Particulate Matter Concentrations in a Middle Eastern City – An Insight to Sand and Dust Storm Episodes

In this study, the particulate matter mass (PM10 and PM2.5) concentrations we measured during May 2018–March 2019 in an urban atmosphere of Amman, Jordan. The results showed that the annual mean PM10 concentration was 64 ± 39 μg m –3 and the PM2.5/PM10 ratio was 0.8 ± 0.2. According to the Jordanian Air Quality standards (JS-1140/2006), the observed PM10 annual mean value was below the limit value but that of the PM2.5 was three times higher than the corresponding limit value. However, both exceeded the World Health Organization (WHO) air quality guideline values. In a larger perspective, the annual mean PM10 concentrations in Jordan were lower than what was reported in other cities in the Middle East but were higher when compared to other Mediterranean cities. During the measurement period, Jordan was affected by Sand and Dust Storm (SDS) episodes on 14 days. The source origins of these dust outbreaks were traced back to North Africa, the Arabian Peninsula, and the Levant. The 24-hour PM10 concentrations during these SDS episodes ranged between 108 and 188 μg m – , which was about 3–6 times higher than the mean values during clean conditions (~33 μg m).


INTRODUCTION
Aerosols affect the Earth's atmosphere directly, e.g., by the scattering of solar radiation, which results in the cooling of the Earth's surface, and indirectly, e.g., by participating in cloud formation. In urban areas, aerosols originate from a vast range of local sources (natural and anthropogenic) and long-range transport. Aerosols have adverse health effects (Pope and Dockery 2006). Cardiorespiratory and lung problems have been often associated with long-term exposure and inhalation of dust particles (Pope et al., 2002;Hoek et al., 2013).
In general, a sand and dust storm (SDS) is by definition an aeolian processes that occur wherever there is a supply of granular material (typically inorganic grains with diameter episodes, which have been reported more frequently during the past decades (Furman et al., 2003;Keramat et al., 2011;Hussein et al., 2011;Hamidi et al., 2013;Hussein et al., 2014;Kchih et al., 2015;Hussein et al., 2017;Munir et al., 2017;Hussein et al., 2018;Bin Abdulwahed et al., 2019;Amarloei et al., 2019;Saeifar et al., 2019;Fountoukis et al., 2020). The increased frequency of SDS episodes in the Eastern Mediterranean region has been referred to the impact of climate change and its consequences by desertification, deforestation, wetland destruction, increased population growth and anthropogenic emissions, food insecurity, and water shortage (Amiraslani and Dragovich 2011;Rezazadeh et al., 2013;Notaro et al., 2015). Notaro et al. (2015) showed that the Eastern Mediterranean region has suffered of warming and a drying episode since the beginning of this century. This led to an increased potential to collapse the Fertile Crescent (namely Iraq and Syria). In the Middle East, and especially in the Arabian Peninsula, a pronounced variability in dust activity was reported with an abrupt regime shift from an inactive dust period (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) to an active dust period (2007)(2008)(2009)(2010)(2011)(2012)(2013) (Aba et al., 2018), which was linked to climate change and global warming impacts in the 2000s (Notaro et al., 2015;Doronzo et al., 2016Doronzo et al., , 2018. The increased dust episodes and atmospheric dust concentrations on top of the atmosphere have a significant impact on the albedo and short-wave radiation over the African and Arabian regions leading to higher surface reflection (Satheesh et al., 2006), especially in Palestine (Singer et al., 2003), Iraq (Al-Hemoud et al., 2020), Kuwait (Al-Dousari 2009. This also has a socioeconomic impact on oil sector , photovoltaic energy efficiency , and health . For example, aerosol dust cause direct and indirect adverse effects for fauna, flora and human health in the regional scale (Abd El-Wahab et al., 2018).
Besides climate change impacts, anthropogenic aerosols have had an increasing trend during the previous decades in the Middle East (Givati and Rosenfeld, 2007). These particles are anticipated to slow down the conversion of cloud drops into raindrops and snowflakes, thus decreasing precipitation from short-lived clouds such as form in moist air that crosses topographic barriers. This in turn, escalated the desertification process in the Middle East causing increased frequency of dust episodes and atmospheric dust particle concentrations.
While aerosol research has been given increased attention in the Eastern Mediterranean, it is still at an early stage in the Middle East (Hamad et al., 2015;Heo et al., 2017;Taheri et al., 2019), especially in Jordan (Hussein et al., 2018). Therefore, the main objective of this study was to investigate the particulate matter (PM10 and PM2.5) mass concentrations over a long-term period from May 2018 to March 2019 in the urban atmosphere of Amman, Jordan. The methods included aerosols sampling using high-volume samplers and gravimetric analysis combined with air mass back trajectories to identify the source origin of SDS episodes.

