Introduction

Dust storms are meteorological phenomena common in arid and semi-arid regions that have occurred throughout the recent history and ancient geological time. Dust can remain in the atmosphere for weeks and influence regional weather and climate (Shi et al. 2005). It is estimated that nearly 50% of troposphere atmospheric aerosol particles are minerals, mainly sourced from the deserts and their boundaries (Andreae 1995), and can carry heavy metals, bacteria for long distance. Aerosols, especially those carried by dust storms, can be transported globally, and have significant impacts on the global environments and climate (Prospero 1999; Clarke et al. 2001; Bishop et al. 2002). Aerosols also play an important role in removal, deposition and transport of atmospheric pollutants (Sievering et al. 1989; Dentener et al. 1996; Carmichael et al. 1996).

Annually, massive dust storms occurring off the African Desert and Asia have blanketed hundreds of thousands of square kilometers of Asian area. Two main types of winds blow on Iraq, these are Al-shamal and Sahara. Al-shamal is a northwesterly wind blowing over Iraq and the Arabian Gulf states (including Saudi Arabia and Kuwait) creating large sandstorms that impact Iraq, most sand having been picked up from Jordan as well as Syria (Farouk and Makharita 2009). Sahara is a western wind that comes off the Sahara Desert in North Africa (Fig. 1), and is locally known as a simoom, occasionally reaching large areas of Asia, including Iraq. Dust storms strongly influence the global environment, because it is capable of transferring and redistributing the fine-grained sediments. Asian dust storms can transport mineral grains thousands of kilometers to Japan, Korea, North Pacific Ocean, Hawaii and even North America (Clarke et al. 2001; Yang et al. 2001; Moore et al. 2003). African dust storms have been studied by Bergametti et al. (1989); Avila et al. (1997); Falkovich et al. (2001).

Fig. 1
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Map shows Iraq location in the way of the dust storm blowing from the west

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Iraq is situated in the northeast of the Arab world in western Asia, between latitudes 29.5°N and 22ºN and longitudes 38.45ºE and 48.8ºE, bordering Turkey to the north, Iran to the east, Syria to the west and Saudi Arabia to the south, comprising an area 438,446 km2. Iraq has hot dry continental weather in summer and cold, wet weather in winter. Dust storms in Iraq occur more frequently during spring and summer, with an average of 20 days per year (Al-Farrajii 1998). Northerly and northwesterly winds sweep the country during the dry months, where wind velocity may exceed 100 km/h, which raises dust storms. Wind speeds may reach their maximum by midday in July (average 3.3 m/s). The measured threshold velocity for the movement of soil and sediment particles was 3.0 m/s (Dougramedji 1999). Sand particles and dust particles will be blown away from the dry land surface and transported for several hundred kilometres to the Atlantic Ocean (Thompson 1984). When a low-pressure system occurs in the north of the Sahara Desert or above the Mediterranean, the turbulence takes place from west to the east. Strong winds will transport “hot sand particles and dust particles” from the Sahara Desert to the Mediterranean coasts and the delta of the Nile. This wind can blow for 50 days without stopping and reduce visibility to a few meters. It is locally termed “Wind Hilleck” in Algeria or “Sirocco” in southern Europe (Thompson 1984).

In Sudan, the strong wind is locally called Haboob (Thompson 1984). It was reported that the wet–warm strong winds in the Northern Sudano, at the southern fringe of the Sahara Desert, normally associated with sand-dust storm and thunder can last, on average, 3 h and can flatten sand dunes and accumulate new barchan dunes. Haboob windstorms sometimes originated from the Northern Sahara Desert and are related to the lower atmospheric pressure in the Mediterranean (Thompson 1984). It blows with dense contents of sands and dusts.

Three major categories of particles have been differentiated in dust storms of China during 20 March 2002: mineral particles, coal fly, and soot aggregates. Soot aggregates were characterized by chain-like and "fluffy" appearance, coal fly ash by a smooth spherical shape and mineral particles by irregular shape. Mineralogy and geochemistry of dust storm particles may be more diagnostic of their sources (Shi et al. 2005).

