Document Type : Full length article
Air Pollution and Dust, Atmospheric Science and Meteorology Research Center
Sand and dust storms are the focus of much attention by governments and the international community because they affect health, agricultural land, infrastructure, and, generally, biological and economic resources. In this paper, critical sources of wind erosion, sand, and dust storms in Ilam province are identified, and environmental factors affecting their activating are determined. Also, the pattern of spatial and temporal changes in wind erosion, sand, and dust storms in this province has been studied according to national and international guidelines. Finally, the priority of critical sources of wind erosion, sand, and dust storm in Ilam province has been classified and determined.
Observed data from the synoptic station of Ilam province were used from 2002 to 2017 to investigate active dust sources in Ilam province. Optical Thickness Aerosol (AOD) At 550 nm, the MODIS sensor product, as a dimensionless parameter, is directly related to the concentration of airborne particles and is considered as an important criterion for detecting aerosols in the atmosphere. In addition, re-analyzed Era5 data are used to examine meteorological patterns. Dust Storm Index (DSI), wind Index, and drought index (SPEI) are calculated during the mentioned period. Then, using the GOCART model, the vertical dust flux was investigated monthly and annually, and the amount of dust emission was obtained from different sources in Ilam province. The HYSPLIT Lagrangian model also is used to determine the transmission path of dust particles through a case study that occurred on March 14 and 15, 2012.
Results and Discussion
A study of the monthly and seasonal pattern of AOD in Ilam province showed that in the warm season, dust formation is strengthened by north, and northwest winds and canalization of wind fields due to geological forcing, especially in the Mesopotamian region. Also, using observation data from synoptic stations in Ilam province dust storm index (DSI) and wind index were calculated and investigated. The results indicate that the trend of dust event changes in Ilam province has decreased over time. The highest wind index in this province was observed in the class of 0-5 m / s. In addition, in the wind class of 5-10 m / s, the maximum values are observed in the northern and western half of the province. The decrease in horizontal visibility due to dust events in the southern and southeastern of the province reaches between 1500-5000 meters. In spring and summer, the number of dusty days with the mentioned decreased visibility in these areas is more than in the other seasons. By calculating the drought index (SPEI) for Ilam province, it was observed that the drought situation had gradually intensified since 2008 when it was in a normal state so that in 2013, moderate drought prevailed in the province, and then with the continuation of this trend moderate to severe drought has been observed in this province from 2015 to 2017. Then, using the GOCART model, the vertical dust flux was investigated monthly and annually, and the amount of dust emission was obtained from different sources in Ilam province. The results showed that the highest amount of vertical dust flux occurs from Dehloran city in spring and Mehran city in summer, while the lowest amount of dust emission occurs in winter. In general, dust flux is observed in Ilam province in the southern half of Dehloran city and a small area located in the center of Mehran city. A comparison of friction velocity and dust flux quantities also showed that in the southern half of Dehloran city, dust flux values are very high despite very low friction velocity values.
Finally, a case study of a dust storm on March 14 and 15, 2012, showed that during these days, the horizontal visibility at Abdanan station had reached less than 1000 meters, and at Darhshahr and Dehloran stations had reached less than 2500 meters. Establishment of the front in the southwestern regions of the country on March 14, 2012, at 12:00 UTC and in the western and southwestern regions on March 15, 2012, at 12:00 UTC, which was well observed with the change of direction and speed of the wind field in these areas, provided the conditions for dust emission in these areas so that a significant reduction in horizontal visibility occurred in many synoptic stations in the country and Ilam province on March 14 and 15, 2012. On the land surface, the establishment of a low-pressure center on the southwestern borders of the country and a high-pressure center on the southern half of the country provide the conditions for creating a strong pressure gradient in the southwestern regions of the country and strengthening the winds that finally, has caused the dust emission. The RGB image of the MSG satellite also shows the presence of dust mass on both days in the central regions of Iran.
Therefore, in Ilam province, two high and very high-intensity sources are located in the south (Mosian city) and southwest (Dehloran city) of this province. The prevailing wind direction in each season indicates the regions affected by each dust source. The dust source located in the south of Ilam, which has the highest intensity, is very active in all seasons. Due to the prevailing wind in spring, which is in the west, southwest, northeast, and northwest, Ilam and Khuzestan provinces are affected by this dust source. In summer, the prevailing wind is in the west, southwest, and northeast; this season, some areas of Ilam and Khuzestan provinces will be affected. Dominant northeasterly, westerly and easterly in autumn, and southwesterly winds in winter in this region cause Ilam and Khuzestan provinces to be affected by dust from this dust source. The second center, which is located in the southwest of Ilam province, is very active in spring, summer, and autumn. Due to the prevailing wind in spring and summer that blows from the west, southwest, northwest, and north, the provinces of Ilam, Khuzestan, and Lorestan are affected by this center. In autumn, northerly, northwesterly, northeasterly, easterly, and southwesterly prevail, and Ilam and Khuzestan provinces are affected by this center. Also, the annual review of the sources' activities showed that both had more intensity in 2013 than the other years.