Mineral dust aerosols coming from the arid and semiarid regions of the world can aggregate and form microspherulites under special atmospheric conditions. This is the case for iberulites, formed in the atmosphere from Saharan dust intrusions into the southern Iberian Peninsula during the summer, prompting a noteworthy case of dust accretion unique in the world. This study consists of a long-term monitoring of Saharan dust outbreaks producing haze that reaches the southern Iberian Peninsula. Aerosol concentration, relative humidity, and temperature time series available at the ground stations in this area indicate sharp variations of these atmospheric variables during the iberulite-forming events. Most of these events occurred during the summer (60%), with 65 episodes for the period 2005–2013, in which 107 plumes reached the Iberian Peninsula. Iberulite episodes lasted 5 days on average, during which an initial increase of particulate matter (PM) levels and temperature, accompanied by a decrease in relative humidity, was registered until the third day. These trends reversed when the plume began to abate. Our data also indicate that iberulites form during dusty episodes when a minimum threshold in the content of large aerosol particles (PM₁₀) reached concentrations above 15 µg × m⁻³. Surface evaporation due to the sharply rising air temperatures give rise to clouds associated with the plume, where the water droplets that formed from condensation capture large amounts of aerosols as they fall. In this sense, muddy raindrop impacts with variable water:dust ratios recorded during red-rain episodes are interpreted as the precursor of the iberulites. A singular process of dust aggregation is here proposed for the formation of iberulites.

Key Points

Investigate a singular process of mineral-dust aggregation in the atmosphere
Correlation of the evolution of Saharan dust outbreaks with aerosol concentration and iberulite-forming events
The establishment of a well-defined threshold in the content of aerosol particles to form iberulite aggregates

1 Introduction

Aerosols play a key role in weather, atmospheric radiative properties, and atmospheric processes [Xi and Sokolik, 2012; Bangert et al., 2012; Zhao et al., 2014], interfering positively or negatively in the development of life [Jickells et al., 2005; Kellogg and Griffin, 2006; Griffin, 2007; Lawrence and Neff, 2009]. Mineral particles, together with marine salts, are the main constituents of the tropospheric aerosol loading, the particle mass content of which is 2 orders of magnitude greater than anthropogenic aerosol species as biomass-burning aerosol or fossil-fuel black carbon [Intergovernmental Panel on Climate Change (IPCC), 2001]. Most mineral aerosols come from the soils of arid and semiarid regions of the world, mainly deserts [Prospero et al., 2002]. Mineral particulate material from North Africa, the Middle East, and Central Asia represents around the 70% of the total mass burden aerosol in the atmosphere [Miller et al., 2004]. The global flux of aerosols has been estimated to be in the range 1500–2600 Tg/a [IPCC, 2007], the Sahara being the world's foremost dust source area [Goudie and Middleton, 2001; Middleton and Goudie, 2001; Prospero et al., 2002; Washington et al., 2003; Tanaka and Chiba, 2006] with 58% of the total emissions from all arid and semiarid regions [Okin et al., 2006]. The assessment of dust production of the Saharan and Sahel region is 40–70% of the total global annual amount [Tanaka and Chiba, 2006; Engelstaedter et al., 2006]. Specifically in the Mediterranean area, d'Almeida [1987] established a dust flux of 80–120 Tg/a, and Barnaba and Gobbi [2004] found that a total seasonal mean value of 43 Tg/a of Saharan dust outbreaks (SDOs) invade the Mediterranean atmosphere.

According to epidemiological studies, airborne suspended particulate matter can affect the human health [Dockery and Pope, 1996; Diaz et al., 2012]. For an assessment of health effects, it is necessary to define the framework of the time course of elevated aerosol emissions [Nastos et al., 2010; Christopher et al., 2011]. Studies dealing with short-range transported dust plumes, near their source, are based on short surveys due to difficulties in performing measurements in remote and extensive areas [Cowie et al., 2014]. Some high natural inputs of mineral aerosols consist of large dust plumes transported thousands of kilometres away from their sources. Tracking the dust emissions from Africa toward Europe based on long-term monitoring allows links to be established between source areas and seasonal Saharan dust passageways.