Dust storms and sand storms are the most natural hazard in Iraq due to extreme heat, low humidity and little precipitation. The deposited dust influences vegetation productivity, atmospheric transparency, heat balance, decrease of photo-radiation received by vegetation, vegetation cover, increasing particles on the plant leaves, which prevents the arrival light to the chlorophyll, and damages the small crops.

This work aims to describe the mineralogical composition, mineral maturity and classify the sediments of sand dune and the African–Asian dust storms that occur in Iraq in their way toward the east, and giving a preliminary estimation on the amount of transported and deposited dust.

Materials and methods

Many dust storms occurred in Iraq in 2008. Five samples were collected from occurrences on 15 March, 11 April, 28 April, 25 May and 26 June at Baghdad city. A total number of 20 sand dune samples were also collected randomly at the surface of four locations (between Al-Ramadi and Rutba) emplaced in the shadow of the south side of the highway which crosses the Western Desert of Iraq and connects Iraq with Jordan and Syria (Fig. 2).

Fig. 2
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Geological map shows the studied area

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In attempt to calculate the mass of transferred fallout dust, a clean glass plate of 1 × 1 m was designed and installed horizontally on the building roof of 3 m high at Baghdad city. Accordingly, the fallout dust was collected from the glass slab and carefully and weighted. The amount of deposited dust in Iraq during only 8 h was calculated by multiplying the area which was covered by dust (75% of total area of Iraq approximately) by weight of dust deposited in an area of 1 m2.

Mechanical sieve analyses for both types of samples (dust storms and sand dunes) were performed (Table 1). In this procedure 50 gm from each sample was taken; from then, the sand fraction was separated by dry sieving; thereafter, the mud fraction was split into silt and clay by the pipette method following Folk (1974). Heavy minerals were separated from dust storms and sand dunes samples using bromoform. The mineralogical identification for heavy and light minerals was carried out petrographically on the prepared thin sections and tested under polarized microscope in the Department of Earth Sciences, College of Science, University of Baghdad. Then, at least 100 grains were identified and counted manually based on their optical properties, then calculated as percent (Table 2). The methods of heavy mineral grain identification are described in Grosz et al. (1990) and Mange and Maurer (1992). One whole sample of both of the dust storm and the light mineral fraction of sand dune were prepared as powdered (less than 4 μ), then identified mineralogically using the X- ray diffraction technique (XRD) (Fig. 5), where the XRD Philips PW1729 was used. Chemical analyses were done by different methods; Fe2O3 and Al2O3 were determined using the Atomic Absorption Spectrophotometer (AAS); which is a computerized-type GBC 933 at the Department of Chemistry in the University of Baghdad. Na2O3 and K2O were determined by Flame photometry; CaO and MgO were determined according to the ASTM (1989) by titration with ethylenediaminetetraacetic acid (EDTA) (0.01 N).

Table 1 Results of grain size analyses of sand dune and dust storm samples
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Table 2 Average mineralogical composition in the dust storms and sand dunes, dash lines represent 0%
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Results and discussion

Grain size analyses

Dust storm and sand dune samples were sieved mechanically. On the basis of the interpretation of the average grain size of the dust storm samples; clay particles are dominant (70%), while the remnant ratio is shared between silt fraction (20.6%) and fine sand fraction (9.4%) (Table 1; Fig. 3a). The grain size of clay is less than 0.0039 mm, whereas the grain size of silt ranges from 0.062 to 0.0039 mm and fine sand ranges from 0.125 to 0.062 mm. The average grain size of sand dunes was distributed as 72.7% sand (0.062–1 mm), 25.1% silt and 2.19% clay (Table 1; Fig. 3b). Dust storm samples appear to be clay to sandy clay sediments, whereas the sand dune samples are silty sand (Fig. 4).

Fig. 3
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Average distribution of grain size in dust storm samples (a); in sand dune samples (b)

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Fig. 4
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Grain size analyses of the average of dust storm and sand dune samples

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Sand storm is basically a wind storm that carries sand through the air, forming a relatively low cloud near the ground. Accordingly and based on grain size, the current studied storms are dust storms, but not sand storms. Fine particles may also be swept upwards hundreds of meters into the air. The average height of a dust storm was 900–1,800 m (Al-Farajii 2000).

