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Advances in Meteorology
Volume 2012 (2012), Article ID 828301, 17 pages
Meteorological Patterns Associated with Intense Saharan Dust Outbreaks over Greece in Winter
Laboratory of Climatology and Atmospheric Environment, University of Athens, Faculty of Geology and Geoenvironment, Panepistimiopolis, 15784 Athens, Greece
Received 14 February 2012; Accepted 21 March 2012
Academic Editor: Dimitris G. Kaskaoutis
Copyright © 2012 P. T. Nastos. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The Mediterranean Basin and southern Europe are often affected by Saharan dust outbreaks, which influence the aerosol load and properties, air quality standards, visibility and human health. The present work examines, mainly of the meteorological point of view, three intense dust outbreaks occurred over Greece with duration of one or two days, on 4 and 6 February and 5-6 March 2009. The synoptic analysis on the dusty days showed the presence of low-pressure systems in the west coasts of Europe and the north Tyrrhenian Sea, respectively, associated with a trough reaching the north African coast. The result of these conditions was the strong surface and mid troposphere winds that carried significant amounts of dust over Greece. During the dusty days extensive cloud cover associated with the dust plume occurred over Greece. The air-mass trajectories showed a clear Saharan origin in all atmospheric levels, while the satellite (MODIS Terra/Aqua) observations as well as the model (DREAM) predictions verified the intense dust outbreaks over eastern Mediterranean and Greece. The ground based particulate matter concentrations in Athens were excessively increased on the dusty days (PM10: 150–560 μg/m3), while significant dry and wet deposition occurred as forecasted by DREAM model.
According to the Earth Observatory website (http://earthobservatory.nasa.gov/), intense dust outbreaks are considered natural hazards, which affect the global and regional radiative balance, cloud microphysical properties, atmospheric heating and stability, tropical cyclone activity, ecosystems, marine environments and phytoplankton, photolysis rates, ozone chemistry, and human health [1, 2]. Mineral and desert dust play an important role in radiative forcing, with an estimated top of atmosphere (TOA) radiative forcing in the range −0.6 to 0.4 Wm−2 . However, the radiative forcing caused by dust particles is very uncertain in both magnitude and sign, mainly triggered by the chemical composition of mineral particles , by the wavelength dependence of their optical properties (like single scattering albedo, asymmetry factor), as well as by the albedo of the underlying surface and also the relative height between the dust layer and the clouds [4, 5]. Desert dust can be transported over long distances from the source regions , with the larger particles to be deposited near the source, while the smaller ones to be suspended in the air for a few days or weeks, thus travelling over large distances.
The Saharan desert is the most important dust source region in the world . Exports of dust plumes to the North Atlantic and Mediterranean Sea occur throughout the year . The occurrence of Saharan dust (SD) events above eastern Mediterranean has a marked seasonal cycle, with a spring maximum and a winter minimum [9–11]. In the summer, dust identification over the region is also frequent due to the longer duration of the dust particles favored by the stable weather conditions, the absence of depressions and precipitation that favor their wet deposition. Many studies [8, 12–14] have shown that the Saharan dust events over Mediterranean are mainly driven by the intense cyclones called Sharav, south of Atlas Mountains (Morocco). These cyclones are generated by the thermal contrast between cold Atlantic air and warm continental air that cross North Africa during spring and summer. Moreover, the thermal lows developed over the desert regions in the warm period of the year consist the beginning of the dust erosion and favoring its uplift at the upper atmospheric levels and its transport over long distances [15, 16]. In general, in the cold period, neither the development of the Sharav cyclones and the thermal lows in Saharan are favored nor the dust transport over long distances. Additionally, due to frequent precipitation and the often presence of depressions and strong winds, the dust particles deposited in the surface near the source region and their atmospheric lifetime is limited compared to the summer . However, under specific weather conditions, mainly consisted of strong surface winds over Saharan desert closely associated with low-pressure systems, the dust outbreaks are also observed in the winter [17, 18]. Furthermore, the close relation of the dust exposure and transport with the prevailing local and regional meteorology is well established. To this respect, Dunion and Velden  have shown that an elevated Saharan dust layer may play a crucial role in suppressing tropical cyclone activity in the Atlantic and the tropical cyclones in the Bay of Bengal cause mineral dust exposure from the continental India over the Bay of Bengal and Arabian Sea [20, 21]. Focusing in the Mediterranean several studies [1, 15, 22] have analyzed the prevailing meteorology during the dust events, while Meloni et al.  and Carmona and Alpert  classified the weather types responsible for the dust outbreaks in Lampedusa and Israel, respectively.