An important tool to gather information on dust-emission phenomena and their effects on extensive areas consists of establishing appropriate correlations between the source emissions and the ground-based measurements in the reception area. The common measurements based only in the distribution of particulate matter (PM) 10 and PM_2.5 fractions may provide an incomplete overview of the phenomenon. For instance, aggregation phenomena in the atmosphere are little known and not currently considered in the analysis of the atmospheric particles. A special case of giant aerosol particles formed in the atmosphere involves the so-called “iberulites,” which are spherical mineral aggregates of relatively large sizes (ranging 50–300 µm) collected in the Granada basin, southern Spain [Diaz-Hernandez, 2000; Diaz-Hernandez and Paraga, 2008]. On the contrary, other giant aerosol particles described in the literature, such as those reported by Jeong et al. [2014], were not formed by aggregation of fine particles via atmospheric process, but transported directly from source.

The aim of this study is to infer the atmospheric environmental factors controlling the genesis of iberulites and thereby gain new insights into the physico-chemical controls and mechanisms of iberulite formation from the evolution of PM₁₀ and PM_2.5 concentration, temperature, and relative humidity in the Granada basin (southern Spain) during dust episodes.

2 Geographical and Meteorological Framework for the Saharan Dust Intrusions Into the Iberian Peninsula

Here we examine the time course of SDOs over a region covering 5 × 10⁶ km² (Figure 1), comprising north-western Africa (as the dust source region) and the southern Iberian Peninsula (dust reception area). In the absence of more precise information on Saharan inputs to the Western Mediterranean areas, the satellite images show that the southern Iberian Peninsula constitutes the first reception area of Southern Europe for Saharan dust (Figure 1). In this region near North Africa, Saharan dust events represent a risk for air and marine transport activities, because they can reduce visibility to less than 300 m (Figure 2). The region shown in Figure 1 has a marked environmental contrast, with mountain ranges (Betics, Rif, and Atlas mountain belts) capturing the moisture brought by westerly winds from the Atlantic Ocean. On the other hand, a low, flat desert with some small mountains forms the Western Sahara.

Details are in the caption following the image — **Figure 1**
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The study area showing a case of SDO that occurred on 2 August 2012 (Image courtesy of MODIS at NASA; ORNL DAAC). Sandstorm blowing off the west coast of Northern Africa and over the Canary Islands (CI) extends northward from the north-western Africa coast in a clockwise direction, reaching the southern Iberian Peninsula. The sampling site is also indicated in this image (GR; Granada). Note the formation of clouds in relation to the dust plume which progressed with the northward movement of the plume.

It is well known that the dust from the western Sahara, transported by the trade winds through the Atlantic Ocean, often reaches the Americas [e.g., Prospero and Carlson, 1972; Karyampudi and Carlson, 1988; Karyampudi et al., 1999]. However, Saharan dust also may be blown along the Atlantic shoreline into Europe (Figure 1). The meteorological scenario explaining such trajectories of the SDOs is controlled by summer anticyclones located to the south-east of the Iberian Peninsula [Coz et al., 2009]. The frequency of the SDOs that reach Southern Europe and the Mediterranean basin is much greater in the summer [e.g., Escudero et al., 2005; Diaz-Hernandez and Paraga, 2008; Coz et al., 2009]. During this period, dust injection at high altitudes is the result of increased atmospheric convection caused by the marked ground heating in the Sahara regions [Prospero and Carlson, 1972; Alpert et al., 1998].

The SDOs that cross North Africa affecting the central region of the Mediterranean are greater in May (38%) than in July (37%), as determined from ground-based radiometer measurements for the period 1999–2005 [Meloni et al., 2007]. High PM₁₀ and TSP events were recorded in southern and eastern Spain for the period 1996–1999 when high-dust Saharan air masses were transported over the Iberian Peninsula [Rodríguez et al., 2001].