Mineralogical composition

Mineralogical composition in dust storm and sand dune samples are presented by X-ray diffractograph (Fig. 5) and polarized microscope (Fig. 6). The identification of mineral assemblages of aerosols participates in diagnostics of their sources (Davis and Guo 2000). The particles of the dust storms were mainly formed from light minerals like quartz (49.2%), calcite (38%), and feldspar (4.9%) which were represented by orthoclase and plagioclase, gypsum (4.8%) and dolomite (0.8%) (Table 2), besides little quantity of the heavy minerals (3.2%) which are represented by hornblende, pyroxene, chlorite, magnetite and zircon (Fig. 5a). Heavy minerals provide valuable evidence about the origin of sediment and interpretation of their provenance and can give insights into the types of geology that formed the target rocks (Walkden et al. 2008). Zircon and hornblende basically reflect the granitic igneous rock origin. Pyroxene and magnetite originated from basic igneous rocks. Chlorite was probably derived from metamorphic rocks. Mineralogical composition indicates that the dust sources are eroded igneous, sedimentary and metamorphic. Since the wind has been blown from the west of Iraq, therefore the probable supplying sources are the dried land in the Saudia Arabia, Jordan, Syria and North Africa. Egypt, Saudia Arabia, Jordan, Syria and even some parts of Iraq are source points of dust (Fig. 7a; MOIRI 2009). The satellite images support that the Sahara Desert in North Africa was a source of the dust storms in case of east direction wind. Arabian shield contains granitic and metamorphic rocks (Glennie 2010). Sahara and dry lands around the Arabian Peninsula are the main sources of airborne dust, with some contributions from Iran (Victor 2007), Basaltic flow exposed on surface in eastern Jordan and ordinary extends to the southeast of Syria (Abed 2000).

Fig. 5
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X-ray diffractograph of: a dust storm sample collected during 11 April, 2008 in Iraq; b light fraction of sand dune sample collected from the Western Desert of Iraq

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Fig. 6
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Microscopic photos of storm dust samples. a Displays the separated heavy minerals from the fine sand fraction (20×); b displays light minerals (5×); (+) represents cross-polarized light

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Fig. 7
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Satellite images. a Middle East with point sources of dust (after MOIRI 2009); b dust storm coming from Sahara Desert in the Africa toward Asia (after UCAR 2008)

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Sahara Desert is the largest desert providing a different type of loss sediments. Satellite images present that the Sahara Desert participated in supplying particles (Fig. 7b). Accordingly, Sahara Desert, Arabian shield, weathered igneous rock in Syria and Jordan as well as the dried land and loss soil are considered as a parent material source.

With regard to the sand dunes, the major constituents are quartz, feldspar and calcite (Fig. 5b). The average mineralogical composition is comprised mainly from quartz (67.1%), feldspar (20.9%) and calcite (5%), whereas gypsum (0.4%) and dolomite (1%) formed a little quantity (Table 2). Heavy minerals contributed 6.6% from the average of mineralogical composition of sand dunes (Table 2).

Heavy mineral suites are represented by zircon, tourmaline, muscovite, garnet and pyroxene. Heavy minerals are concentrated within the sand fraction; therefore, they existed much more in sand dune samples than in dust storm samples. The metamorphic source is indicated by existence of epidote, garnet, kyanite, staurolite, chlorite, andalusite, and blue-green hornblende (Milner et al. 1962). In the dust storm samples, chlorite reflects the metamorphic origin. Magnetite, chromite, chromian-spinels, pyroxene and rutile commonly refer to basic igneous sources (Zimmerle 1984). Magnetite and pyroxene reflect the basic igneous rocks. Large bodies and occurrences of acidic igneous rocks (granite) and basic igneous rocks (gabbro and diabase) are well exposed in the western side of the Arabian shield in Saudia Arabia (Hussein and Omer 2011). The occurrence of Lower Paleozoic rocks in Saudi Arabia crop out in three regions, in the northwestern, central and southern portions of Saudi Arabia (Wanas and Abdel-Maguid 2006). In the northwestern region, outcrops of the Lower Paleozoic rocks extend from Great Nafud to Jordan (Helal 1968). In the central area, exposures of Lower Paleozoic rocks were recorded by AL-Laboun (1986) in the Hail and Qasim regions. In the southern region, exposed Lower Paleozoic rocks extend from Gabal Wajid to North Yemen (Kellogg et al. 1986). The sandstone in southern Saudi Arabia that occurred directly above the Precambrian Arabian Shield has been described as ‘Nubian type’ (Hardley and Schmidt 1975) because of its similarity to other similar sandstones exposed in North Africa, northern Saudi Arabia and Jordan (Beydoun 1988). Zircon, muscovite and hornblende reflect the acidic source.

