the study of desert dust in mashhad metropolis using satellite images and synoptic datasets (2009 - 2013)

1. introduction in recent years, desert dust frequency has increased in the global system. due to lack of vegetation in the regions prone to dust, the earth’s surface air becomes warm enough in these regions, moves upward and causes a cold downward circular air flow when it collides with upper tropospheric winds. when these winds strongly hit the earth’s surface, they create dust storms (xuan et al, 2004). many studies from all around the world have been conducted on the source regions of this environmental phenomenon (kai & gao, 2007). the studies on asian dust storms with sea sources during 2000-2002 showed 64% of storms originate from china seas and become more severe from west to east and carry dust on the land. wang, stein, draxler, rosa & zhang (2011) studied sand and dust storms in 2008 and defined north africa, middle east, mongolia, and north-west china with high frequency of dust phenomenon, using hysplit model. shamsipour and safar-rad (2012), analyzed the july 2009 dust in the west of iran. in the chosen sample, the zagros hillsides contain the highest amount of pollutants. also, the results indicate that the location of naveh, upper-level divergence of 500 and forming a thermal low-pressure system on earth’s surface play the main role in flowing the dust toward iran. 2. study area study area includes the mashhad metropolis which is located in the northeastern part of iran. as the second most populated metropolis in iran and due to its vicinity to northeast deserts and strong winds of summer, mashhad is threatened by this environmental hazard. therefore, to be aware of this disaster, knowing about its trend and source locations are essential for the study. the studies on particulates and dust in mashhad are based on analysis of synoptic or atmospheric circulation patterns or statistical methods using air quality monitoring equipment or weather stations. in addition, no study has clearly demonstrated the source location of dust in mashhad. because of the necessity of air pollutant observation and control in metropolises and dissipation or lack of weather stations, it is necessary to use other data sources to check the air quality such as remote sensing data and satellite imagery. therefore, the aim of this study is to use hysplit model outputs by analyzing modis satellite images in combination with synoptic methods and tracing the dust with retrograde method. 3. material and methods 3-1. data the study materials include meteorological records relating to 06 dust code, synoptic maps and modis satellite images in dusty days. the data of the first group was obtained from khorasan razavi meteorological organization for the period of 2009-2013 and mashhad synoptic station at intervals of 8 hours. the second group includes gridded climate data with 2.5×2.5° longitude and latitude taken from ncep/ncar website. the synoptic maps of dusty days in gradis setting in 25-55° north latitude and 35-85° east longitude were drawn and analyzed. the third group includes calibrated level-1 modis atmospheric product with 1 km resolution. totally, 16 concurrent images of dusty days were obtained between march 3, 2009 and october 30, 2013 from goddard nasa website. the fourth group includes the average daily data of pm10 from air quality monitoring stations in 2011which were used in the same year in order to assess the validity of aod daily data of modis. the fifth group is the hysplit lagrangian output model with retrograde method for tracing the dust source of mashhad. in this research, we used hysplit model to locate the path and source of dust that reach mashhad outside the station. the model input data were extracted from ncep/reanalysis data. also, the flow measurement method was isobaric lines with a 6-hour time step and altitudes of 50 and 1000m above sea level. 3-2. methods the present research is an analytic-descriptive case study with a quantitative method for data analysis and uses descriptive statistics and spatial analysis. based on meteorological records, we have first analyzed temporal trend of dust phenomenon in hourly, monthly, seasonally and annually durations. then we documented spatial distribution of dust on urban areas by modis images which is obtained from nasa website. these images were processed by two indicators: 1- brightness temperature index (btd31,32) which is suggested by ackerman (1997) and, 2- normalized difference dust index (nddi). btd measures between 11-12 micrometer and mndvi parameter were used to separate the desert arid regions and threshold temperature of 290° kelvin in the band 32 for separating cloudy from dusty regions. 4. results and discussion 4-1. frequency of dusty days the observation hours on july 3rd, 6th, 9th, june 12th, march 12th and 15th had the highest average dust. in contrast, january, february and december had the lowest dusty hours. july, june, march and august had the highest and january, february and december had the lowest monthly dusty days during the above-mentioned period. also, the highest number of dusts in this period occurred in 2009. 4-2. satellite image analysis 4-2-1. validation of aod modis images and pm10 georeferenced data in this analysis, about 69% of pm10 dependent variable changes are defined by the modis aod 550nm independent variable which shows a fine correlation between the two validated parameters. 4-2-2. dust identification index with btd31, 32 the implementation of the index algorithm led to clarification of dust on the image bands in four calculation steps. according to the models mentioned here, the dust local algorithm in mashhad shows that the northwest, west and south parts of this city were covered with dust. therefore, it seems the districts 1, 8, 9, 10, 11, 12 located in these regions experience more dust compared with other districts. also, the analyses show that compared with city center, outskirts of mashhad are subject to more dust pollution. 4-3. synoptic analysis of dust in order to study the synoptic conditions in dusty days, the synoptic maps concurrent with images at 12 pm local time were analyzed. in dusty days, the middle level of high-pressure atmosphere with negative omega dominates iran which leads to dust due to lack of humidity. considering the path and speed of the wind at the earth’s surface and domination of flows in a north-south pattern, it seems the dust moves from its focal points towards north-east of iran and mashhad. generally, according to the dusty days map we can say that the main and closest source of mashhad dusts lies to the east of the region and afghanistan and east of turkmenistan. 4-4. tracing the path of dust as soon as the firs dust was reported, the path of particles was examined since then back to 24 hours. the model’s output maps show 3 main paths which transfer the dust to mashhad. in most cases with turkmenistan and central asia as the source, the main path of dust was from north-east to south-west mostly during the warm seasons of the year. the second path is from north-west to south-east in cold seasons. the third path is from south-west to north-east which has a low frequency with the cold seasons domination. 5. conclusion according to meteorological dust records of mashhad synoptic station, the maximum and minimum frequencies of daily dusts occur at morning and midnight hours respectively in local time in most seasons during the statistical period of the study. in monthly view, most of mashhad dusts occur in july, june, august, and march, respectively. the minimum number of monthly records without dust in december occur during the cold period of the year. the btd index properties was able to clarify the regions covered with dust on satellite images in mashhad. it also provided a synoptic analysis of dusty days to identify the dust and find the way by which it is transferred. it was specified from a dusty days sample with a synoptic approach that afghanistan and part of turkmenistan located in the north-east of mashhad are the main focal points of dust that reach mashhad. the hysplit model maps output overlap synoptic maps of dusty days. generally, during the period of study, north-west, west and south of mashhad were covered with dust. the districts in these regions experience more dust conditions compared with other mashhad districts.