Mineral dust is the most abundant aerosol, having a profound impact on the
global energy budget. This research continues our previous studies performed
on surface soils in the Arabian Peninsula, focusing on the mineralogical,
physical and chemical composition of dust deposits from the atmosphere at
the Arabian Red Sea coast. For this purpose, aerosols deposited from the
atmosphere are collected during 2015 at six sites on the campus of the King
Abdullah University of Science and Technology (KAUST) situated on the Red
Sea coastal plain of Saudi Arabia and subjected to the same chemical and
mineralogical analysis we conducted on soil samples. Frisbee deposition
samplers with foam inserts were used to collect dust and other deposits, for
the period December 2014 to December 2015. The average deposition rate
measured at KAUST for this period was 14 g m
X-ray diffraction (XRD) analysis of a subset of samples confirms variable amounts of quartz, feldspars, micas, and halite, with lesser amounts of gypsum, calcite, dolomite, hematite, and amphibole. Freeze-dried samples were re-suspended onto the Teflon® filters for elemental analysis by X-ray fluorescence (XRF), while splits from each sample were analyzed for water-soluble cations and anions by ion chromatography. The dust deposits along the Red Sea coast are considered to be a mixture of dust emissions from local soils and soils imported from distal dust sources. Airborne mineral concentrations are greatest at or close to dust sources, compared to those through medium- and long-range transport. It is not possible to identify the exact origin of deposition samples from the mineralogical and chemical results alone. These aerosol data are the first of their kind from the Red Sea region. They will help assess their potential nutrient input into the Red Sea, as well the impact on human health, industry, and solar panel efficiency. These data will also support dust modeling in this important dust belt source area by better quantifying dust mass balance and optical properties of airborne dust particles.
Dust emission and deposition modeling and measurements are required for the assessment of the dust mass budget. Both emission and deposition are under-constrained in atmospheric dust models, leading to large uncertainties (Bergametti and Forêt, 2014; Schulz et al., 2012). To improve simulations, the above authors and others suggested the establishment of dust deposition networks in the vicinity of and away from dust source regions, operating throughout the year. In this paper we are presenting results from a network of dust deposition samplers located on the campus of the King Abdullah University of Science and Technology (KAUST) along the Red Sea coast of Saudi Arabia. This is an important dust source region (Ginoux et al., 2012; Prospero et al., 2002), the effect of which extends thousands of kilometers downwind. To better characterize optical, microphysical, and health effects of dust aerosols we conducted detailed chemical, mineralogical and particle size analysis of deposition samples collected from the air.
Mineral dust is the most abundant atmospheric aerosol, primarily from suspended soils in arid and semi-arid regions on Earth (Buseck et al., 2000; Washington and Todd, 2005; Goudie, 2006; Muhs et al., 2014), including deserts of the Arabian Peninsula (Edgell, 2006). Dust aerosols profoundly affect climate (Haywood and Boucher, 2000; Hsu et al., 2004; Kumar et al., 2014), cloud properties (Twomey et al., 1984; Wang et al., 2010; Huang et al., 2006), visibility (Kavouras et al., 2009; Moosmüller et al., 2005), air quality (Hagen and Woodruff, 1973), atmospheric chemistry and mineralogy (Sokolik and Toon, 1999; Kandler et al., 2007), biogeochemical cycles in the ocean and over land (Jickells et al., 2005; Mahowald, 2009), human health (Bennett et al., 2006; Bennion et al., 2007; De Longueville et al., 2010; Menéndez et al., 2017), and agriculture (Fryrear, 1981; Nihlen and Lund, 1995).
A further important implication of dust emission/deposition processes is associated with the harvesting of the solar renewable energy in the desert areas. Dust deposits on solar panels are known to have a severe detrimental effect on the efficiency of photovoltaic systems (Goossens and Van Kerschaever, 1999; Hamou et al., 2014; Mejia et al., 2014; Rao et al., 2014; Sulaiman et al., 2014; Ilse et al., 2016), with its adverse effects depending on mineral composition and atmospheric conditions (Supplement B).