Measurement Location
The long-term aerosol measurement campaign was performed during May 2018-March 2019 on the roof top (about 20 m above the ground) of the Department of Physics at the campus of the University of Jordan [32. 0129N, 35.8738E]. The campus is situated at an urban background location in the northern part of Amman, which is the capital city of Jordan. The surrounding area of the campus is a mixture of residential area and road network with one of the main roads (Queen Rania street) passing parallel to the west side of the campus (Fig. S1). The downtown was about 10 km south of the campus area.

High-volume Sampling
Two high-volume samplers (model CAV-A/mb, MCV, S.A., Spain) were used; one for PM10 and another one for PM2.5. The high-volume samplers were operated at 30 m 3 h -1 and record the overall mean temperature and pressure during the sampling session. The cascade heads (model PM1025-CAV, MCV, S.A.) were used for sampling particles with aerodynamic diameter lower than 10 µm (PM10) and lower than 2.5 µm (PM2.5) respectively. These sampling heads can accommodate a 15-centimetre diameter round filters, which were quartz filters (Pallflex, PALLXQ250ETDS0150, TISSUQUARTZ 2500 QAT-UP).
Each sampling session (including PM10 and PM2.5) lasted for 24-hours every 6 days. In total, 51 PM10 samples and 48 PM 2.5 samples were collected. Blank sampling was performed several times during the campaign by following the same procedure as for the active sampling except that the highvolume sampler was turned off (i.e., zero flow rate). The blank samples were necessary for the accuracy check of the sampling procedure and analysis (Querol et al., 2004).

Gravimetric Analysis
The gravimetric analysis was according to the European directive EN1234-1 (20°C and 50% HR) at the Institute of Environmental Assessment and Water Research (IDAEA-CSIC, Barcelona, Spain). The gravimetric analysis included pre-sampling and post-sampling weighing of each filter (including blanks). The pre-weighing and post-weighing were done with the same procedure: conditioning temperature 20°C and relative humidity 50%. The conditioning time was 2 days. Each sample was weighed twice (24 hours interval in between) and the average value was recorded. The weighing was made by using a microbalance (Mettler-Toledo, model: XP105 with electrostatic charge detection, Switzerland).
The 24

Meteorological Conditions
The weather conditions were monitored on-site with a weather station (WH-1080, Clas Ohlson: Art. no. 36-3242), which was set to record the reading with 5-minute interval. The weather data included: ambient temperature, pressure, relative humidity, wind speed and direction, and precipitation.

Back Trajectories
Air mass back trajectories were calculated by using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Hess, 1997;Draxler et al., 2012;Stein et al., 2015), which provides detailed information about the origin and path of air masses that arrived at the measurement site. Four-days back trajectories were calculated for each hour at arrival heights 100, 500, and 1500 meters above ground level.
Back trajectories crossing maps were generated in terms of the frequency of air mass crossing over each grid cell of the domain. Here the domain was taken to cover the west-south Asia, North Africa, and Europe (i.e., longitude -20°-60° and latitude 15°-55°). The crossing map resolution was set to 0.5° for the 4-day back trajectories calculated for each hour.