While progress has been made in characterizing the importance of mineral dust in global-scale processes, there has been less progress in identifying the sources of dust, the environmental processes that affect dust generation in the source regions and the meteorological factors that affect the dust transport. This can be achieved by the development of the regional and synoptic weather forecast models, also able to predict the emission, the amount, the transport, as well as the deposition of the dust [15, 22].
Saharan dust outbreaks also affect human health [24–27], because such episodes are closely associated with increases in particulate matter concentrations on the surface, especially when the synoptic conditions favor advection of the dust within the boundary layer. The health burden due to particulate matter (PM) air pollution is one of the biggest environmental health concerns, especially over areas directly affected by intense dust storms .
The present study focuses on the analysis of three dust events occurred over Greece in the cold period of the year (February 4, 6, 2009 and March 5-6, 2009), mainly from the meteorological aspect based on NCEP-NCAR reanalysis. The dust transport is also monitored by the satellite remote sensing, through observations from MODIS, while the use of regional atmospheric models, such as DREAM, is established as a powerful tool for the dust forecasting. Ground-based PM concentrations recorded in the University Campus of Athens are in close agreement with satellite observations and model forecasts showing that the three dust events affect strongly the aerosol concentrations at the ground.
2. Data Analysis
The Saharan dust episodes over Greece on February 4, 6 and March 5-6, 2009 were analyzed using satellite observations (MODIS Terra/Aqua, TRMM) model forecasting (DREAM), air mass trajectories, NCEP-NCAR reanalysis datasets, and ground-based PM measurements.
2.1. Satellite Observations
The data used in this study include both Terra and Aqua MODIS aerosol products (AOD550, Angstrom exponent at 550–865 nm over ocean and aerosol mass concentration), calculated using separate algorithms over land and ocean. The “Level 3” MODIS products are available on daily and monthly intervals, globally, on a grid. Further details of the MODIS aerosol algorithm, products, and validation are presented in the studies of Remer et al.  and Levy et al. . In the present work, Level 3 C005 retrievals were used centered over Greater Athens Area covering the period February 1, 2009 to March 10, 2009.
Except of the aerosol retrievals, satellite data of the accumulated precipitation over eastern Mediterranean during the dusty days were obtained via Tropical Rainfall Measurement Mission (TRMM) with a spatial resolution of . Since the dust events are associated with low-pressure systems and extensive cloudiness above the study area, precipitation has also taken place. The TRMM spatial distribution of the precipitation can also be compared with the predictions of DREAM regarding the wet deposition of dust. Therefore, the TRMM observations can also be a validation tool for the DREAM predictions.
2.2. The DREAM Model
In the last years, an integrated modeling system, the Dust Regional Atmospheric Modeling (DREAM) model (http://www.bsc.es/projects/earthscience/DREAM/) , is widely used to simulate the 3-dimensional field of the dust concentration and its cycle in the atmosphere. It is based on the SKIRON/Eta modeling system (http://forecast.uoa.gr/) and the Eta/NCEP regional atmospheric model . The dust model takes into account all the major processes of dust life cycle, such as dust production, convection and advection, as well as wet and dry deposition. It also includes the effects of the particle-size distribution on aerosol dispersion, lifetime, and transportation from their source regions. The dust production is parameterized using near-surface wind and thermal conditions, as well as soil features. The dust-production mechanism is based on the turbulent mixing, shear-free convection diffusion, and soil moisture . In addition to these mechanisms, very high-resolution databases, including elevation, soil properties, and vegetation cover are also utilized. In the operational version, for each soil texture type, fractions of clay, small silt, large silt, and sand are estimated with typical particle size radii of 0.73, 6.1, 18, and 38 μm, respectively . Transport mixing and deposition processes are online driven by an atmospheric model and the predicted meteorological parameters. The atmospheric model is updated every 24 h with newly observed data, but the simulated dust-concentration field produced in the previous-day run initializes the dust model at the same time intervals. This model has been extensively used for the identification of dust events in the Mediterranean [10, 22, 31, 32].