The importance of dust mineralogy has long been recognized (Engelbrecht et al., 2016), but only recently has the explicit transport of different mineralogical species been implemented in climate models (Perlwitz et al., 2015a, b; Scanza et al., 2015)
The mineralogy and chemical composition of dust generated from the Red Sea coastal region remains uncertain. The Red Sea coastal plain is a narrow highly heterogeneous piedmont area, and existing soil databases do not have the spatial resolution to represent it adequately (Nickovic et al., 2012).
The specific objective of the present study is to examine mineralogical, chemical and morphological information of deposition samples collected on the KAUST campus. This will help to better quantify the ecological impacts, health effects, damage to property, and optical effects of dust blown across this area (Engelbrecht et al., 2009a, b; Weese and Abraham, 2009). Knowledge of the mineralogy of the dust deposits will provide information on refractive indices, which can be used to calculate dust optical properties, providing input into radiative transfer models, and to assess the impact of dust events on the Red Sea and adjacent coastal plain.
This research complements our dust studies performed in the Arabian Peninsula (Engelbrecht et al., 2009a; Kalenderski et al., 2013; Jish Prakash et al., 2015, 2016) and globally (Engelbrecht et al., 2016).
The Arabian Peninsula is one of Earth's major sources of atmospheric dust,
contributing as much as 11.8 % (22–500 Mt a
Minerals previously identified in continental soils from Middle East dust-generating regions include quartz, feldspars, calcite, dolomite, micas, chlorite, kaolinite, illite, smectite, palygorskite, mixed-layer clays, vermiculite, iron oxides, gypsum, hornblende and halite (Engelbrecht et al., 2009b, 2016; Goudie, 2006; Jish Prakash et al., 2016; Pye, 1987; Scheuvens and Kandler, 2014). It could be expected that similar mineral assemblages would occur in variable proportions in the dust deposition samples collected in the region.
Position of
With the exception of the area around Jazan in the south, which is impacted
by the Indian Ocean monsoon, the Red Sea coastal region has a desert climate
characterized by extreme heat. Temperatures measured at the KAUST campus
reach 43
Vegetation is sparse, being restricted to semi-desert shrubs, and acacia trees along the ephemeral rivers (wadis), providing forage for small herds of goats, sheep, and dromedary camels.
During infrequent but severe rainstorms, run-off from the escarpment along
wadis produces flash floods in lowland areas. With such events, fine silt and
clay deposits are formed on the coastal plain, which are transformed into
dust sources during dry and windy periods of the year. The resultant dust is
transported and deposited along the coastal plain itself and adjacent Red
Sea by prevailing northwesterly to southwesterly winds, with moderate
breezes (wind speed
This study is meant to complement the recently published papers by our research group that characterize the effect of dust storms (Jish Prakash et al., 2015; Kalenderski et al., 2013), evaluate radiative effect of dust (Osipov et al., 2015), and analyze soils from the Red Sea coastal plain (Jish Prakash et al., 2016) and dust emissions in the same region (Anisimov et al., 2017). Mineralogical, physical and chemical results are presented of deposition samples collected largely during 2015 at six sites on the campus of KAUST, located approximately 80 km north of Jeddah, along the central part of the Red Sea coastal plain of Saudi Arabia (Fig. 1).
The coastal plains of the Arabian Peninsula along the Red Sea and Persian Gulf are among the most populated areas in this region, hosting several major industrial and residential centers. Airborne dust profoundly affects human activities, marine and land ecosystems, climate, air quality, and human health. Satellite observations suggest that the narrow Red Sea coastal plain is an important dust source, augmented by fine sediment accumulations, scattered vegetation, and variable terrain. Airborne dust carries the mineralogical and chemical signature of a parent soil (Jish Prakash et al., 2016). The purpose of a previous study on 13 soil samples from the Arabian Red Sea coastal area (Jish Prakash et al., 2016) was to better characterize their mineralogical, chemical, and physical properties, which in turn improve assessment of dust being deposited in the Red Sea and on land, affecting environmental systems and urban centers. It was found that the Red Sea coastal soils contain major components of quartz and feldspar, as well as lesser but variable amounts of amphibole, pyroxene, carbonate, clays, and micas, with traces of gypsum, halite, chlorite, epidote and oxides. The mineral assemblages in the soil samples were ascribed to the variety of igneous and metamorphic provenance rocks of the Arabian Shield forming the escarpment to the east of the coastal plain.
