) revealed that particle agglomerates were abundant, most of them having sizes < 2 μm. Aluminosilicates were predominant in dust particles also rich in calcium which was distributed between calcite, dolomite, gypsum and Ca–Si particles. These results were consistent with the origin of the dust particles and the elemental composition results. Sulfur and heavy metals were associated to very fine particles < 1 μm.

A Case Study: The Indirect Aerosol Effects of Mineral Dust on Warm Clouds

Abstract The indirect aerosol effect (Twomey effect) is studied during a Saharan dust-transport event that presented an unusually favorable combination of a dust-loading gradient across clouds with warm cloud-top temperatures. Standard retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS), the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E), and the Clouds and the Earth’s Radiant Energy System (CERES) provide cloud-top temperature (a surrogate for height), liquid water path (LWP), classification of precipitation regime, and radiation flux. The authors correlate a retrieved mean effective droplet radius (re) versus the number concentration of cloud condensation nuclei (NCCN), using the regressed slope d lnre/d lnNCCN as the estimator of the aerosol indirect effect (AIE). Results demonstrate statistically significant AIE for only some of the segregated cloud classes. For nonprecipitating clouds (the most direct test of Twomey effect), the estimated AIE is effectively −0.07 over all wider temperature bands and is statistically significant from 1.1 to 1.9 σ. Further classification by LWP strengthens both the AIE (for all LWP &amp;gt; 150 g m−2) to approximately −0.16, and substantially increases the statistical significance, to better than 5σ. Shortwave radiation forcing of dust aerosols is also estimated directly from satellite measurements. The direct shortwave (SW) radiation effect of Saharan dusts at solar zenith angle 21.6° is 53.48 ± 8.56 W m−2 per unit aerosol optical depth, with a correlation coefficient of 0.92. The indirect SW forcing of Saharan dust is 29.88 ± 2.42 W m−2 per unit AOD for clouds with LWP of 100 g m−2.

Multi-spectral Lidar characterization of the vertical structure of Saharan dust aerosol over southern Spain

Measurements during Saharan dust transport events have been performed at the Andalusian Centre for Environmental Studies (CEAMA) (37.16°N, 3.58°W, 680 m a.s.l.), located in the city of Granada (south-eastern Iberian Peninsula). During 2005, as many as 22 dust outbreaks took place over the study area in south-eastern Spain. The arrival of Saharan air masses induced sharp changes in the background aerosol optical depth (AOD) and the Angström exponent during the analysed events. This fact, in addition to a back-trajectory analysis, identified the Saharan dust events. In order to improve knowledge of Saharan dust over the southern Iberian Peninsula, vertical characterization of temporal evolutions has been performed in terms of the backscatter coefficient, aerosol depolarization ratio and colour ratio. Two severe events in July 2005 have been selected for detailed analysis. Both events lasted several days, allowing the characterization of the evolution of the dust layers over our station. The impact of Saharan dust outbreaks has been followed with the Lidar, and their effects at the surface have been determined in the air quality network of the regional government.

Estimating the impact of Saharan dust on the year 2001 PM 10 record of Rome, Italy

This paper evaluates the role of Saharan dust advection in the exceeding of the PM 10 thresholds in the city of Rome, Italy. To this purpose, a series of observations and model forecasts recorded in the year 2001 are analysed and discussed. Lidar profiles collected over 168 days of the year are employed to both assess the presence and magnitude of Saharan dust layers over the city and to evaluate the depth of the planetary boundary layer. Backtrajectories are used to verify the Saharan origin of the lidar-sounded air masses. Model predictions of the presence of Saharan dust over the area are employed to fill the time gaps between lidar observations. PM 10 and carbon monoxide records of both a city background (Villa Ada) and a heavy traffic station (Magna Grecia) are cross-analysed with the dust events record and meteorological data. The analysis shows that: (1) Saharan dust was advected over Rome on about 30% of the days of 2001; (2) mean contribution of Saharan dust transport events to daily PM 10 levels was of the order of 20 μg m −3; (3) at the urban background station of Villa Ada, the Saharan contribution caused the surpassing of the maximum number of days in excess of 50 μg m −3 fixed by the current legislation (35 per year). Conversely, at the heavy traffic station of Magna Grecia the Saharan contribution was not determinant at causing the observed large exceeding of that limit, as well as of the maximum yearly average of 40 μg m −3; (4) 25% of the Saharan advection days (of the order of 100/year at Rome) led to a PM 10 increase >30 μg m −3, 4% caused an increase >50 μg m −3, thus leading on their own to surpassing the 50 μg m −3 daily limit.

Optical properties of Saharan dust layers as detected by a Raman lidar at Thessaloniki, Greece

Measurements during Saharan dust transport events were performed at several stations of the European Aerosol Research Lidar Network. During the period 2001–2002, 12 cases were captured at Thessaloniki, Greece (40.5°N, 22.9°E) with a 355 nm Raman lidar. For these cases the vertical profiles of the extinction and backscatter coefficients were determined and examined, as well as profiles of extinction‐to‐backscatter ratio (lidar ratio) and the backscatter‐related Ångström exponent between 355 and 532 nm (color index). Model calculations from the DREAM model confirmed the existence of Saharan dust in all cases examined. Within the dust plumes the lidar ratios ranged from 20 to 100 sr and the color index ranged from −1 to 3. An anticorrelation was found between the lidar ratio and the color index during the Saharan dust events. In certain cases the results suggest that the mixing of dust particles with boundary layer aerosols reduces the dust ‘signature’ and thus, makes difficult the separation of the dust particles from those of mostly urban origin.