2.3. Air Mass Back Trajectories
The surface-level aerosol characteristics during dust events can be quite different compared to the columnar ones as different types of aerosols vary depending on their scale heights . For the above reasons, the 72-hour air mass back trajectories for each one of the examined SD events were calculated using the HYSPLIT-4 model of Air Resources Laboratory of NOAA (http://www.arl.noaa.gov/HYSPLIT_info.php). The back trajectories analysis was carried out for 3 distinct levels, namely, 500 m, to give representative origins of air masses near the surface, 1500 m, which can serve as a representative height for the boundary layer, and 4000 m, representative of the free troposphere, where the Saharan dust is usually transported . The Air Resources Laboratory’s Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model is a complete system for computing both simple air parcel trajectories and complex dispersion and deposition simulations. The model calculation method is a hybrid between the Lagrangian approach, which uses a moving frame of reference as the air parcels move from their initial location, and the Eulerian approach, which uses a fixed three-dimensional grid as a frame of reference. In the model, advection and diffusion calculations are made in a Lagrangian framework following the transport of the air parcel, while pollutant concentrations are calculated on a fixed grid. The model is designed to support a wide range of simulations related to the atmospheric transport and dispersion of pollutants and hazardous materials, as well as the deposition of these materials (such as mercury) to the Earth’s surface. Some of the applications include tracking and forecasting the release of radioactive material, volcanic ash, wildfire smoke, and pollutants from various stationary and mobile emission sources [34, 35].
2.4. NCEP-NCAR Reanalysis
The National Center for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) have cooperated in a project (denoted “Reanalysis”) to produce a record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involved the recovery of land surface, ship, radiosonde aircraft, satellite and other data, then quality controlling and assimilating these data with a data assimilation system which is kept unchanged over the reanalysis period . NCEP uses satellite-based temperature retrievals from the National Environmental Satellite Data and Information Service (NESDIS). These retrievals commence in November 1978 and are assimilated over all ocean areas, and above 100 hPa over land areas with poor radiosonde coverage .
2.5. Ground-Based PM Measurements
Ground-based PM concentrations were collected using a real-time photometric sampler (Aerocet 531-MetOne instruments), based on light-scattering method at 0.780 μm. The data consists of 3-minute recordings (operation process for 2 minutes followed by 1 minute interval) of particulate matter concentrations with different aerodynamic diameters: PM10, (with diameter less than 10 μm), PM7 (with diameter less than 7 μm), PM2.5 (with diameter less than 2.5 μm), and PM1 (with diameter less than 1 μm). The accuracy of the Aerocet 531 detector is ±10% to calibration aerosol. The sampling site was located at the Laboratory of Climatology and Atmospheric Environment (UOA) in the campus of the University of Athens (longitude: 23°47′E, latitude: 37°58′N, altitude: 257 m a.m.s.l.), a suburban region with low traffic and far from inhabited areas. This location is at the east edge of the Athens basin, at the foothills of Hymettus Mountain. Calibrations, including flow rate and zero tests, were carried-out weekly, according to the manufacturer’s instructions.
Remotely sensing aerosols via satellite images gives general picture and can provide information of the spatial distribution of the aerosol properties. The Aqua-MODIS true color images on 4 and 6 February are shown in Figure 1. Both images show extensive cloud cover over Greece and, especially, over Balkan countries (February 4, 2009) associated with the cyclonic conditions. However, a great part of the images is free of clouds and, therefore, the dust plumes on both days are clearly depicted. More specifically, on February 4 (Figure 1(a)), the dust plume is observed over Aegean Sea, while it is more intense south of Crete in the Libyan Sea. Towards northern Greece, the dust plume intensity decreases due to the gravitational deposition of the largest particles. After the passage of a clear day (not shown), a new dust outbreak starts from the western Libyan desert and transported towards Greece (north-northeasterly) following the cyclonic circulation. This dust plume was more intense as clearly can be seen from the Aqua-MODIS image on February 6 (Figure 1(b)). It is extended from the Libyan coast towards the south Ionian and Libyan Seas also covering the western and central Greece. In this image, the eastern Aegean Sea is free from dust aerosols, but the dust plume was transported easterwards affecting these areas in the next hours and day, February 7 (not shown). In Athens, the visibility was limited to few hundred meters on February 4 and February 6-7, while February 5 was a day with clear sky and good visibility. In the midday hours of February 6, a slight precipitation (0.7 mm) took place, but not capable to dillute the atmosphere, which was remained very turbid due to dust presence until the evening hours of February 7, when an intense rainfall (29 mm) took place.
