نرخ فرونشست گرد و غبار اتمسفري و ارتباط آن با برخي از پارامترهاي اقليمي در استان خراسان رضوي

Dust Deposition Rate and its Relationship to Some Climatic Parameters in Khorasan Razavi Province

ذرات معلق اتمسفري به عنوان نتيجه اصلي فرسايش بادي، اثرات بسيار مهمي بر محيط پيرامون خود دارند. هدف از اين پژوهش، تعيين نرخ فرونشست گرد و غبار در استان خراسان رضوي و ارتباط آن با پارامترهاي اقليمي است. به اين منظور تعداد 50 ايستگاه در سطح استان انتخاب و طي 12 ماه (مجموعا 600 نمونه) ، توسط تله هاي شيشه اي نمونه هاي گرد و غبار به صورت ماهيانه جمع آوري شد. حداقل مقدار ميانگين نرخ فرونشست مربوط به ماه آذر با ميزان 97/9 گرم بر متر مربع و حداكثر آن در ماه خرداد با ميانگين 96/20 مشاهده شد. دامنه مقدار فرونشست گرد و غبار در طول مدت نمونه برداري 3/0 تا 42/150 گرم بر متر مربع در ماه بود. شهرستان هاي گناباد و قوچان، به ترتيب، با متوسط 14/313 و 62/74 متر مربع بر گرم در سال بيشترين و كمترين ميزان فرونشست گرد و غبار را در سال داشتند. بر اساس نقشه هاي پراكنش مكاني، بيشترين مقدار فرونشست در ماه هاي مربوط به فصول بهار و تابستان در غرب و جنوب با اقليم خشك و فرا خشك بود و در قسمت هاي شمالي استان به دليل رطوبت بيشتر و كوهستاني بودن كمتر بود. تحليل نقشه هاي سينوپتيك اقليمي طوفان، حاكي از شيب فشار حاصل از استقرار همزمان كم فشاري در مرز هاي جنوبي كشور و مركز پرفشار بر روي درياي خزر و تركمنستان بوده كه سبب وزش بادهاي شديد و ايجاد طوفان هاي گرد و غبار در فصل بهار شده است. با تغيير جهت باد غالب از شرق و شمال شرق در ماه هاي خشك سال، به غرب و جنوب غرب در ماه هاي با بارش بيشتر، نرخ فرونشست ذرات معلق در اكثر مناطق به جز مناطق شرقي استان كاهش يافت. همچنين، نتايج نشان داد كه بين متوسط فرونشست گرد و غبار در 12 دوره نمونه برداري رابطه مثبت و معني داري با حداقل و حداكثر دما، سرعت باد و رابطه منفي با رطوبت نسبي در سطح 5% وجود داشته است. بالا بودن سرعت باد و خشك تر بودن خاك و در نتيجه بالا بودن رخداد طوفان، در فصل بهار نسبت به ساير فصول باعث شده مقدار گرد و غبار بيشتري توسط باد و تا فواصل طولاني حمل شده باشد كه مي تواند به منابع محلي و برون مرزي مربوط باشد.

