ردیابی منابع گردوغبار در سطوح مختلف جوّ تهران با استفاده از مدل HYSPLIT

1. Introduction Dust phenomenon which occurs in arid and semiarid lands of the world is closely related to climatic characteristics of the region. According to the world meteorological organization (WMO), whenever wind speed exceeds 15 meter per second and horizontal visibility reduces to less than 1 km in a station, dust storm is reported (Goudie & Mideleton, 2006). Great storms are created when long-term droughts occur and the soil is quite dry (Azimzadeh et al, 2002). Sometimes, dust particles affected by physical and chemical processes are combined with other pollutants in a long- distance transition and form new compounds (Zhao, 2010). Toxic pollutants which travel with dust clouds can be absorbed into the skin or entered into the respiratory tract and consequently cause skin irritations and respiratory illnesses (Goudie & Mideleton, 2006). The process of studying and analyzing storm events and tracing dust sources can be performed through different methods. Concerning the frequency of dust storms in the world, Engelstadler (2001) stresses the important role of dry bed lakes and African great desert as the major producers of dust, and considered Sahara to produce dust more than any other deserts in the world. Wang (2005) studied the formation of dust storms in northeast Asia from synoptical view point and found that dust storms in this region are always accompanied by a cyclone or a low-pressure system and the amount of dust is maximized in the warm sector of the cyclone. Each year, Iran suffers serious casualties and damage from natural disasters due to its great breadth, diversity of climate, and geographical conditions. Kutiel and Furman’s study (2003) indicates that the highest frequency of dust storm in the Middle East belongs to Iran, Sudan, Iraq and Saudi Arabia. Therefore, the aim of studying the dust events influencing the capital of Iran is to identify the areas prone to particle emission by using statistical-synoptic approaches and dust source tracing (HYSPLIT) model. 2. Study area This study was conducted performed through statistical analysis on observational data from Tehran synoptical stations including: Karaj, Abali, Firoozkooh, North Tehran and Chytgr. 3. Material and Methods In this study, three different phases with statistical, synoptic and modeling approaches were adopted, respectively. The first phase began by analyzing the observational data collected from five synoptical stations, including Tehran, Firozkhoh, Chitgar, Karaj and Abali during the period of 1981-2005. The daily data of dust, the visibility of less than 2 km from Tehran meteorological organization, monthly, seasonal and annual frequency distribution of dust were calculated. The vegetation coverage and the topology of Tehran were studied in the second phase by using GLCC land use data and GTOPO elevation data adopted from the Abdus Salam international centre for theoretical physics (ICTP). Wind speed and atmospheric pressure systems play a major role in causing dust storms (Alijani, 1997); therefore, atmospheric condition at pressure levels of 500, 700 and 850 hPa was analyzed using atmospheric parameters, including zonal wind, meridional wind and geopotential height with a grid size of 2.5×2.5 degree at three pressure levels provided the national nenter for atmospheric research (NCAR/NCEP). In third phase, In order to identify the dust sources of the mentioned stations, backward movement of dust particles was traced by applying HYSPLIT software within 48 hours before entering the region during May 1-5, 2000 at pressure level with 100, 500, 1000 meters in elevation. 4. Results and Discussion The results indicate that within the period of 1981-2005, the highest frequency of dust occurred in May, 2000. In spring, the highest frequency of dust was observed in all stations under study. The 24 year old dust frequency of five synoptic stations shows that Abali station had the highest number of events (109 days) and Chitgar station had the lowest number of events (12 day). Since in transition period of spring pressure systems of north latitudes are still active over Iran, the present study, conducted in May, was affected by these systems. The observations at the level of 700 hPa indicate that one of the pressure systems causing dust and determining its direction is the pressure system which influences Saudi Arabia with had little displacement during all dusty days, and it was thus considered as a permanent system in the region. Other effective pressure system in relation to Tehran’s dust storms was the low-pressure system in the north of Iran. This system affected the region along with high-pressure system over Saudi Arabia on May 1st and 2nd. However, on May 4th and 5th, due to the movement of the mentioned system toward the north, cut-off low-pressure system was formed over the north of Mediterranean, which partly affects pressure lines, speed and direction of flows. 5. Conclusion Studying particles transition directions of HYSPLIT model outputs indicates that the main dust sources of tehran are generally located at 25N-37N latitudes within range of Iraq, Saudi Arabia and Syria. Surveying the elevation data of the emission particles shows that dust particles in higher layers flowed toward Iran and reached Tehran at lower levels. In exploring the pressure systems, firstly, it seems that dust particles were transmitted to the higher levels by low-pressure system, drawn to the high-pressure over Saudi Arabia and then were descended to the ground level. Overall, anti-cyclonic flows dominate this area.