This study is based on satellite imagery and ground-based measurements from three monitoring stations (two aerosol ground stations and one weather station) located in the Granada basin (GR in Figure 1). The selected sampling site represents the immediate European border of dust invasions from the Sahara desert. The aerosol samplers and the other weather instruments located in an intramountain depression of the Betic Cordilleras prevent the influence of sea-salt aerosol fluxes, high atmospheric humidity, and other related parameters associated with the marine boundary layer (MBL) in the reception area.

The three ground-station sampling sites were located in rural areas of the Granada basin, which is surrounded by young mountains of high relief composed of calcareous dolomite and schists.

Giant aerosols (iberulites) were sampled in the Vega of Granada, a fertile valley located in the center of Granada basin (37°10′N–3°3 l′W, 640 m above sea level (asl)). The soils in this depression are mainly Calcareous Fluvisols [IARA-CSIC, 1989], with irrigated crops in a radius of several tens of kilometres around the monitoring station, and forestry farther away. This area is not industrialized.

3 Sampling and Methodology

3.1 Ground-Based Monitoring

Iberulite was monitored over the period August 1999-October 2013. The sampling method was based in the weekly recovery of atmospheric dust with passive collectors [Diaz-Hernandez and Paraga, 2008], noting meteorological incidences. In all cases, medium-sized trays (0.064 m² of surface area and 5 cm deep) were used to collect sample quantities suitable for study. The trays were placed in rural areas, on the roofs of buildings ~8 m above the ground, and on structures ~2 m above the roof (total height ~10 m). The dust collectors were further protected from local dust contribution by surrounding irrigated land and by a dense tree barrier. The weekly sampling period represents the minimum viable resolution because the quantity of sample recovered in fewer days is usually very small. Because iberulite sampling was weekly, the diverse types of data presented here were cross-checked on a monthly basis (see below). Iberulites were detected with a binocular microscope and in most cases were separated. The number of iberulites sampled during a productive event can reach tens of thousands of specimens and can represent between 5 and 40% of the total aerosol weight, which ranges between 0.01 and 0.2 gr m^-2 day^-1.

The iberulite aggregates were handpicked after careful examination of sample under the stereomicroscope and then classified according to their presence and abundance. Carbon-coated samples consisting of isolated iberulites were analyzed (secondary electron images and energy dispersive X-ray spectroscopy) with a field-emission Scanning Electron Microscope Auriga (Carl Zeiss).

During the summer period, iberulite-forming episodes sometime coincide with muddy rains (red dust rains). As long as rainfall is not heavy, both the iberulites and their precursory muddy raindrops would be preserved. Muddy raindrop impacts during red rain were collected by using circular glass disks 16 cm in diameter. The disks were screened and photographed using a stereomicroscope with oblique illumination at different magnification. Colour photographs were taken using a stereo-microscope, opposing mixed light and a black background to remove shadows and assist in particle selection. Morphometric features of aerosol particles were performed only on those images from muddy-raindrop impacts where the cohesion between their constituent particles is practically nil due to the relatively high water:dust content. In these cases, length, surface area, perimeter, and circularity of the particles were determined. The largest and smallest dimensions of each particle were measured, representing the primary and secondary axes of the best-fit ellipse; the area of each iberulite was represented in square pixels; the perimeter was measured as the length of the outside boundary of each selection; finally, circularity was defined as 4π × (area/perimeter²), where a value of 1.0 indicates a perfect circle.

The analysis of PM₁₀ and PM_2.5 levels were recorded in the weather ground station of Viznar also located in the Granada basin (north-eastern border, 37°13′N-3°33′W, 1079 m asl) over a period of 14 years (2000–2013). The monitoring of daily ambient particulate matter (PM₁₀ and PM_2.5 levels, in µg × m⁻³) was taken from public European databases (European Monitoring and Evaluation Programme (EMEP), Cooperative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air pollutants in Europe— Agencia Estatal de Meteorología (AEMET), Spanish Agency of Meteorology). PM₁₀ data follow the UNE-EN 12341:1999 normative, using a TISC captor with filters 203 × 254 mm and analyzed by the procedure PNE_CNSA-CA_02 in the Instituto de Salud Carlos III. PM_2.5 data follow the UNE-EN 14907:2006 normative, using a MCV CAV-A captor with filters of 15 cm in diameter, also analyxed in the Instituto de Salud Carlos III following the procedure PNE_CNSA-CA_49.

