Transport routes and potential source regions of the Middle Eastern dust over Ahvaz during 2005–2017

Highlights

• A new Middle East dust origins map was produced based on previous studies.

• The main corridors through which dust particles travel and arrive in Ahvaz were delineated.

• The fluctuation of the dust occurrence in Ahvaz was investigated in different seasons.

• Main contributing sources to the dust of Ahvaz were determined during dusty days on a seasonal basis.

Abstract

Ahvaz is among the most polluted cities in the world in terms of particulate matter (PM) concentration. It is a prevailingly vulnerable city to the dust storms intrusion experiencing nearly 98 dusty days per year. The main aim of this study is to determine the transport pathways of dust particles reaching Ahvaz and investigate the dust sources that potentially contribute to dust loading over this city. To this end, temporal and spatial characteristics of Ahvaz dusty days, with a focus on associated air parcel trajectories are investigated in tandem with station observation, potential source contribution function (PSCF) and aerosol optical depth (AOD). Four main dust corridors were identified, based on the trajectory results; one northwesterly and two westerly routes, passing mainly over Tigris Euphrates Plain and Mesopotamian marshlands before hitting Ahvaz. These transport routes are associated with Shamal winds and are active all year long, especially in the summer. Another dust corridor influencing Ahvaz is a southerly route related to the prefrontal dust-storm mechanism. This corridor is active mainly in spring and winter and lifts the dust from Zubair Desert, Ad Dahna Desert and arid land of southern Khuzestan Plain. The forward calculation, PSCF and AOD analysis are consistent with backward air masses trajectory results and showed that eastern Syria, central and southeastern Iraq, and northwest of Ahvaz are the primary sources for dust affecting Ahvaz. Other intense dust sources are located in the south of Ahvaz including the eastern coastal zone of the Arabian Peninsula, southern Iraq, and southeast of Ahvaz.

Introduction

Constituting more than half of global aerosol loadings, mineral dust is the amplest aerosol content in the atmosphere with an annual global emission estimation of 1000 to 2000 MT. (Choobari et al., 2014; Middleton, 2017). Dust storms, usually emanating from arid and semi-arid regions, are reckoned as the primary source of mineral dust (Nabavi et al., 2017; Rashki et al., 2012). Nevertheless, dust storms' hazards are not restricted to the arid areas and could affect extraneous areas owing to long-range transport (Barkan and Alpert, 2010; Makra et al., 2011; Middleton, 2017; Prospero et al., 2014). The influence of mineral dust on both earth system and humans is abundant and diversified (Goudie and Middleton, 2006; Middleton, 2017). Dust particles not only affect earth solar radiation budget (Bi et al., 2017; Fernández et al., 2017; Gehlot et al., 2015; Noh et al., 2016; Wu et al., 2018), but also impact the environment in many facets indirectly including global warming (Kok et al., 2017), rainfall patterns adjustment (Fan et al., 2012; Gu et al., 2016; Min et al., 2009; Price et al., 2018), and hastening snow melting (Axson et al., 2016; Li and Flanner, 2018; Skiles et al., 2015). Dust storms also threaten urban environments (Goudie, 2014) as they can convey a tremendous amount of PMs (Escudero et al., 2006; Givehchi et al., 2013), heavy metals (Lyu et al., 2017; Naimabadi et al., 2016), organic matters (Hou et al., 2006; Zhang et al., 2010), and allergens (Goudarzi et al., 2014; Griffin, 2007; Soleimani et al., 2016). They may engender huge ramifications on residents health, most notably respiratory and cardiovascular diseases as the relationship between the increment of PM₁₀ concentration and the number of hospital admissions and death rate has been determined to be significant (Khaniabadi et al., 2017; Marzouni et al., 2016; Morman and Plumlee, 2013).

Accounting for 20–25% of dust emission globally (Hamidi et al., 2017; Moridnejad et al., 2015a), the Middle East is recognized as the second-largest mineral dust source in the world just after Sahara (Alizadeh-Choobari et al., 2016; Rezazadeh et al., 2013). The dust storms take place all year long in the region (Hamidi et al., 2014), while the activity's peak may shift (Furman, 2003; Notaro et al., 2013). Many Iranian urban districts are placed downwind of dust-bearing winds (Alizadeh-Choobari et al., 2016) and exposed to Middle Eastern dust storms frequently. Among them, Ahvaz, declared as the most polluted city in the world based on its PM₁₀ concentration (Broomandi et al., 2017b; Farsani et al., 2018; Maleki et al., 2016), is subjected to dust storms frequently owing to the adjacency of Saudi Arabia, Iraq and Kuwait's arid lands (Naimabadi et al., 2016). This city lies in the southwest of Iran and is the capital city of Khuzestan province with a population of 1.3 million in 2016. Although there are different aerial pollutant emission sources in Ahvaz including motor vehicles, fossil combustion, and petrochemical industries, previous studies highlighted that majority of Ahvaz PM₁₀ are of aeolian origin (Broomandi et al., 2017b: Sowlat et al., 2013: Zarasvandi et al., 2011). For instance, Sowlat et al. (2013) showed that crustal dust comprised more than 40% of Ahvaz PM₁₀ and they concluded that the natural sources contribute most to PM₁₀ particles in this city. Ashrafi et al. (2018) also showed that the crustal dust comprises about 30% of the total suspended particles (TSP) over Ahvaz and is the leading source for its PM. Moreover, there is a large body of documentation that reveals dust storms' impact on different pollutants level on air quality of Ahvaz urban area (Goudarzi et al., 2014; Goudarzi et al., 2019; Naimabadi et al., 2016; Najafi et al., 2014; Farsani et al., 2018; Soleimani et al., 2013; Soleimani et al., 2015; Soleimani et al., 2016). Department of Environment (DOE) of Ahvaz reported that hourly PM₁₀ concentration surpassed 9000 μgr/m³ in extreme cases of dust storms such as September 9th, 2015, February 20th, 2016 and January 27th, 2017. Frequent dust outbreaks with high intensity have led to the vast hospitalization in Ahvaz (Geravandi et al., 2017; Maleki et al., 2016) and may cause significant emigration (Farsani et al., 2018).