Meteorological Conditions
The hourly, daily, and monthly means of the meteorological conditions (ambient temperature (T), relative humidity (RH), absolute pressure (P), wind speed (WS), and hourly precipitation) are presented in Fig. 1. The monthly mean, standard deviation, minimum, and maximum values are listed in Table S2.
The ambient temperature (T) showed a clear seasonal variation with high values during the summer (June-August) and low values during the winter (December-February). During the summer, the monthly mean T was around 24°C and during the winter it was around 9°C ( Fig. 1(a)). During May 2018-March 2019, the daily mean T was in the range 3-30°C (overall mean 17 ± 7°C ).
The seasonal variation of the relative humidity (RH, Fig. 1(b)) and the absolute pressure (P, Fig. 1(c)) was opposite to that of T. For example, the monthly RH was about 55% and 82% during the summer and the winter; respectively. As for P, it was about 896 hPa and 901 hPa during the summer and the winter; respectively. During May 2018-March 2019, the daily mean RH was in the range 20-100% (overall mean 68 ± 21%) and the daily mean P was in the range 890-908 hPa (overall mean 899 ± 4 hPa).
The wind speed (WS) showed a different monthly variation than T, RH, and P. The monthly mean wind speed was minimum during the autumn (September-November) and maximum during the summer ( Fig. 1(d)). The maximum monthly WS value was about 2.1 m s -1 (August) and the minimum was about 0.8 m s -1 (November). During May 2018-March 2019, the maximum daily mean WS was about 3.6 m s -1 (March 1, 2019).
The rain season started in October 2018 with a small amount (cumulative ~13 mm) ( Fig. 1(e)). During December 2018, the cumulative precipitation was about 180 mm. During January-February 2019, the cumulative precipitation was about 120 mm. By the end of the measurement campaign (i.e., March 2019), the cumulative precipitation was about 470 mm.

Back Trajectories
The 4-day back trajectories crossing maps are presented in Fig. S2 (Supplementary Material) for arrival heights 100 m and 1500 m. The spatial extent of the trajectories crossing for 1500 m arrival height was broader than that for 100 m arrival height. For back trajectories arrived at 1500 m height, they covered the whole Mediterranean Sea Basin and included north Africa, Red Sea, north and middle region of the Arabia Peninsula, the Levant with an extension to the Caspian Sea, and Europe. As for arrival height at 100 m, the back trajectories covered the middle and eastern parts of the Mediterranean Sea, northeast Africa, north Red Sea, north Arabian Peninsula, the Levant, and southeast Europe. Furthermore, trajectories arrived at 100 m showed a predominant crossing path over the eastern part of the Mediterranean Sea.
Surprisingly, the PM concentrations did not show a clear seasonal variation. This is contrary to previous observation reported via on-line continuous long-term measurement of the particle number size distribution, which showed a clear seasonal variation for the coarse mode particle number concentration with higher concentrations during the winter than summer and showed specific peaks in spring and autumn (Hussein et al., 2018(Hussein et al., , 2019. A reason for not observing this seasonal variation in this study can be due to the sampling protocol conducted here; collecting a sample every 6 days. Therefore, it is recommended to perform the sampling every other day if not possible on daily basis; i.e., higher time resolution of the sampling sessions is recommended. According to the Jordanian standards (JS-1140/2006) for ambient air quality, the annual mean the PM10 and PM2.5 must not exceed 70 µg m -3 and 15 µg m -3 ; respectively. This means that the observed PM10 annual mean value is below its annual limit value but the PM2.5 annual mean three times higher than its limit value. As for the 24 h mean limit value, PM10 and PM2.5 must not exceed 120 µg m -3 and 65 µg m -3 ; respectively. According to this, the exceedance of PM10 was 6 times and that of PM2.5 was 7 times. As will be shown in the next section, these exceedances were during the reported SDS episodes. Compared to the World Health Organization (WHO) air quality guidelines for PM10 (annual and 24h must not exceed 20 µg m -3 and 50 µg m -3 ; respectively) and PM2.5 (annual and 24h must not exceed 10 µg m -3 and 25 µg m -3 ; respectively), the observed annual concentrations here are exceeded both annual guidelines. According to the 24 h WHO guidelines, only 6 days did not exceed the PM2.5 limit value and 25 days did not exceed the PM10 limit value.
The WHO (2018) released an update for the global ambient air quality database that reported the annual mean PM10 and PM2.5 concentrations during 2008-2016. Recalling the data for three Jordanian cities (Al-Zarqa', Amman, and Irbid) in 2017, the annual mean PM10 was 82, 68, and 53 µg m -3 ; respectively. This is consistent with our observation here with an annual mean PM10 ~64 µg m -3 . The world overall annual mean PM10 was ~72 µg m -3 during 2008-2016, which is slightly higher than what was observed during our measurement campaign.