Meteorological data come from the weather ground station of the Granada airport (State Meteorological Agency, AEMET; 37°11′N–3°46′W, 567 m asl. For the period from January 2005 to December 2013, the mean annual precipitation and temperature values were 404 mm and 15.3°C, respectively, and the annual mean relative humidity (RH) was 61.5%, which corresponds to a semiarid Mediterranean climate. Average daily relative humidity was determined from a continuous recording with a platinum-resistance thermometer and capacitive hygrometer (CVS-HMP-450, Vaisala). Conventional wind-speed data show that gusty winds under 5 km/h prevail (63%) up to 10 A.M., accelerate from midday to 4 P.M. up to 20 km/h, and slow approximately to 0 km/h for the rest of the day.

3.2 Satellite Imagery

Dusty episodes coming from Africa were monitored with satellites images that enabled us to detect SDOs and follow their spatial and temporal shifts. Spaceborne sensors, such as Moderate Resolution Imaging Spectroradiometer (MODIS (http://earthdata.nasa.gov/data/), wavelengths 470, 670, and 212 nm, resolutions 2 km, 1 km, and 500 and 250 m/px) [Remer et al., 2005], provide adequate information from polar orbiters Terra and Aqua, marking two daily passages (EOS A.M. and P.M.) over the study area. Specifically, north-western Africa (emission area) and Europe-3-01 subsets (Iberian Peninsula, reception area) were used [Oak Ridge National Laboratory Distributed Active Archive Center, 2011]. The recorded period for north-western Africa subsets was 2007–2013, and for Europe-3-01 it was 2005–2013. Thus, it was feasible to complete the north-western Africa subsets using the data from AERONET Dahkla subsets for the period 2005–2007, because the former only amplify the area of the latter. Thus, the monitoring was extended to the period 2005–2013. The failure of NASA servers on December 2013 caused the loss of much of the information presented here.

The monitoring of SDOs by satellite images covered the area between the following coordinates: longitude 2°E–20°W, latitude 21°–44°N (equivalent to around 5 × 10⁶ km²; Figure 1). The study area includes the trajectories of SDOs moving off the west coast of North Africa, and extending northward from Cabo Blanco (south-western Sahara). At the end of its trajectory, most of these large outbreaks spread over the southern Iberian Peninsula (Figure 1). Other SDOs over the Iberian Peninsula follow northward trajectories across the Western Mediterranean [Loÿe-Pilot et al., 1986].

As displayed in Figure 1, SDOs are visualized after they pass the shoreline, due the optimal contrast between water masses on the surface of the oceans and aerosols. In this sense, the SDOs of interest for this study can be easily recognized by direct satellite observation because of African shoreline has two main segments: the Atlantic (2390 km) and the Mediterranean (910 km), which are crossed by the North Atlantic Ocean and northward (across the Mediterranean) trajectories, respectively.

The Terra satellite making an orbit from the north to south crosses the study area shown in Figure 1 in the morning, while Aqua passes south to north over this area in the afternoon. This enabled (1) the specification of the moment of the day of the production of plumes in the source area and of their arrival to the reception area, (2) the determination of the trend and lifespan of each dust episode, (3) the tracing of their precise trajectory, and (4) the inference of the relation between the SDOs and generation of iberulites. Thus, diverse types of SDOs were distinguished: short episodes (just one morning or one afternoon), episodes of 12–24 h in duration (morning-afternoon or afternoon-morning), or great plumes (≥2 days).

4 Results

4.1 Giant Aerosol Aggregates (Iberulites) and Red Rain

Iberulites are pinkish mineral microspherulites with spherical morphologies and mean diameters between 60 and 90 µm (Figure 3). Three main structural elements usually characterize them: vortex, core, and rind. The morphology and structural elements of the iberulites have been explained by the interaction of aerosol particles and water droplets modeled by hydrodynamic forces during their fall [Diaz-Hernandez and Paraga, 2008].

