The most important physical, chemical and mineralogical properties of atmospheric dust deposited on Yazd city (Central Iran)

Introduction
One of the most important phenomena in arid and semi-arid regions of the world is dust, which is one of the most important environmental issues in these areas. Low rainfall in these areas, have somewhat reduced water erosion, while the lack of vegetation and other factors contributing to soil erosion has provided the ground for the development of wind erosion and the occurrence of dust storms (Ali Sufi and Shahriari, 1399). The integration of these sources of atmospheric particle production with dust from the soil increases the amount of organic matter and heavy elements in the subsided dust in dry ecosystems (Jafari and Khademi, 2017). Rashki et al. (2013) showed that silica, calcium, aluminum, sodium, magnesium and iron oxide compounds are the most important oxide compounds and quartz, calcite, muscovite and plagioclase minerals are the most abundant minerals in Sistan dust particles. Ali Sufi and Shahriari (2020) examined some chemical properties and the amount of some nutrients along with dust in Sistan plain. Calcium and phosphorus were the most abundant nutrients in the region, with phosphorus and sodium being the highest and lowest enrichment ratios, respectively. Salahi and Behrozi (2020) in the Dezful region of Khuzestan province, Iran showed that among the soluble elements in dust, calcium, potassium, sodium and magnesium had the highest concentration and the tracing of the dust showed that the alluvial sediments of Tigris and Euphrates in Iraq has been the main source of dust in Dezful. Over the past few years, the city of Yazd has witnessed many severe dusts, and it is known that the discovery of the source and other characteristics of these particles has helped to better combat this phenomenon or reduce its effects and its amount. This is despite the fact that no community studies have been conducted on these features in the city of Yazd. Understanding how particles are distributed and their chemical dust and composition to determine the physical / chemical properties, resources and mechanisms of formation and behavior, as well as determining strategies to control it is useful and valuable. Therefore, this study was performed to study the chemical and physical properties of dust during different seasons and to determine the possible source of particles and dust.
Materials and methods
The present study was conducted in Yazd city, the most populous city and center of Yazd province with an area of 131600 Km2. In order to sample dust and surface soil, 30 sampling sites were randomly selected in the study area to provide adequate coverage throughout the area. For sampling of atmospheric dust, from Marble Dust Collector which is made of a plastic tray with several rows of marbles (at least 2 rows) with a tray diameter of 31.5 cm and a height of 5 cm and a diameter of 1.6 cm glass marbles were used. Sampling of atmospheric dust was carried out in four seasons of autumn and winter of 2018, spring and summer of 2019 in Yazd city at a height of three meters above the ground (roofs of one-story houses). Surface soil sampling was performed at a depth of 0 to 10 cm once during dust sampling and close to dust sampling points. The particle size distribution of dust samples was determined. Then there are some chemical and physical properties such as texture, pH, electrical conductivity, nitrate, bicarbonate, the concentration of P, S, Na, K, Mg and Cl were determined in dust and soil samples, as well as the enrichment ratio of nutrients in the dust. The mineralogy of dust particles was also determined by XRD analysis. The data obtained from the analysis of dust and surface soil samples were analyzed using SPSS 16 software. A comparison of the mean of the studied parameters and the significance of their differences was performed using Duncan's test at the level of 5%. Also, spatial distribution maps of the parameters studied in the study were plotted using the inverse distance weighting method (IDW) in Arc GIS 10.1 software.
Results and discussion
In all seasons, silt particles (2 to 50 µm) make up the largest percentage of dust particles. Sand particles (more than 50 µm) make up the largest part of the particles after silt, and clay particles (less than 2 µm) have the lowest frequency of dust in the study area. According to the results, the pH of dust samples has changed from 7.10 to 9.74 with an average of 7.83, which compared to the results of soil samples fewer were found in all seasons. However, the minimum amount of pH was found in winter. Many researchers attribute the decrease in pH in cold seasons to a decrease in temperature and an increase in heating devices and fossil fuels, which causes CO2 and SO2 emissions to be released into the environment, thereby reducing pH (Jafari and Khademi, 2017). The average salinity in dust samples is 4412.93, which is less than the average salinity of the soil samples (minimum, maximum and average salinity of the soil 1990, 18060 and 6852.94 µS/cm respectively) while HCO3 (with an average of 23729.47 mg / kg) and K (with an average of 18972.82 mg / kg) were found in dust samples higher than surface soil samples (with an average of 1239.39 and 16633.06 mg / kg, respectively). HCO3 has the highest amount in summer (2770 mg / kg) and the lowest in spring (1720 mg / kg). NO3 has maximum (4017 mg / kg) and minimum (10.5 mg / kg), respectively, in spring and autumn. Except for the autumn season, nitrate in all seasons of sampling was higher than the amount in surface soil (231.27 mg / kg).The results showed that the salinity of the soil with anions and cations of nitrate, chlorine, potassium and sodium was significant at the level of 1%. However, there was no significant relationship between salinity of dust solution and bicarbonate, phosphorus and magnesium. The results showed that chlorine and sulfur elements were significantly enriched in dust compared to surface soils, and the southern study area with the highest concentration of chemical elements was found. Quartz, calcite, and albeit were the main minerals, and chlorite, elite, muscovite, and dolomite were identified as sub-minerals.
Conclusion
The results indicate that the chemical properties of dust have different time trends in the seasons. These changes can be attributed to changes in the probable origin of dust particles in different seasons and changes in the natural and human resources of dust production. High and positive correlation of chlorine ions, nitrate, and potassium and sodium ions with salinity in dust can be a reason for the origin of dust particles from soils of saline areas. Mineralogy of dust samples showed that the dust of the region contains minerals such as quartz, silica, calcite, albite, sodium feldspar, chlorite, dolomite, illite and calcium carbonate, some of which are similar. Clay mineralogy is topsoil in the study area. Minor changes in major and minor minerals in dust samples indicate the same and similar origin of dust production in the study area.
Key words: chemical properties, mineralogy, atmospheric dust, enrichment ratio, particle size.