Sand and Dust Storm (SDS) Episodes
As indicated in Fig. 2, SDS episodes were considered when PM10 > 70 µg m -3 . This arbitrary threshold was selected based on the distribution of daily PM10 concentrations, which showed two distinct groups of samples below and above this threshold (Fig. S3 in Supporting Material). In practice, this threshold is slightly higher than the annual PM10 mean value and it separates two groups of the PM10 concentrations distributions. Based on this threshold, 14 days were identified and listed in Table 1. According to the air mass back trajectories analysis, the atmospheric SDS was transported from three main source regions: (1) long-range transport from north Africa (Sahara), (2) medium range transport from the Arabian Peninsula, and (3) short-range transported from the Levant. Sometimes, the transport was a combination of two or three regions. Accordingly, type identification was suggested: S-type originated from Sahara region, SL-type originated from Sahara region and the Levant region (i.e., SDS combined from these two regions), and SLA-type originated from all three regions. The SLA was the most common SDS type because the back trajectories originated from north Africa crosses or circulates over the northern part of the Arabian Peninsula and the Levant region.
During the measurement period, a single S-type SDS episode (on July 25, 2018; PM10 ~121 µg m -3 and PM2.5 ~109 µg m -3 ) was identified, which was solely originated from the Sahara region (Fig. 3). Interestingly, during this episode the back trajectories arrived at 1500 m (Fig. 3(c)) were originated and crossed over North Africa but those arrived at 100 m and 500 m (Figs. 3(a)-3(b)) were from the Mediterranean Sea.
Two SL-type SDS episodes were identified (Table 1) when the air masses originated from the Sahara region and circulated over the Levant region (Fig. 4). During these episodes, the back trajectories arrived at 100 m and 500 m originated from the Mediterranean Sea and circulated over the western parts of Syria, but the trajectories arrived at 1500 m circulated originated from the Sahara region and circulated over Syria, Iraq, and Jordan (i.e., Levant). During the first SL-type episode (May 26, 2018) the PM10 concentration was about 108 µg m -3 and during the second one (June 7, 2018) it was about 127 µg m -3 . The average for SL-type episodes was 117 µg m -3 and 112 µg m -3 for PM10 and PM2.5, respectively, with a PM2.5/PM10 ratio of 0.95. The fine size distribution measured during the S and SL types, can be due to size segregation during transport with preferential deposition of coarser particles.
During the autumn, winter, and spring, more intense SDS episodes were observed and they spanned over long time periods (Table 1). These episodes SLA-type SDS (Fig. 5). The back trajectories analysis at all arrival heights confirmed the origin of these SDS to be from the three regions dust sources: Sahara, Arabian Peninsula, and Levant. During these SLA-type episodes, the PM10 concentration was in the range 88-188 µg m -3 . An intensive SLA-type episode was observed during several weeks in October, 2018 (Fig. 2). During this episode, the PM10 concentrations were higher than 100 µg m -3 and also recorded the highest concentration (as high as 188 µg m -3 ). The average for SLA-type episodes was 122 µg m -3 and 77 µg m -3 for PM10 and PM2.5, respectively, with a PM2.5/PM10 ratio of 0.63. The coarser size distribution of PM during the SLA types compared with the S and SL   Fig. 2) at arrival heights (a) 100 meters, (b) 500 meters, and (c) 1500 meters. The arrival location was the campus of the University of Jordan, Amman, Jordan. These maps were generated from the hourly back trajectories during the sampling dates (+ following day).
types, is probably due the proximity of the source area. Notice that S-type and SL-type episodes occurred during the summer. They indicate that some SDS episodes can be lifted up to the upper atmosphere while being transported from their source region to the receptor region, where they settle down. The SLA-type episodes occurred during the Autumn, winter, and spring and they recorded higher PM10 concentrations than the S-type and SL-type episodes, which occurred in the summer. Furthermore, SLA-type episodes transported dust at any arrival height and the back trajectories crossed over a larger spatial extent than that for the S-type and SL-type episodes.
As a comparison to the SDS days, the PM10 concentrations less than 50 µg m -3 occurred during rainy days or accompanied with air masses originating and crossing over the Mediterranean Sea and the eastern part of Europe (Table 1, Fig. 2, Fig. 6).
Sahara SDS crossing over the Mediterranean Sea was reported in the literature Solomos et al., 2018). Solomos et al. (2018) analyzed a record-breaking dust episode observed on Crete on March 22, 2018 that recorded 24h mean PM10 concentration as 206, 850, and 1125 µg m -3 in Chania, Finokalia, and Heraklion; respectively. Gkikas et al. (2018) focused on the direct radiative effects of 20 intense and widespread dust outbreaks originated in North Africa and affected the Mediterranean basin during March 2000-February 2013. Similarly, Alam et al. (2014 and Nabavi et al. (2016) focused on the aerosol optical depth and climatology of some dust outbreaks originated in the Middle East and the Arabian Peninsula that affected west-south Asia. According to these studies, the dust transport in the Middle East and North Africa (MENA) region is transported Fig. 4. Back trajectories (96 hours) crossing maps during SL-type SDS-episodes (indicated on Fig. 2) at arrival heights (a) 100 meters, (b) 500 meters, and (c) 1500 meters. The arrival location was the campus of the University of Jordan, Amman, Jordan. These maps were generated from the hourly back trajectories during the sampling dates (+ following day). from west to east (Alam et al., 2014;Nabavi et al., 2016;Naimabadi et al., 2016;Khaniabadi et al., 2017;Rashki et al., 2017;Gkikas et al., 2018). This consistent with previous observation in Jordan that most of the SDS was mainly started from North Africa and transported to the Middle East after crossing/circulating over the Arabian Peninsula and the Levant. Nevertheless, some episodes started within the Levant and the southern region of the Arabian Peninsula.