The formation of iberulites is closely associated with haze, sometimes accompanied by red rain. Generally, iberulites develop from aerosols during periods without rain (dryfall or dry deposition), although aerosol deposition during red rains (wetfall) produces other mineral aggregates that, as will be shown below, can be interpreted as precursors of iberulites. The sequence of impacts shown in Figure 4 indicates a connection between the specimen collected during red-rain episodes (muddy raindrops) and the iberulites formed under dry deposition conditions. As evidence of the action of the modeling forces on muddy raindrops, a depression or vortex arises in some iberulites (upper specimen of Figure 3), which is also observed in some muddy raindrops with very low water content (Figure 4b, arrow). Its preservation in these last cases is interpreted as being due to a dampening of the impact force.

Impact traces of raindrops with low dust content produced during a typical red-rain episode would correspond to the early stage of iberulite formation, where the cohesion between its constituent particles is practically absent (Figure 5a). The aerosol particles contained in this type of precursory droplets of iberulites are circular and show a low positively skewed size distribution with a mode of around 2.5 µm. These sizes are slightly lower than those of the fine particles constituting the iberulites and their associated dust (3.5 µm [Diaz-Hernandez and Paraga, 2008]) and would correspond to the small aerosol (PM_2.5) with diameters near 2.5 µm usually collected in the air-quality monitoring stations. The size distribution of the aerosol particles contained in the droplet of Figure 5b shows that particles with diameter higher than 2.5 µm represent around a 50% of the cases. In the case of iberulites constituents, coarse particles with diameters greater than 2.5 µm constitute more than 50% (Figure 10 in Diaz-Hernandez and Paraga [2008]). This coarse fraction found within the droplets and iberulites studied corresponds to the large aerosols, and its distribution in the troposphere can be monitored by PM₁₀.

4.2 Timing of Iberulite-Forming Events and SDOs

The monitoring of iberulite-forming events over the period August 1999 to October 2013 is shown in Figure 6a. A well-defined annual periodicity is evidenced with the maximum number of iberulite episodes occurring during the summer season. The same periodicity is observed for the arrival of SDOs to the Granada basin reception area from MODIS satellite images, reaching up to six SDOs in summer months for some periods (case of the year 2012; Figure 6b). However, this type of periodicity is not easily evidenced in the time course of outbreaks emitted from the source area throughout the period 2005–2013 (Figure 6c). Most of dust episodes are also produced during the summer months. For the common period from 2005 to 2013, the number of iberulite episodes was 65, most of which were concentrated in two maxima (curve 1, Figure 6d). The largest one, found during the dry period (June–August), represents the 60% of the iberulite episodes. The other maximum occurred in the spring season, with 17 iberulite episodes registered in this period. These two maxima were also well defined in our observations of SDOs, both for the reception area and the source area (curves 2 and 3 in Figure 6d, respectively). As expected, a major number of SDOs was registered in the source area (246 events) with respect to the reception area studied (107 events) because many outbreaks coming from the west coast of North Africa did not reach the southern Iberian Peninsula. The base level, given as the average of the left and right minimum values, was around 15 events in curve 3, whereas it was very low (1 event) in the case of curve 2, indicating that dust episodes occurred practically year round in the source area.

4.3 Monitoring of PM Levels and Relative Humidity

The distribution of the PM₁₀ levels for the study period (2001–2013) shows a periodic pattern with maxima in summer (Figure 7). Compared to PM₁₀, PM_2.5 levels are clearly lower, showing a similar periodic pattern with maximum values (a third of those of PM₁₀) also corresponding to summer periods (not shown). The distribution of the daily PM₁₀ fraction shows many individual peaks within the summer period, in addition to other scattered peaks for other seasons, such as those for March (years 2003, 2004, 2005, and 2010), October (2006, and 2008), and December 2007 (Figure 7a). A calculation of PM₁₀ daily means for the overall study period revealed three maxima (Figure 7b): (1) the well-defined broad peak corresponding to the summer months, (2) a lower peak at around the spring equinox (March), and (3) a sharp peak at the autumn equinox that is caused by a spurious intrusion in October 2008. The former two peaks are also clearly evidenced in Figure 7c (bar graph), where monthly means of daily means are represented.