In Figure 2, the analysis is focused on the dust event on March 5-6, 2009, which was one of the most intense above Greece in the last years, with maximum values of AOD550 and PM (Figures 6 and 7) comparable to those reported by Kaskaoutis et al.  and Gerasopoulos et al. . The images were obtained by the Aqua-MODIS sensor. On March 6, the dust storm originated from Libya was transported northwards affecting Greece and was mainly driven by strong surface and middle troposphere winds in a cyclonic pathway. This was also the case during the intense dust event on April 17, 2005 [1, 38]. The maximum intensity of the dust plume is depicted between the North African coast and Crete; then it becomes more vague as being diluted by the removal processes (dry and wet deposition) (Figure 4). Significant cloud cover is also obvious over Greece on both days, and specifically on March 5; as a consequence, the elevated dust plume may interact with cloud microphysical properties (cloud condensation nuclei, cloud albedo, water-vapor content) altering them in the process. It is well known that dust interacts with clouds, after absorbing hygroscopic material , and affects photolysis rates and ozone chemistry by modifying the spectrum of UV radiation . The clouds in the left part of the image on March 6 are characteristic of a cyclonic circulation and reveal the cold front approaching Greece.
The air mass back trajectories at 4000 m, 1500 m, and 500 m on the dusty days (Figure 3) verified the vertical transport of dust, from near the surface to the middle of troposphere; so did DREAM simulations, which showed dry and wet deposition of dust particles over Athens on these days.
The DREAM forecasts regarding the dry and wet deposition over north Africa, Mediterranean, and Europe are presented in Figure 4. Concerning the dust deposition on February 4, 2009 (Figures 4(a) and 4(b)), the dry mechanism clearly dominated on this day, since the precipitation over the regions affected by the dust plume was limited. As expected, the dry deposition covered the north African arid regions, the continental Greece and the adjoining seas, while wet deposition was limited over areas with precipitation, as in Albania and western Balkans. On February 6, 2009, the dry mechanism dominated again except from the northern African regions, large amounts of dust dry deposition were observed in central Mediterranean and the western part of Greece (Figures 4(c) and 4(d)). The wet deposition was limited mainly in the northwestern Greece and Albania, as well as in a small area in southern Aegean Sea. On March 6, 2009 there is a clear evidence of an intense dust event influenced the majority of Greece. Significant dry deposition (100–500 mg/m2) and wet deposition, which on regional scale over west Greece ranged from 500–1000 mg/m2 indicated the intensity of this SD episode (Figures 4(e) and 4(f)).
The findings resulted from DREAM products were in close relation with those from TRMM analysis (Figure 5). Light precipitation appeared on February 4, 6, 2009 over Greece, with some exceptions over central mountainous regions (>40 mm). A different pattern appeared on March 5, 2009, when moderate-to-strong precipitations came up the country and mainly the western regions (>70 mm). On the next day, March 6, 2009, the precipitations were limited on local scale (mountainous regions of Peloponnese), allowing the dry deposition of the dust, which reached at maximum levels, as recorded on the ground.
Before focusing on the PM concentrations over Athens during the intense dust events of February 4, 6, 2009 and March 5-6, 2009, the temporal variation of some aerosol properties obtained from Terra and Aqua-MODIS sensors is analyzed. More specifically, Figure 6 depicts the daily values of mass concentration, and AOD550, from Terra and Aqua-MODIS sensors, within the period February 1, 2009 to March 10, 2009, averaged over the area 37.0°–38.5° N and 23.0°–24.5° E including the Greater Athens Area and its surroundings. This also allows a qualitative comparison between the satellite retrievals and ground-based PM concentrations. During the period, the AOD550 varied widely from 0.06 to 2.67 for Terra-MODIS and from 0.05 to 1.87 for Aqua. AOD550 values higher than 0.5 were closely related to the SD events as marked with arrows in the figure. The remarkable increase in AOD550 values on March 5-6, 2009 clearly revealed the intensity of this dust event.