Lidar and in situ observations of continental and Saharan aerosol: closure analysis of particles optical and physical properties

Abstract. Single wavelength polarization lidar observations collected at Mt. Cimone (44.2º N, 10.7º E, 1870 m a.s.l.) during the June 2000 MINATROC campaign are analyzed to derive tropospheric profiles of aerosol extinction, depolarization, surface area and volume. Lidar retrievals for the 2170-2245 m level are compared to the same variables as computed from in situ measurements of particles size distributions, performed at the mountain top Station (2165 m a.s.l.) by a differential mobility analyzer (DMA) and an optical particle counter (OPC). A sensitivity analysis of this closure experiment shows that mean relative differences between the backscatter coefficients obtained by the two techniques undergo a sharp decrease when hygroscopic growth to ambient humidity is considered for the DMA dataset, otherwise representative of dry aerosols. Minimization of differences between lidar and size distribution-derived backscatter coefficients allowed to find values of the "best" refractive index, specific to each measurement. These results show the refractive index to increase for air masses proceeding from Africa and Western Europe. Lidar depolarization was observed to minimize mainly in airmasses proceeding from Western Europe, thus indicating a spherical, i.e. liquid nature for such aerosols. Conversely, African, Mediterranean and East Europe aerosol showed a larger depolarizing fraction, mainly due to coexisting refractory and soluble fractions. The analysis shows average relative differences between lidar and in-situ observations of 5% for backscatter, 36% for extinction 41% for surface area and 37% for volume. These values are well within the expected combined uncertainties of the lidar and in situ retrievals. Average differences further decrease during the Saharan dust transport event, when a lidar signal inversion model considering non-spherical scatterers is employed. The quality of the closure obtained between particle counter and lidar-derived aerosol surface area and volume observations constitutes a validation of the technique adopted to retrieve such aerosol properties on the basis of single-wavelength lidar observations.

Influence of Black Sea and local biogenic activity on the seasonal variation of aerosol sulfur species in the eastern Mediterranean atmosphere

[1] Methanesulfonate (MSA) and non-sea-salt (nss) sulfate concentrations were measured in bulk aerosol samples collected during January 1996 to December 1999 at two stations along the Turkish and Cretan coastline of the eastern Mediterranean. The data set enabled the origin and variability of biogenically derived sulfate concentrations in the eastern Mediterranean atmosphere to be defined. Although similar seasonal patterns of MSA and nss-sulfate concentrations for both stations were detected, the mean concentrations at Erdemli (Turkey) over the sampling period (42 ± 52 ng m−3 MSA, and 6.8 ± 5.2 μg m−3 nss sulfate) were found to be almost twice that of those measured at Finokalia, Crete (25.8 ± 14.9 ng m−3 MSA and 3.9 ± 1.7 μg m−3 nss sulfate). Analysis of the air mass back trajectory and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data suggests that the majority of the biogenic contribution at Erdemli is originated from summer coccolithophorid Emiliania huxleyi blooms developed in the Black Sea. The data further points to a significant relationship between Saharan dust transport events and local oceanic production of MSA through occasional fertilization of the eastern Mediterranean during wet deposition events. This process accounts for episodic, strong weekly changes in MSA concentrations during the spring months. Considering that Erdemli and Finokalia are approximately 1000 km apart from each other geographically, large differences in their MSA and nss-sulfate concentrations indicates a considerable role of regional mesoscale atmospheric transport processes on the spatial structure of biogenically produced atmospheric sulfur aerosols.

On the dynamic of the atmospheric input of copper and manganese into the Western mediterranean sea

Daily 24-h aerosol samples have been collected at a coastal location in northwestern Corsica, between February 1985 and March 1988. Total deposition has been sampled between February 1985 and October 1987 with a time span of about 15 days. Three-dimensional air-mass trajectory analysis indicates that atmospheric Cu and Mn over the western Mediterranean are long-range transported from continental source regions. We show that the scavenging of atmospheric Cu and Mn by rain generates a seasonal cycle with high aerosol concentrations during the dry season and lower aerosol concentrations during the wet season. Sporadic but intense Saharan dust transport events are responsible for the highest atmospheric concentrations of Mn. The ratios between the atmospheric deposition of Cu and Mn and the corresponding mean daily precipitation rate reach a maximum during the Mediterranean summer. We attribute this behaviour to the wet scavenging of a more loaded atmosphere during the dry season than during the wet season. The precipitation frequency (Fp) is a major factor influencing the seasonal variability of the atmospheric Cu deposition and, this is in agreement with what we observe for the atmospheric Pb deposition. Our results also suggest that the relationship between the seasonal variability of Mn deposition and the Fp is less direct. This is due to the occurrence of Saharan dust transport events which strongly influence the variability of the Mn total deposition.