Anisimov et al. (2017) estimated that the eastern Red Sea coastal plain
emits about 5–6 Mt of dust annually. Due to its close proximity, a
significant portion of this dust is likely to be deposited into the Red Sea,
which could be comparable in amount to the estimated annual deposition rate
from remote sources during major dust events (Jish Prakash et al.,
2015). Therefore, we expect that the total dust deposition into the Red Sea
is on the order of 10 Mt a
In the past few decades, wind tunnel and field tests have been performed on
different designs of deposition samplers and sand traps to compare their
efficiencies. The samplers and traps included marble dust collectors (MDCOs),
inverted Frisbees, and glass surfaces (Goossens and Rajot, 2008; Sow et
al., 2006; Goossens et al., 2000; Goossens and Offer, 2000). Most of the
experiments performed in wind tunnels failed to completely mimic the field
conditions, which resulted in an underestimation of the dust deposition,
more so for the
Inverted Frisbee-type deposition sampler
At each sampling site the particulate deposits were collected into a 227 mm diameter inverted Frisbee dust deposit sampler, each with a polyester foam insert and bird strike preventers (Hall et al., 1993; Vallack and Chadwick, 1992, 1993; Vallack and Shillito, 1998) (Fig. 2). The purpose of the foam insert is to enhance the particulate collection capacity of the dust gauge (Vallack and Shillito, 1998) by better collecting and retaining wet (from fog, dew, rain) and dry as well as fine and coarse particles, under stable meteorological conditions, during severe dust events, northwesterly shamal, and daily coastal winds.
For the period December 2014 to March 2015, four Frisbee samplers were located at the New Environmental Oasis (NEO) site, about 50 m apart. The gravimetric information from the four samplers were similar, with small variations amongst them ascribed to the impact from local construction activities. Due to the similarity of these gravimetric results, and to obtain a better representation of dust deposition onto the KAUST campus, two of the samplers (DT1 and DT2) were moved in March, the first (DT1) to a residential area and the other (DT2) to the quay adjacent to the Coastal & Marine Resources Core Lab (CMOR) (Table 1). (Site meta-data provided in Supplement A.)
Locality of deposition samplers at six sites on the campus of KAUST.
The deposition samples were collected for intervals of a calendar month, starting in December 2014 and ending December 2015. At the end of each month, the samples are retrieved by flushing the dust deposit with distilled water from the foam insert and collection dish into the downpipe and plastic bottle. Both the insoluble particles and dissolved salts in the water suspension are retrieved in the laboratory by a freeze-drying (sublimation) procedure.
A total of 52 deposition samples were collected at the six sampling sites on the KAUST campus (Fig. 1b) over a period of 13 months, largely in 2015. Representative subsets of these samples were selected for electron microscopy (12 samples), XRD (27 samples) and chemical analysis (29 samples).
Freeze-dried sample splits were re-suspended in the laboratory onto
Teflon® filters, for elemental analysis by
XRF spectrometry using a miniaturized version of a dust
entrainment facility (Engelbrecht et al., 2016) (
The samples re-suspended onto the Teflon® filters
were chemically analyzed for elemental content by XRF, including for Si, Ti,
Al, Fe, Mn, Ca, K, P, V, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, and Pb (US EPA, 1999). Splits of about 2 mg from each freeze-dried sample were analyzed
for water-soluble cations of sodium (Na
A subset of 27 samples from the total of 52 samples, representing all months
of the year, was selected for X-ray diffraction (XRD) analysis. XRD is a
non-destructive technique particularly suited to identify and characterize
minerals such as quartz, feldspars, calcite, dolomite, clay minerals, and
iron oxides, in fine soil and dust. Dust reactivity in seawater as well as
optical properties depends on its mineralogy; e.g., carbonates and sulfates
are generally more soluble in water than silicates such as feldspars,
amphiboles, pyroxenes, or quartz. A Bruker D8®
X-ray powder diffraction system was used to analyze the mineral content of
the dust deposition samples. The diffractometer was operated at 40 kV and 40 mA, with Cu K
A likely bias in the results from applying the X-ray diffraction (XRD) technique, together with the RIR method is widely recognized, and therefore our methodology is considered to be semi-quantitative at best. Chung (1974) recognized that if the RIRs of all the crystalline phases in a mineral mixture are known, the sum of all the fractions should add to 100 %. However, XRD is effective at measuring crystalline phases such as quartz, calcite, and feldspars, and less so for partly crystalline and amorphous phases, including some layered silicates such as clays as well as many hydrous minerals. This could lead to an overestimation of the abundance of the crystalline mineral species in the dust, compared to partly crystalline and amorphous phases (Formenti et al., 2008; Kandler et al., 2009). Other discrepancies could occur from preferred orientation of layered silicates in the sample mounts. To minimize this effect, the dust samples were loaded into side-mount holders.