Despite the difference in orbiting time and the fact that the data pixels from the two satellites are not always the same (due to differences in cloud cover), the Terra and Aqua-MODIS sensors give similar AOD550 values, while a strong linear relation () holds. Furthermore, the SD events are clearly identified from the low values obtained from Aqua. In the measuring period, the took values from relatively low (~0.2-0.3), during the dust events (February 4, 6, 2009 and March 5-6, 2009), to high (>1.0) on certain days of anthropogenic aerosols and local pollution. The values show presence of fine and coarsemode particles, with the fine mode to be dominant (mean value of ). The large variability in values is attributed to the variety of air masses, aerosol types, and characteristics. Furthermore, on the dusty days, the aerosol mass concentration enhanced, especially on March 5-6, 2009. This also reveals the strong influence of the Saharan dust outbreaks on the aerosol load in southern Europe [12, 32, 41].
4. Ground-Based PM Concentrations
The daily Box and Whiskers plots for PMs recorded on 3 min basis during the period February-March 2009 are presented in Figure 7. The sampling station at the University Campus of Athens, outside the urban area, could be considered as background station with respect to PM concentrations. In general, significantly higher PM concentrations were recorded on the dusty days against the rest days of the examined period, while the temporal pattern for PM concentrations was similar to that observed from Terra/Aqua MODIS AOD500 over Athens (Figure 6).
More specifically, on February 4, 2009, the mean daily PM concentrations were calculated as follows: μg/m3 for PM1, μg/m3 for PM2.5, μg/m3 for PM7, μg/m3 for PM10, and μg/m3 for TSP. The maximum values were recorded in the early afternoon hours, which is in agreement with DREAM dust forecast (Figure 8(a)) indicating that the majority of aerosol particles were within the boundary layer. The PM concentrations on February 6, 2009 were found at lower levels; thus, μg/m3 for PM1, μg/m3 for PM2.5, μg/m3 for PM7, μg/m3 for PM10, and μg/m3 for TSP. In this case, the maxima were recorded at night hours (~21:00), in agreement with dust forecast (Figure 8(b)).
The intense signature of the Saharan dust episodes on March 5-6, 2009 appeared in PM concentrations at the ground. The mean values of PMs, on March 5, 2009 were calculated as follows: μg/m3 for PM1, μg/m3 for PM2.5, μg/m3 for PM7, μg/m3 for PM10, and μg/m3 for TSP, with maxima during afternoon hours (see also Figure 9(a)). On the next day, a further increase in PM concentrations was recorded, thus μg/m3 for PM1, μg/m3 for PM2.5, μg/m3 for PM7, μg/m3 for PM10, and μg/m3 for TSP; the primary maximum appeared in the morning hours, and a secondary one in the late afternoon. This is depicted clearly in Figure 9(b), where the vertical profile of the dust concentration shows that the dust advected within the boundary layer. In Mediterranean coastal urban areas, such as Athens, which are close to north African arid regions, the effects of dust outbreaks play a key role in PM concentrations and further contribute to the urban air quality and human health [12, 25]. Many studies [1, 32, 37, 41, 42] have established that dramatically PM10 enhancements in Mediterranean are associated to Saharan dust outbreaks.
The dust events over Greece occur either in an upper atmospheric level or in the whole atmospheric column [17, 43, 44], with the latter to be more intense, directly influencing the PM concentrations at the surface. On the other hand, dust transports only within the boundary layer, and it is characterized by lower AOD and PM concentrations, since the majority of the particles have already been deposited near the source regions . According to the EU standards, since January 2005, the exceedance of the PM10 daily average threshold of 50 μg/m3 at any station is allowed for a maximum of 35 times per year, while the PM10 annual average should not exceed 40 μgm−3. In this point, it must be noticed that according to the 2008/50 EU directive, the exceedances of the 50 μg/m3 limit value due to Saharan dust are not included in the number of exceedances. The PM10 concentrations exceeded the daily European Union limit of 50 μg/m3 for 4 days associated with dust transport, during the study period. It is characteristic that the SD contribution to PM10 levels is about 200 μgm−3 on March 6, compared to the mean PM10 value in the study period. However, it should be noted that the PM monitoring station is located in a suburban area and is not so characteristic for the Athens urban environment. Nevertheless, as Gobbi et al.  shown, the remote areas allow for a better dust monitoring than the urban environments, where the anthropogenic emissions contribute significantly to the background PM levels.