Although Ahvaz is frequently affected by dust intrusion, day in day out, very few studies were focused on identifying the potential sources of Ahvaz dust storms, and also no study investigated the effects of regional sources on seasonal variability of dust concentration over this city. The current understanding of dust sources affecting Ahvaz is based on the simulation of six extreme dust storms occurred on May, Jun and July 2010 (Ashrafi et al., 2014; Sotoudeheian et al., 2016) and July 2016 (Khalidy et al., 2019) and the analysis of backward trajectory of warm period in 2010 (Broomandi et al., 2017a). The results stated that northern and central parts of Iraq and eastern Syria are the most influential sources for Ahvaz dust loading. Ashrafi et al. (2014), Khalidy et al. (2019), and Sotoudeheian et al. (2016) quantitatively determined the contribution of different Middle Eastern dust sources to the PM₁₀ level of Ahvaz. These studies are very profitable for the precise determination of contribution of different source regions to PM₁₀ level over a specific receptor, but they did not determine the transport pathways of dust particles reaching Ahvaz. Moreover, these studies focused on the extreme cases (PM₁₀ concentration of over 300 μg/m3), and moderate and weak dust storms were neglected. As the occurrence of non-extreme dust storms are frequent, and they also influence the air quality of Ahvaz, more dust storm episodes should be included to increase the results' reliability. A recent study conducted by Broomandi et al. (2017a) tried to locate the origins of dust storms affecting Ahvaz by analyzing backward air masses during the warm period of 2010. The backward trajectory calculation is easy to perform. Thus when it comes to dust source identification, it should be used in a broad temporal extent to cover a meaningful period, especially for Ahvaz, encountering about 98 dusty days each year. Therefore, performing limited backward calculations significantly reduce the reliability of the results. Moreover, Ahvaz receives a considerable mass of mineral particles even in spring, fall, and winter. The number of dusty hours in this city is roughly equal for both spring and summer, and extreme cases of dust storms were recorded in winter and fall. All the reviewed studies not only just considered a few dusty periods, they mainly focused on significant dust storms that occurred on warm months of Ahvaz (late spring to summer). Besides, previous studies primarily relied on one tool or method (e.g., dust storm simulation module of HYSPLIT, or backward trajectory calculations) to conduct their research. As every single tool has limitations, the combination of different available methods and data could significantly enhance the results. This long-term study tried to fill the mentioned gaps and address the unanswered questions in the previous works by considering multiple tools, methods, and datasets, including Aerosol Optical Depth from MODIS (MODerate-Resolution Imaging Spectroradiometer), meteorological data, backward and forward trajectory calculations, and a receptor model named PSCF through a long-term calculation. Air mass trajectory calculations can be designated as the most popular, yet effective tool to identify the possible sources and transport pathways of different aerial pollutants such as dust, SO₂, ash, and radionuclides (Salmabadi and Saeedi, 2019; Willis et al., 2017). We have used both backward and forward trajectory calculations to minimize the uncertainties of the trajectory calculations. In addition to trajectory calculations, MODIS AOD, used in many studies to investigate dust storms (Ginoux et al., 2012; Nabavi et al., 2016; Prospero et al., 2002; Yu et al., 2018), has been employed. Moreover, through a detailed and long-term study, we tried to increases the results' reliability as enough dust storms have been included in our calculations.

The main objectives of this study are: 1) Investigation of the fluctuation of dust occurrence over Ahvaz for the period between 2005 and 2017 using synoptical data 2) Determination of the transport pathways of dust storms reaching Ahvaz in different seasons using air mass backward trajectory. 3) Discovering the potential dust source regions contributing to dust particles loading over Ahvaz using a receptor model (potential source contribution function), forward calculation from Middle Eastern dust sources and AOD estimations from MODIS instrument. To the author's knowledge, identification of transport pathways as well as source of dust loading reaching Ahvaz has not been addressed based on the large dataset, covering 13-years, up to the present time. Besides this, for the first time, seasonal climatologies (2005–2017) of air parcel trajectories from the each of the Middle Eastern dust sources were created to further investigate the main contributive dust sources of Ahvaz.