The well-defined summer maximum for the PM₁₀ distribution is associated with the numerous SDOs observed for the reception area in this season (compare Figure 7c and curve 2 in the Figure 6d). In addition, SDOs that reached the reception area during spring could also be monitored by PM₁₀. According to this cause-and-effect relationship between the number of SDOs and PM levels, summer dust events at the reception area (Figure 6b) are well represented by the PM₁₀ curve of Figure 7a for the years 2003, 2004, and 2007 and appear more depressed in 2009, 2011, and 2013. It is also deduced from Figure 7c that summer dust events were responsible for some 50% of the total atmospheric aerosol load over the Granada basin, with a background slightly higher than 5%. The mean annual distribution of iberulite episodes for the period 2001–2013 is also indicated in this figure, showing a coincidence of its main peaks for spring and summer seasons (line graph of Figure 7c) with those of monthly means of PM₁₀.

The connection between raindrops and iberulites evidenced by our results of section 4.1 suggests that the humidity represents another potential factor in the genesis of iberulites. Thus, the daily relative humidity levels were monitored for the period 2004–2014. These data, together with daily means and monthly means calculated from daily means, are shown in Figure 8. As expected, a marked decline occurred in the hot summer months, with values approaching 40%.

5 Discussion

5.1 Iberulite-Forming Events and Variation of PM and RH During Dust Episodes

Distribution curves 1 and 2 of Figure 6d show that most of the iberulite episodes and SDOs reaching the study area occurred during summer, with well-defined positive correlations between them: y = −0.0123x² + 0.8576x − 0.3612, R² = 0.819, P = 0.0002, x = the number of SDOs reception area, and y = the number of iberulite events. In the study case, the correlation between dusty episodes and iberulite-forming event is the result of a cause-and-effect relationship. The occurrence of dusty episodes is a necessary condition for the formation of iberulite, as evidenced by the fact that 107 SDOs occurred in the reception area, with only 65 of them producing iberulites (Figure 6d). However, unraveling the potential factors favoring the formation of these giant aerosols is not a trivial task, requiring PM, RH, and iberulite-events data from ground-based measurements to be combined with those from MODIS observations. In the study case, RH data and MODIS observations are not available for the entire study period (prior to 2004 and 2005, respectively). In addition, data correlation requires the comparison of homogeneous means (corresponding to the same period) of the number of SDOs, iberulite episodes, and PM₁₀-RH values.

The correlation between iberulite episodes and PM measurements (Figure 7c) indicates that this type of aggregate forms in relation to particularly strong dust episodes occurring in summer. Some spurious dust episodes such as that of October 2008 also generated iberulites (compare Figures 6a and 7a). This single episode produced 560 µg × m⁻³ (PM₁₀) in 3 days.

Figure 9a shows the distribution of PM₁₀ levels for dust events exclusively, during a period of 13 years (2001–2013). A significant correlation between PM₁₀ and PM_2.5 levels (Figure 9b) indicates that the content of the former was 4 times that of the latter during dust events. However, the maximum concentration of small aerosol particles (10 µg × m⁻³) proved only slightly lower than that of coarser PM₁₀ particles when no dust events occurred. The number of iberulite-forming events for each month of the period 2004–2013 shows statistically significant positive and negative correlation with monthly PM and RH values, respectively (Figure 9c). As expected, the maximum number of iberulite events, major PM levels, and minor RH corresponded to the summer season, and the contents of PM₁₀ were greater than those of PM_2.5 for the same months. In addition, it should be remarked that the PM₁₀ value for intersection point abscissa (around 15 µg × m⁻³) corresponds to PM₁₀ baseline level indicated in Figure 7b. This value is interpreted as the minimum threshold in the content of large aerosol particles for iberulite formation during the dusty episodes. This threshold is also evidenced from the occurrence of a marked gap close to the origin in the elongated cluster of scattered points in Figure 9b, where the minimum value of the PM₁₀ levels measured during dusty episodes was 15 µg × m⁻³ (see also Figure 9a).