4.1. Synoptic Conditions
The two recorded SD episodes on February 2009 were associated with alike synoptic conditions (Figure 10). More specifically, on 4 February 2009 a barometric low on the surface was established with its center not too far from the north-west coasts of France linked to a trough towards the Adriatic Sea (Figure 10(a)). This barometric turbulence resulted in south-southwest air flow in the region of western Greece with near gale-to-gale winds (15–20 m/s) at 850 hPa level (Figure 11(a)) against severe gale winds (20–25 m/s) at 500 hPa level (Figure 11(c)). As far as the middle atmosphere (500 hPa) is concerned, the existence of a trough over the western Mediterranean, the Iberian Peninsula, and the north-west coast of Africa had originated western air flow towards Greece (Figure 10(a)). The significant negative anomaly of the 500 hPa geopotential heights from the mean value of 1981–2010 Climatology was very characteristic (−300 m), expanding from southern England westwards the Iberian Peninsula (Figure 10(c)). This synoptic condition related to high pressure system on the surface, over the region of north-west Africa, resulted in the advection of dust from these areas (Tunis, Algeria) towards Italy and western Greece, affecting partially the eastern Greece, as well. This is shown by the lower PMs concentrations against the values measured during the second dust episode on March 6, 2009.
The synoptic conditions and the 500 hPa negative anomalies appeared on February 4, 2009, shifted eastwards on February 6, 2009, without significant weather pattern modification (Figures 10(b) and 10(d)). The trough in the middle atmosphere appeared more acute and violent storm winds (29–32 m/s) dominated over north-west Africa weakening in the region of Greece (Figure 11(d)). At the level of 850 hPa, moderate breeze (6–8 m/s) was established over Greece against gale winds over Tunis (Figure 11(b)).
Regarding the Saharan episode on March 5-6, 2009, the synoptic conditions have been differentiated according to the fact that the barometric low on the surface was established with its center firstly over Corsica in northern Tyrrhenian Sea and in the process over northern Italy (Figures 12(a) and 12(b)). This pattern combined with high pressure system on the surface at the south-east Libya, resulted in the prevalence of south air flow towards the region of Greece with gale winds (~20 m/s) at 850 hPa level (Figures 13(a) and 13(b)) against violent storm winds (29–32 m/s) at 500 hPa level (Figures 13(c) and 13(d)) becoming hurricane winds (>33 m/s) on March 6, 2009. The atmospheric circulation in the middle atmosphere (500 hPa) showed a trough over the central Mediterranean and the north-west coast of Africa, which caused an advection of southern air flow towards Greece (Figures 12(a) and 12(b)). The significant negative anomaly of the 500 hPa geopotential heights from the mean value of 1981–2010 Climatology was very characteristic (−300 m), over central Mediterranean Sea. The anomaly shifted eastwards from south England, France and west Mediterranean Sea to Italian Peninsula during the two-day atmospheric episode (Figures 12(c) and 12(d)). This established synoptic condition was the driver of strong south transport of dust from central Libya and north-eastern Algeria towards Greece.
Dust storms originated from Libya are usually transported northward throughout the atmospheric column, from near the surface to the middle of troposphere. Viewing thesatellite images for a long-time period (~6 years), these dust storms are more intense than those originated from the northwestern Africa (mainly Algeria, Mauritania, and Mali), which are the most frequent in the central/eastern Mediterranean and south Europe [10, 14, 18, 45]). Except of their intensity, other significant differences between the dust events coming over Greece from Libya and those from northwestern Saharan are (1) the regional and synoptic meteorology favoring their transport, (2) the seasonality, (3) their vertical extend, and (4) their association with clouds. More specifically, the dust events affecting Greece from southwestern directions, with a source in the desert regions of northwestern Africa, also cover a great part of the central Mediterranean, before reaching Greece; as a consequence, their intensity is decreased since the larger particles were deposited near the source. These events are more common in summer and are mainly driven by an anticyclone over northwestern Africa, which transports desert particles over central and eastern Mediterranean with air masses following an anticyclonic pathway; this is the case on June 29, 2002  and on August 20–31, 2000 . The duration of these dust events is rather large, especially in the central part of the Mediterranean, where Meloni et al.  found durations of 13 consecutive days occurred in two episodes, in August 1999 and from 6 to 18 July 2002. The latest was persistent over a large part of the Mediterranean also affecting Greece. These dust events are mainly detected at an elevated layer into the atmosphere [10, 46] having a clear signal in AI values [43, 47]. For this reason, the deposition of the smaller and lighter dust particles still suspended on the air is not favored, enhancing the duration of the dust plumes. The stable weather and the nearly absence of precipitation over Mediterranean in the summer also favor the dust-aerosol residence time. The DREAM forecasts of the dust events on June 29, 2002  and on August 31, 2000  show a large vertical extent of the dust plumes, at altitudes as high as 6-7 km. Since the dust particles within the boundary layer are easily deposited, while those at middle and upper atmosphere are not due to the reasons mentioned above, these dust events are mainly detected in the upper atmosphere. Similarly, Kalivitis et al.  found that the dust events over Crete in summer are mainly transported in the upper atmosphere. Finally, dust events originated from northwestern Africa in summer are associated with sunny and cloudless conditions over the largest part of the Mediterranean and Greece.