Electron microscopy provided information on the individual particle size and
shape of micron-size particles, important for determining the optical
parameters for modeling of dust (Moosmüller et al., 2012). The scanning electron microscope (SEM)-based individual particle
analysis was performed on a subset of 12 deposition samples collected
for each month of 2015. For each sample, a portion of the deposition sample
was suspended in isopropanol and dispersed by sonication. The suspension was
vacuum filtered onto a 0.2
Particle size distribution plots of 12 deposition samples collected monthly at the KAUST campus throughout the 2015 period are shown by volume in Appendix A and by number in Supplement C. The chemical abundance tables are in Appendix B. The mineralogical results from XRD are described under Sect. 5.5 and the normative mineralogy calculated from the chemistry, presented as histogram plots in Fig. 11.
Wind (m s
Northwesterly shamal winds prevailed during all 12 months of 2015 (Fig. 3). Four to five severe dust storms lasting 3 to 5 days each, contributed to hot humid conditions during the summer months. Weaker northeasterly winds were experienced in October and November of that year. Although the northeasterly winds were more frequent in November, they did not reach the maximum strength of the northwesterly winds.
Monthly averaged temperatures, humidity measurements, and calculated dew points at KAUST during 2015.
The first 4 months in the second half of 2015 experienced the highest
ambient temperatures (Table 2), with an average temperature of 35
Monthly averaged minimum (blue,
Monthly deposition rates (g m
With a few exceptions, the monthly gravimetric measurements from the four
samplers (DT1–DT4) are comparable (Fig. 5), changing similarly by month
and season. The deposition rates were at their lowest for December 2014
(avg. 4 g m
Source apportionment is considered to be a following step in the Red Sea
dust research program. As an approximation of source contributions, the
sampler with the lowest deposition rate can be considered to have negligible
or contain the least amount of local dust and sea salt (Fig. 5). In the
months of December 2014 and January, April, March, June, July, and December
2015, the deposition rates at the four sampling sites were similar and
considered to have similar but negligible amounts of dust from local
construction, roads, marine salt, or other particulates. In August, it is
estimated that 24–56 g m
Dust deposition measurements from the Middle East and other global dust regions.
Bearing in mind that the dust deposition samplers, sampling procedures, and conditions and sampling periods were different to those of this
study, some comparisons to similar studies in desert regions are listed in
Table 3. The deposition rates from this study, both on average
(14 g m
Average monthly deposition rates for all four samplers (DT1-DT4) on
the KAUST campus, together with
Average particle size distributions and standard deviations of
The aerosol optical depth (AOD) is one of the best observed aerosol
characteristics. It defines the aerosol radiative effect and reflects the
abundance of aerosols in the atmosphere. A CIMEL robotic sun photometer is
installed on the rooftop of the CMOR building on the campus of the KAUST and
operated by our group since 2012, as a part of the NASA AERONET, providing
aerosol optical depth (AOD) and aerosol-retrieved characteristics
(
Furthermore, a comparison between the deposition samples and the visibility is made with measurements taken in 2015 at the Jeddah airport meteorological station, approximately 70 km to the south of KAUST. Visibility is expressed as the frequency of dust events with reported weather codes 06–09 or 30–35, grouped as dusty or non-dusty days, for each month (Notaro et al., 2013; Anisimov et al., 2017), expressed as percentages. The bimodal monthly distributions seen with the deposition rates and AERONET monitoring are also mirrored by the visibility measurements collected at Jeddah (Fig. 6b). The linear correlation coefficient between the monthly deposition rates and monthly averaged visibility measurements is 0.48, clearly suggesting a causal relationship between the two variables.