Figure 10 shows the variation in PM levels, relative humidity, and temperature during the course of dust episodes forming iberulites. First, this figure presents the time course of these parameters during two discrete iberulite-forming events: one with aerosol deposition occurring through rain washout and other of dry deposition (Figures 10a and 10b). Figure 10c corresponds to a mean dusty event from 80 dust episodes forming iberulite aggregates, during the period 2004–2013. As shown, the mean event lasted 5 days and was characterized by a rise in PM levels and temperature, as well as by a RH decline as the dust plume approached, until the third day when the dust began to let up. This trend is common to all heat waves registered in southern Spanish regions associated with high-dust Saharan air mass intrusions. As expected, temperatures were slightly lower in the case of SDOs that produced iberulites under wet deposition conditions compared to those events of dry deposition, but the RH measured at ground level were lower in the former than in the later.

SDOs force the heating of the lower troposphere [Alpert et al., 1998; Kuo-Ying and Chao-Han, 2014], a fact which was also observed during the dusty iberulite episodes (Figure 10). The decrease in the RH at the ground level during the dusty episodes studied resulted from the strong increase in the air temperature, which produced surface evaporation. Water-vapor evaporation processes give rise to clouds associated with the plumes. This phenomenon is well visible in the satellite image of Figure 1, where the dust plume is accompanied by the formation of clouds resulting from the water evaporated from the oceanic water surface. Meteorological data representing the environmental conditions occurring during iberulite formation in the troposphere were not available in this study. Thus, the exact effect that the water-vapor condensation in the plume exerted on the formation of iberulites from mineral aerosols cannot be determined. However, the estimation of the dew point or a better water-vapor mixing ratio at the ground level during the dusty episodes indicated that water was transferred into the atmosphere from days 2 to 4, after which it disappeared again (Figure 11). Although the signal was quite weak at the surface levels, that effect must be amplified within the plume at the medium levels of the troposphere. This implies that condensation processes responsible for water-droplet formation (and iberulite formation?) within the plume should occur from days 4 to 6.

5.2 Iberulite-Forming Mechanism

Mechanisms responsible for water-droplet formation also play an important role in the genesis of iberulite aggregates, as suggested by the sequence of droplet impacts with variable dust content shown in Figure 4. In addition, there are marked analogies between the morphology of the iberulites (Figure 3) and that of the drops of diluted polymer solution (viscoelastic drops) falling through a quiescent viscous Newtonian fluid [Sostarecz and Belmonte, 2003] and of the water droplets modeled by hydrodynamic forces during their fall [Pruppacher and Klett, 1997]. Diaz-Hernandez and Paraga [2008] proposed that iberulites are formed by the interaction of dust and water droplets falling in the air.

It is known that the certain types of aerosols such as smoke from biomass burning and urban and industrial air pollution act as small cloud-condensation nuclei (CCN), which increase cloud albedo and in turn inhibit precipitation from clouds [Rosenfeld, 2000]. The occurrence of soot aerosol and sulphur in the iberulites suggests a wet scavenging of the air pollutant, because these aggregates are generated by the interaction of dust aerosol and water droplet, presumably close to the reception area [Cuadros et al., 2015]. A clay content of around 50% was measured in iberulites collected in the Granada basin, which is significantly higher than 30% found in the total dust [Cuadros et al., 2015]. Sulphate particles, and probably some type of clays (smectites), constitute a hygroscopic material, which can act as CCN [Kumar et al., 2011]. Rosenfeld et al. [2001] contended that the occurrence of desert dust particles in the atmosphere considerably reduces the size of droplets in clouds, which do not reach the minimal size required for the onset of precipitation. During iberulite-forming dusty episodes, at least minimum coalescence of droplets and absence of significant precipitation are necessary conditions for the formation of these giant aggregates because it would otherwise exclusively produce the muddy raindrops that form during red-rain episodes (Figures 4, and in particular 5a). Probably, many small water droplets would need to be formed in the genesis of iberulites.