Wind speeds may reach their maximum by midday in July (Al-Farajii 2000). The measured threshold velocity for the movement of soil and sediment particles was 3.0 m/s (Dougramedji 1999). Wind speed, in addition to the mineral hardness, is control factor of the shape of the mineral surface. The resistant minerals such as quartz possessed smooth surface during weathering and transportation processes, whereas the surface of non-resistant minerals (calcite and feldspar) are rough and uneven.

Geochemistry

The results of geochemical analyses for dust storm and sand dune samples are listed in Table 3. The concentration of three major oxide groups such as silica and alumina, alkali oxides and iron oxides plus magnesia were used to classify sandstones. The enrichment of SiO2/Al2O3 by mechanical and chemical processes produces quartz arenites (Orthoquartzites; Obiefuna and Orazulike 2010). Sand dunes appear to be more mature than dust storm because they have more quartz, where quartz is a function of the intensity of weathering. Abundance of alkalis (Na2O and K2O) characterizes immature sandstones such as Arkoses and greywackes, whereas the ratio of Na2O/K2O determines both the provenance and diagenesis of the sandstone (Akinmosin and Osinowo 2008; Ibe and Akaolisa 2010). Most of the Alkalis (K2O and Na2O) reflect high quantity of feldspar relatively indicating immature sediments.

Table 3 Chemical composition of the dust storm and sand dune in Iraq
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According to Pettijohn et al. (1972), sandstone classification is presented in Table 4; as arenite, greywacke, arkose and lithic arenite. In dependence on the comparison between the results in Table 5 with the results in Table 4, dust storms and sand dunes are classified as arkosic sediments.

Table 4 General classification of sandstone according to logarithmic oxide ratio (after Pettijohn et al. 1972)
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Table 5 Classification of dust storm and sand dune
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The environmental impacts

Dust storms in Iraq are most prevalent in the spring and summer when a prevailing northwesterly wind known locally as the Shamal kicks up the fine desert sand and the silt along the Tigris and Euphrates river basins (Al-Farajii 2000). A dust storm which occurred in 11 April, 2008 continued to blow on Iraq for 8 h, covering 75% of the total area of Iraq. This storm deposited 20.987 g per square meter, where it was collected on a glass sheet of 1 × 1 m2; and therefore, the total weight of deposited dust equaled 6.9 million tons, which is calculated through the following equation:

$$ D_{\text{w}} = { 75}\% \, A \, \times \, W $$

where, D w is the weight of dust, A is the total area of Iraq; and W is the weight of the deposited dust on 1 m2 during 8 h.

The 75% of the Iraq area equaled 328,834.5 km2; 20.987 g of dust was deposited on each square meter approximately. Accordingly, the total weight of fallout dust is calculated to be 6.90125 million tons (rounded to 6.9 million tons). It was deposited as a thin layer during just 8 h. This phenomenon displays how wind power can rapidly influence the weather, climate and environment. The atmospheric temperature declined in Baghdad (the capital of Iraq) from 36 to 30°C during 8 h. During the episodes of dust storms, visibility decreased enormously and became no more than 30 m. Dust components play a certain role in the decay of solar radiation by decreasing the transparency of the atmosphere (Muminov and Mnagamova 1995; Chube 1998. High levels of airborne dust reduce the amount of sunlight that reaches the land; if it occurs over the ocean, there is a lowering of sea surface temperatures and, generally, hurricane probability. Depositing dust on traffic road highway and railway sections confused traffic flow, causing road accidents, and increasing maintenance costs.