In contrast, the dust events originated from Libya affect only the eastern Mediterranean, as in the present case and on April 17, 2005. Furthermore, a different meteorological pattern is responsible for their exposure, mainly driven by a cyclone centered on Adriatic Sea and Italy and an extent trough reaching the Libyan coast. The result of this atmospheric circulation is a northward flow associated with strong surface and middle-troposphere winds carrying significant amount of dust over eastern Mediterranean and Greece . The dust plume in these cases is usually transported throughout the atmospheric column, resulting in both large AOD in the vertical and increased surface PM concentrations. These dust events are more frequent in late winter (although rare) and early spring, since this period favors the depressions above the Mediterranean. To this respect, Kalivitis et al.  found that the vertical dust transport is more frequent in winter and spring over Crete, while a similar dust transport mechanism took place on February 26, 2001, as reported by Kaskaoutis et al. . The duration of these dust events is 1-2 days, since the depressions favoring them are quickly moved and attenuated. They are associated with extent cloud cover over eastern Mediterranean and Greece, which is generated by the presence of the depression and the uplift of water vapor from the sea. The dust plume is mainly transported below the clouds, at altitudes lower than 4 km as shown in the present cases. Additionally, such were the cases on 4th April, 1988 and on March 27, 1992. Both phenomena are associated with the appearance of depressions that are generated in northwest Saharan, to the south of the Atlas Mountains especially during spring .
The present work gives evidence of the synoptic conditions which were drivers of intense dust outbreaks over Greece, during the period February-March 2009. The performed analysis was based upon satellite (MODIS Terra/Aqua) observations, model (DREAM) forecasts, ground-based particulate matter concentrations, and NCEP/NCAR reanalysis datasets. The synoptic conditions prevailed during the examined SD events revealed two major synoptic patterns. The first (on February 4, 6, 2009) steered the dust plume over Greece following anticyclonic track, influencing firstly the west and central Mediterranean and Italy. Late spring and summer are favorable seasons for this type of advection, mainly appeared in the upper layers of troposphere (~4000 m). In these cases, the dust transport does not affect significantly the PM concentrations on the ground, while there is not extensive cloud cover over the region, with an exception of the development of icicle clouds. Regarding the second pattern (on March 5-6, 2009), the dust plume was transported from Libya northwards, and associated with strong winds following cyclonic track, while extensive cloud cover over the region was observed as a direct result of depression activity. The duration of this type of episodes is small, one or two days, and these SD are characterized by great intensity. The advection of dust concerns mainly the lower tropospheric layers and significantly affects PM concentrations on the ground. The appearance of such episodes is experienced mainly during winter, against low frequency during spring as well. Finally, the overall agreement in the temporal variation between the AOD550 derived from MODIS and ground-based PM concentrations, over Athens, both on dusty and nondusty days, suggests that the majority of aerosol particles are within the boundary layer.
The present work was funded by SYNERGASIA 2009 PROGRAMME. This Programme is cofunded by the European Regional Development Fund and National Resources. (Project code: 09ΣYN-31-711). The author gratefully acknowledges data and/or images from the BSC-DREAM8b (Dust REgional Atmospheric Model) model, operated by the Barcelona Supercomputing Center (http://www.bsc.es/projects/earthscience/DREAM/), the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.arl.noaa.gov/ready.php) used in this publication and the NCEP/NCAR reanalysis project scientific teams.
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