After a critical radius is reached (Figure 12), water droplets capture aerosol particles as they fall. During fall, the water in the drops evaporates because the droplets precursory of iberulites leave a cloudy region and reach lower and warmer levels of the troposphere. The sequence of raindrop impacts with decreasing water:dust ratio of Figure 4 represents the development from precursor water droplet to the iberulite aggregate; that is, as the water droplet falls, it traps dust and dehydrates.

Water droplets containing mineral particles are modeled by complex hydrodynamic interactions during their fall where the rheological properties of colloidal suspension formed by water drop-aerosol particles develop from those of Newtonian fluids toward viscoelastic behavior (Figure 12). The flow of matter within the water droplets relocates particles, with a major concentration of fine-grained (mainly clays) and coarser materials in the rind and the core, respectively (Figure 12). It produces spheroids together with the typical morphological features (vortex) observed in both iberulites and some muddy raindrop impacts with a low water:dust ratio (Figures 3 and 4b [Diaz-Hernandez and Paraga, 2008]) similar to those generated in the context of airflows circulating water droplets [Beard and Chuang, 1987; Pruppacher and Klett, 1997]. In addition, heterogeneous reactions involving gas species, together with water retention in the evaporation stage by clays, explain the concentration of diverse types of salts, mainly sulphates, in the pinkish rind of the iberulite, composed by Fe-oxyhydroxides and smectites (Figure 12).

6 Conclusions

Iberulite-forming events result from SDOs affecting the southern Iberian Peninsula. From the long-term trends found through diverse combined monitoring (ground-based data of PMs, RH, temperature, and iberulite-sampling together with monitoring of dust episodes by satellite imagery), several significant findings arise: (1) To form iberulite aggregates, high atmospheric dust concentration is necessary, with a well-defined minimum threshold in the content of large aerosol particles. The supply of large amounts of mineral aerosol to the study area is guaranteed by the proximity of the Sahara desert, which is the largest dust-source area worldwide. (2) Major PM levels and minor RH measured in the study region correspond to the summer, when the maximum number of iberulite events was registered for the study period. (3) The time course of PM, RH, and temperature during dust episodes indicates that the genesis of iberulites occurs in relation to the formation of minor clouds during haze. (4) The presence of appropriate amounts of water vapor in the plume drastically influences iberulite formation, which results from the interaction of mineral aerosols and water droplets falling through the air. (5) The morphology and the structure of these giant mineral aggregates result from the flow of matter within the water drops due to the action of hydrodynamic forces and from water evaporation during their fall.

Although this special type of aerosol aggregate has been described only in the study region, the monitoring of iberulite-forming event, the determination of its relation to the trends of the overall aerosol content, and the knowledge of the processes involved in its formation will help identify other potential iberulite-forming areas in the world. In addition, if remains are preserved, it will also enable important dust events in the past to be recognized.

Scientific Abbreviations Used

A.M./P.M.: ante/post meridiem
asl: above sea level
CCN: cloud-condensation nuclei
MBL: marine boundary layer
MODIS: Moderate Resolution Imaging Spectroradiometer
PM: particulate matter
RH: relative humidity
SDOs: Saharan dust outbreaks
TSP: total suspended particles

Acknowledgments

We wish to thank the AEMET (Agencia Estatal de Meteorología, Spain) for meteorological and aerosol data. (URLs: https://sede.aemet.gob.es/AEMET/es/GestionPeticiones/home and http://sds-was.aemet.es/, respectively). Satellite imagery was obtained from MODIS, NASA (http://daac.ornl.gov/MODIS/modis.html). Data of ground-based monitoring of iberulite episodes can be requested at the following e-mail address: [email protected]. This work was supported by the P11-RNM-7067 project of the Junta de Andalucía (C.E.I.C-S.G.U.I.T.). Both the anonymous reviewers are gratefully acknowledged for their valuable comments and suggestions, and Lynn Russell for the manuscript's editorial handling. David Nesbitt revised the English version of the manuscript.

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Citing Literature

Volume121, Issue12

27 June 2016

Pages 7064-7078

Saharan dust outbreaks and iberulite episodes

Abstract

Key Points

1 Introduction

2 Geographical and Meteorological Framework for the Saharan Dust Intrusions Into the Iberian Peninsula