Document Type : Research Paper
Authors
1 Associate Professor, University of Tabriz
2 PhD student Department of Climatology, University of Tabriz
Abstract
Introduction
In recent years because of decrease of precipitation, use of water for agriculture, construction of hydraulic structures and etc, Urmia Lake surface area has been decreased. Considering the salinity of Urmia Lake and direction of wind, the costal and even further area of Urmia Lake is seriously in danger of salt intrusion. Then knowledge of the spatial-temporal distribution of aerosol characteristics is critical for quantification of salt intrusion impacts. Aerosol optical depth (AOD) is a column-integrated measure of extinction coefficient, representing the attenuation of solar radiation by aerosol scattering and absorption. Satellite images of AOD are useful for studying dust storms owing to the large spatial nature of such plumes. Lack of an AERONET station makes studying dust storms difficult in this area. The present study was conducted to understand spatial AOD patterns and the variability and intensity of inter- and intra-annual MODIS AOD for the longest possible period of 14 years (2000–2015).
Methodology
In this study, monthly AODs from average MOD08 are used to investigate the spatial and temporal distribution of dust storms over Urmia lake for the period between 2000 and 2015. Monthly average MOD08 product files are available at spatial resolution of 1 degree by 1 degree (http://ladsweb.nascom.nasa.gov/data). This study focuses on AOD at 550 nm over land, as this is close to the peak of the solar spectrum and is, therefore, associated with major radiative effects (Papadimas et al. 2009). MODIS data are compared to AERONET data at the nearest station (Kuwait University) for the period between 2005 and 2014 (http://giovanni.gsfc.nasa.gov/aerostat/). The AERONET site shows better AOD correspondence with MODIS Terra (RMS = 0.028, R = 0.916) than with MODIS Aqua (RMS = 0.166, R = 0.646); therefore, hereafter AOD data are discussed from Terra. In this study, monthly mean aerosol optical depths (AODs) from MODIS are used to investigate the spatio-temporal distribution of aerosol in these affected areas (2000-2015). The monthly and annual mean AOD trends has been extracted. With the aim of displaying and analyzing the spatial distribution of particulate matter concentrations, the mean change map was extracted and each map was classified according to the standard deviation method. Using the standard deviation method, the amount of change in each of the pixels can be determined from the mean of the region.
Results and discussion
The changes in dust concentrations for shows that in June, July and April, there is the most similarity is between the trend of change in order in West Azerbaijan and East Azerbaijan. There are two provinces under study, and in February, November and December there is the most differences between the two provinces, which has declined sharply since 2009. Also, the trend of changes in all months shows that the slope of AOD changes has been increasing during the study period. Most monthly AOD fluctuations are seen in January, February and December during different years; It is worth noting that in these months, in terms of dust concentration, AOD also shows low values. The increasing trend of fine dust is much more pronounced at the end of the warm season and the beginning of the cold season (August, September, October and November). Most AOD values are observed in spring and early summer, ie in March, April, May, June and July. Until 2008, the amount of AOD in the southwestern part of the study area was high, indicating that fine dust observed in the southwestern part of the region could be carried by westerly winds from the deserts of neighboring countries during these years. From 2009 to 2014, the average amount of fine dust in Pixel of including Lake Urmieh, increased sharply over the entire region, which cannot be attributed to dust carried by western winds due to the AOD status in the west and southwest of the lake.
Conclution
In this study, annual and monthly averages were used to examine how dust changes in the last 16 years in the provinces of East Azerbaijan and West Azerbaijan, which are adjacent to Lake Urmia. One of the main objectives of this study was to monitor the oscillations of fine dust in the area of Lake Urmia and its adjacent areas to show the presence of salt dust in Lake Urmia, which has been the result of the drying up of large parts of the lake in recent years. The monthly and annual mean AOD trends show the increasing trend in AOD values. Then to show the spatial distribution during the period of study, mean annual maps for each year was extracted. Results show there is two seperated period in area of study for AOD spatial pattern. First during 2000 to 2009 there is higher AOD in south-western part of area and the existence of Urmia lake had caused reduction in AOD in western part of lake. Second period started from 2010 there is significant high AOD above Urmia lake. This fact shows the lake as a source of aerosols. In next step to show the spatial distribution of AOD changes during time, based on AOD value two years with high (2014) and low (2004) AOD was selected. The difference between these two years shows the most changes in area of study has occurred over Urmia lake and also around the lake. Based on the result of this study the increase of salty aerosols that originated from Urmai lake is one of major aspect of drought of the parts of lake and must be considered.
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