Introduction Airborne dust is an important environmental hazard in arid and semi-arid regions. Dust deposition rate is one of the most important characteristics of airborne dust which needs to be measured for risk assessment and source determination. During the past decade, by increasing the dust deposition in Iran, many investigations have been conducted to document the dust deposition characteristics and identify the regional atmospheric and land surface processes controlling dust transport. The objective of this study is to measure the dust deposition rate and investigate the effect of climatological factors on airborne dust in Khorasan Razavi Province located in the northeast of Iran. Materials and Methods Khorasan Razavi Province is located in the northeast of Iran. Elevations range from 235 m in Sarakhs in east to 3211 m at Binaloud Mountain in north of the area. Mean annual precipitation varies from 111.5 to 306.3 mm and the mean annual temperature is between 12.7°C to 18.9°C. The Kopeh Dagh and Binaloud Mountains are in the northern part of the area. There are several playas (including clay flats, salt crusts, and sand dunes) in the south and western of the Province. Airborne dust samples were collected monthly from May 2014 to April 2015. A dry flat collection tray with an area of 1 m2 was used to sample airborne dust. Fifty trays were placed on the roof of buildings ~3- 4 m above the ground level. Dust trapped on the trays were collected by a rubber spatula and then were weighted. Totally, 600 samples were collected from 50 sites in the Province. Interpolation in this study was carried out using surfer software 14. Furthermore, the Grads software was used to draw the average monthly wind speed, direction patterns, and synoptic pattern of dusty days. The Pearson coefficient was used to determine the correlation between the characteristics with normal distribution. Results and Discussion The measured dust deposition rate indicated the significant spatial and temporal variations during the studied period. The lowest and highest average monthly dust deposition was 9.97 and 20.96 g m-2 which occurred in December and June, respectively. The annual dust deposition rate showed high variations from 313.14 g m-2 y-1 in Gonabad with desert climate regime in southern to 74.62 g m-2 y-1 in the city of Quchan with temperate-mountainous climate in northern part of the area. In spring and summer, the dust deposition rate was the highest in the west and the south of the Province and also indicated the high range of variations. In autumn and winter, the range of dust deposition rate decreased and also the highest amount of deposition occurred in the east. The lowest amount of deposition was found in the mountainous northern part of the area. There was a significant positive correlation between the average dust fallout and the mean wind velocity and the maximum and minimum monthly air temperatures, while there is a significant negative correlation between this parameter and relative humidity. The spatial distribution analyses showed the highest amount of dust fallout in the southern and western parts of the Province, which includes the desert and sub-desert climate and also the most storm occurrences in spring. Also, in the months with higher rainfall, the amount of fallout rates decreased in most areas except the east of the Province. The high speed of wind, the dryness of the soil and the high occurrence of dust storm in June and July have caused the dust to be carried a long distance as compared with other months. Conclusion The results of this study indicated the hazard of dust deposition rate, especially in the south and west of the Khorasan Razavi Province in spring and summer. The dust deposition was the highest in the areas at margin of the desert with low relief, sparse vegetation and high wind velocity. Monthly variations in dust fallout rates were related to some climatological parameters. There was a significant positive correlation between the average atmospheric airborne dust fallout in 12 sampling intervals, the maximum and minimum temperatures, and wind speed and there was a significant negative correlation between this parameter and relative humidity at P-value<0.05. These findings are closer to the results of Noroozi and Khademi (2015) (in Isfahan), Al-Harbi (2015) (in Kuwait), and Kaskaoutis, et al. (2016) (in the southwest of Asia). Furthermore, through the change of the direction in prevailing winds from east and northeast in dry months to the west and northwest, the rate of airborne dust fallout declined in most regions in the months with more precipitation, but not in the east of the Province where the area was exposed to the highest wind speed in spring and summer. It is noted that the increase of temperature causes a decrease in soil moisture; this issue is, in turn, considerably important in decreasing the speed of the threshold, which consequently increases the wind force and soil movement (Rahimi, 2015). In addition, rainfall and temperature can indirectly play an effective role in amount of the fallout rate through affecting vegetation due to the effect of these two parameters on the plant coverage (Makhdoum, 2006). The results of the analysis of climatic synoptic maps related to stormy days in the sea level pressure map showed that the establishment of the cyclone center in Afghanistan and the south of Khorasan, and the anticyclone center on the Caspian Sea and Turkmenistan cause heavy winds and dust storms in the region. Considering the geographical location of the Province (sharing borders with both Turkmenistan and Afghanistan) and the wind direction, it could be suggested that the sources of these storms are found in the surrounding areas. However, the determination of the source of the atmospheric suspended particles in Khorasan Razavi Province requires further study and more comprehensive laboratory analyses.