CONCLUSIONS
Air quality issues related to sand and dust storms (SDS) in the Middle East are one of the critical issues that require more attention because the frequency and the intensity of SDS episodes have increased recently due to escalating climate change impacts and increased anthropogenic emissions in the region. Here, particulate matter (PM 10 and PM 2.5 ) concentrations were measured and investigated during May 2018-March 2019 in the urban atmosphere of Amman, Jordan. The methods included aerosols sampling using high-volume samplers and gravimetric analysis combined with air mass back trajectories.
The annual mean PM10 concentration was 64 ± 39 µg m -3 (20-190 µg m -3 ). The PM2.5/PM10 ratio was 0.8 ± 0.2, which means that about 80% of the PM10 was within the fine fraction. According to the Jordanian Air Quality standards (JS-1140(JS- /2006, the observed PM10 annual mean value was below its limit value but that of the PM2.5 was three times higher than its limit value. However, both exceeded the World Health Organization (WHO) air quality guideline. According to the WHO global ambient air quality database during 2008-2016, the annual mean PM10 concentrations in  Fig. 2) at arrival heights (a) 100 meters, (b) 500 meters, and (c) 1500 meters. The arrival location was the campus of the University of Jordan, Amman, Jordan. These maps were generated from the hourly back trajectories during the sampling dates (+ following day).
Jordan were lower than what was reported for other cities in the Middle East but were higher when compared to other Mediterranean cities.
During the measurement period extending over 11 months, Jordan was affected by SDS episodes on 14 days. The source origins of these dust outbreaks were traced back to North Africa, the Arabian Peninsula, and the Levant. The 24-hour PM10 concentrations during these SDS episodes ranged between 108 µg m -3 and 188 µg m -3 , which is about 3-6 times higher than the mean values during clean conditions (~33 µg m -3 ).
The limitation of this study is the sampling protocol as collecting a 24-hour sample every 6 days. It is recommended to perform the sampling with higher time resolution. In the future, we continue monitoring with this method and also include on/line sampling with an optical particle sizer.
Combining ground-based monitoring with satellite observation would provide an insight into the SDS episodes characteristics and calibration of the satellite observations.  Fig. 2) at arrival heights (a) 100 meters, (b) 500 meters, and (c) 1500 meters. The arrival location was the campus of the University of Jordan, Amman, Jordan. These maps were generated from the hourly back trajectories during the sampling dates (+ following day).
No 272041), ERA-PLANET (www.era-planet.eu), transnational project SMURBS (grant n. 689443 EU Horizon 2020 Framework Programme and Academy of Finland via the Center of Excellence in Atmospheric sciences and NanoBioMass (project number 1307537). The eCOST action (inDUST, project number CA16202) is acknowledged for supporting this research via the Short Term Scientific Mission (STSM) mobility grant. This manuscript was written and completed during the sabbatical leave of the first author (Tareq Hussein) that was spent at the University of Helsinki and supported by the University of Jordan during 2019.

DISCLAIMER
The authors declare no conflict of interest. Data will be available upon request.

SUPPLEMENTARY MATERIAL
Supplementary data associated with this article can be found in the online version at https://doi.org/10.4209/aaqr.2 020.05.0195