Dust deposition rates depend on the meteorological conditions, and dust properties such as particle size distribution, their vertical distribution, and abundance.
Monthly measured deposition rates and assessments of
Semi-quantitative XRD mineral analyses of monthly Frisbee samples collected at the three sites DT1–DT3, for the period December 2014 to December 2015.
Summary plots of results from SEM-based individual particle analysis for each
month of 2015, expressed by number are presented in Supplement C to this
paper. From these particle size and shape measurements, equivalent
shape-dependent volumes for the particles were calculated, the summary plots
of which are shown in Appendix A. The volume of each particle is calculated
from the measured maximum and minimum diameters, and assuming a prolate
spheroid. Also, assuming a similar average density of, for example, 2.65 g cm
The average size distribution of the 12 deposition samples (Fig. 7a) is
compared to that of the 13 surface soils (Fig. 7b) from potential dust
source regions along the Red Sea coastal plain (Jish Prakash et al., 2016). The
deposition samples with an average diameter of 0.9
XRD analysis of the 27 samples (Fig. 8) shows variable amounts of quartz (6–38 %, avg. 22 %) and feldspars (plagioclase, K-feldspar) (5–34 %, avg. 20 %), clays (10–18 %, avg. 13 %), micas (6–31 %, avg. 13 %), halite (1–53 %, avg. 7 %) with lesser amounts of gypsum (1–8 %, avg. 4 %), calcite (0–8 %, avg. 2 %), dolomite (0–7 %, avg. 3 %), hematite (0–8 %, avg. 3 %), and amphibole (and pyroxene) (0–4%, avg. 1 %).
From the XRD, four broad mineral assemblages are distinguished: the first and major assemblage is comprised of feldspars, clays, and micas, as well as hematite and gypsum; the second group is of quartz, the third of halite, and the fourth of calcite.
There is an increase in the halite concentrations at sites DT1–DT3, from about 2 % (DT1) in December 2014 to about 53 % (DT2) in July 2015 (Fig. 8). From August onwards there is an abrupt decrease in halite content to less than 5 %, except for samples collected at the DT2 (CMOR, quay-side) site alongside the ocean. There was a simultaneous increase in the proportion of quartz to a maximum of 38 % in April (DT3), decreasing to less than 25 % at all sites after July 2015. The silicate mineral group decreased systematically from about 72 % (DT1) in December 2014 to about 25 % (DT2) in July. Except for two samples from the DT3 site collected in September and October 2015, the dominant minerals after July, 2015 included the silicate assemblage, with concentrations of up to 80 %. The variation in the proportions of the four mineral assemblages, especially the halite, is ascribed to seasonal fluctuations in wind, humidity, and precipitation, as well as the proximity of the sea to the sampling sites.
SEM-based SEIs of individual dust particles show that the larger particles being composed of mineral aggregates and coatings on other mineral particles. Examples (Supplement D) include particles composed of coatings of clay minerals on quartz and feldspar; clusters of clay minerals, calcite, gypsum, and halite; and particles and clusters of iron oxides and clay minerals. Similar coatings and aggregates in re-suspended soil samples are reported by Engelbrecht et al. (2016).
As expected, the chemically analyzed deposition samples contain major amounts
of SiO
The water-soluble cations (Appendix A, Fig. 10a, b) account for 1–19 %
and the anions for 1–30 % of the total mass, respectively. These account
for variable amounts of halite (1–32 %), and gypsum (1–9 %), with
lesser amounts of other chlorides and carbonates. Of importance as dust-borne
nutrients likely to be deposited in the Red Sea, are the low concentrations
of both water-soluble NO
The sum of chemical species, including elements expressed as oxides, as well as ion
concentrations, varies from 35 to 78 %, with an average of 56 % of the
measured chemical mass. The shortfall from 100 % is attributed in part to
components not analyzed for, including H
The chemical abundances were recalculated as normative minerals (Fig. 11a, b), comparable in composition to those identified by XRD (Fig. 8) and optical microscopy. The relative normative mineral abundances (Fig. 11b) show variable amounts of quartz (avg. 52.4 %) feldspar (avg. 3.9 %), kaolinite (2.6 %), calcite (8.8 %) dolomite (0.2 %), and hematite (8.0 %), as well as the evaporate minerals gypsum (12.1 %), halite (12.1 %), sylvite (0.2 %), and bischofite (0.2 %). There is also, as shown by XRD, an increase in halite content from about 7.8 % in January to about 25.9 % in July, followed by a sharp drop to about 4.6 % in August, with greater abundances at the CMOR quayside site in September (51.0 %) and October (31.6 %), ascribed to sea spray from stormy conditions during those 2 months.