Air quality standards for suspended particulate matter generally are designated as PM10 standards for inhalable particulate matter and PM2.5 standards for fine particulate matter (Alonso et al. 2002). Public health concerns focus on the particle size ranges likely to reach the lower respiratory tract or the lungs. Inhalable particulate matter (PM10, 10-μ component) represents particle size categories that are likely to reach either the lower respiratory tract or the lungs after being inhaled. Fine particulate matter (PM2.5) represents particle size categories likely to penetrate to the lungs after being inhaled. Size, shape, and density are important physical characteristics of suspended particulate matter. Because of the large relative size of sand (210–62.5 μ; Table 1). It does not affect the respiratory system, while clay affects the respiratory system because of its fine size (3.9–0.006 μ; Table 1). With regard to the silt fraction (53–7.8 μ; Table 1), the soft and very soft parts affect the respiratory system. Accordingly, clay fraction is considered the worst part within the dust storm which forms 70% (Table 1). Air quality considerations focus on the smaller component of particle suspension (Buchdahl 1999; Alonso et al. 2002). Coarse dust particles generally only reach as far as the inside of the nose, mouth or throat. Smaller or fine particles can, however, get much deeper into the sensitive regions of the respiratory tract and lungs. These smaller dust particles have a greater potential to cause serious harm to human health. However, some people with pre-existing breathing-related problems, such as asthma and emphysema, may experience difficulties. Many people were taken to hospitals after sustaining breathing problems. Some of them died.

Wind cannot erode moist soil, but the dry soil can easily be eroded. Wind erosion physically removes the lighter, less dense soil constituents such as organic matter, friable and loss constituents like clays, silts and sometimes fine sand. Thus, it removes the most fertile part of the soil and lowers soil productivity (Al-Farrajii 1998). Therefore, dust can also have beneficial effects where it deposits. Central and South American rain forests get most of their mineral nutrients from the Sahara; iron-poor ocean regions get iron (Ilan 2006) when dust blows in a westward direction; the transported dust added fertile matters may dilute salinity, and redistribute the pollen participating in plant pollination.

Conclusions and recommendations

In terms of grain size, dust storms were classified as clay to clayey sand sediments, whereas sand dunes are classified as silty sand. Due to differences in the grain size, the mineral components in dust storms and sand dunes have varied. Geochemistry and mineralogy revealed that the sand dune samples tend to have much more maturity than the dust storm, but both are classified as arkose.

Heavy minerals as well as the satellite image refer that the Sahara of North Africa, dry lands in Saudi Arabia, Syria and Jordan are sources of the dust. Sand and dust storms are a visible natural process on the African Continent; the vast areas and distribution and extent of the desert landscape, including the Sahara Desert, indicate that this region is the very source of material for sand and dust storms in historical time (Yang et al. 2001). Dust storms from the Sahara are a source of minerals to the Mediterranean and Black Seas (UCAR 2008).

Sahara, North Africa and the Arabian Peninsula and west Asia are going to be desertification, while new useful material is added to the lands in some parts of Asia, including Iraq, and redistributed to refreshing the soil. From the dust storm that occurred in Iraq on 11 April, 2008 settled 6.9 million tons of clay and clayey sand during 8 h as a thin sheet layer covering 75% of the total area of Iraq.

Dust storms affected a direct impact on the climate where the temperature dropped 6°C during 8 h and also especially affected the respiratory system in humans, causing an increase in the numbers of people who died or were admitted to hospitals. Many components of suspended particulate matter are respiratory irritants (Alonso et al. 2002), 70% of dust storm samples are clay (Table 2) and have a size less than 0.0039 mm. Dust storm samples are not carcinogenic because they do not prove the existence of fibrous minerals. Suspended particulate matter or compounds adsorbed on the surface of particles can also be carcinogenic or mutagenic chemicals (Alonso et al. 2002). Fine particulate matter (PM2.5) represents particle size categories likely to penetrate to the lungs after being inhaled (Alonso et al. 2002). Accordingly, the clay fraction is the harmful factor on human health due to penetration to the lungs.

Dust storm-affected plants, causing a pulling from their places or breaking some of their parts, and deposition of a thin layer of dust on the leaves, which partially prevents sunlight, reducing the efficiency of photosynthesis. As plant roots and wet lands reduce erosion (Gerasimov 1986), and for the purpose of combating desertification and land reclamation, this study recommends that the desert land should be planted and irrigated to become resistant to desertification.