Elemental mass ratios for the deposition samples from this study, compared to those of soils from the Red Sea coastal plain (Jish Prakash et al., 2016) and TSP samples from other countries of the Middle East (Engelbrecht et al., 2009a). The TSP filter samples were collected by low-volume aerosol samplers without size selective inlets, for 24 h sampling periods.
Elemental mass ratios of the Frisbee deposition samples are compared to the
This study provides new mineralogical, physical, and chemical information on deposition samples collected at the KAUST campus during 2015, as well as an assessment of the seasonal variability of the regional dust deposition rates onto the Saudi Arabian coastal plain.
Inverted Frisbee samplers with foam inserts are found to be robust, easy to
use, and provided comparable results for the collection of wet and dry
deposits. Once a month the samples are retrieved by flushing the deposits
into plastic flasks followed by freeze drying of the slurry and recovery of
all suspended particles and dissolved salts. The average deposition rate at
KAUST for 2015 was 14 g m
Chemical analysis, confirmed by XRD, points to a consistent silicate mineral
fractions for the deposition samples, at all sampling sites for the entire
sampling period. The Si
For 2015, there are marked similarities between monthly distribution patterns of the deposition samples and AOD measured at KAUST, as well as visibility measurements from Jeddah airport, 70 km to the south. This shows that both the AOD and visibility measurements mirror fluctuations in dust deposition, although it may not be justified to calculate quantitative interrelationships without further research.
Except for the variable halite fractions and local construction dust, there are minor variations in the mineralogical content of the dust samples collected on the KAUST campus. To better model the dust being deposited in the Red Sea and coastal plain, the sampling campaign should be extended to sites beyond the KAUST campus. Such a sampling site was recently set up on an island off the coast from KAUST. Inclusion of particle size with mineralogical and chemical measurements provides more effective data for the modeling community.
The deposition samplers collect all particle sizes; however, bin aerosol
models usually consider only PM
As an approximation of source contributions, the sampler with the lowest deposition rate can be considered to have negligible or the least amount of local dust or sea salt (Fig. 5). In the months of December 2014, January, April, March, June, July, and December 2015, the deposition rates at the four sites were similar and considered to have no or negligible amounts of dust from local construction, campus roads, marine salt, or other particulates.
The gravimetric, mineralogical, and chemical data from this study are available upon request from Georgiy Stenchikov (georgiy.stenchikov@kaust.edu.sa).
SEM-based particle volume distribution curves for the first 6 months of 2015 (January–June).
SEM-based particle volume distribution curves for the last 6 months of 2015 (July–December).
Chemical abundances of deposition samples (January–May 2015).
Note: CaO
Chemical abundances of deposition samples (May–July 2015).
Note: CaO
Chemical abundances of deposition samples (August–October 2015).
Note: CaO
Chemical abundances of deposition samples (October–December 2015).
Note: CaO
JE was responsible for the sample analysis and data compilation; GS formulated the problem, designed the research project, and supported experimental activities; JP performed collection and conducted the freeze drying of the samples, as well as performing part of the XRD analysis. TL performed the SEM-based individual particle analysis; AA assembled the meteorological and visibility data; IS assembled the AERONET optical data; and JE, GS, JP, AA, and IS compiled different parts of the manuscript.
The authors declare that they have no conflict of interest.
This research, including the chemical and mineralogical analysis, is supported by internal funding from the King Abdullah University of Science and Technology (KAUST). We acknowledge the contributions from the collaborating Core Labs at KAUST, the Desert Research Institute, and RJ Lee Group, Inc. This research is supported by the Supercomputing Laboratory at KAUST. The valuable comments by the co-editor and three anonymous reviewers towards improving the manuscript are greatly appreciated. Edited by: Yves Balkanski Reviewed by: three anonymous referees