Spatial and Temporal Variability of Aerosol Indices over East Khorasan, Iran based on Satellite Observation

1. Introduction
Satellite remote sensing provides an undoubtedly unique opportunity to extract spatial patterns of regional, international and seasonal aerosol properties. In order to understand the effects of air particles on climate and human health over the lands, it’s necessary to have spatial and temporal distribution of aerosol. Ground-based measurement data do not provide the spatial pattern of particles and therefore, satellite data are used. Aerosol indices such as Aerosol Index (AI) and Aerosol Optical Depth (AOD) are commonly used to identify the amount of atmospheric particles. East Khorasan is under the influence of dust emission since the strong Levar winds (120-day wind) blow over the region. The aim of this study was to investigate the temporal and spatial variability of aerosol in the East Khorasan, Iran based on satellite data.
2. Study Area
East Khorasan with an area of 100,161 km2 is limited from the north and east to Turkmenistan and Afghanistan and from south to Sistan and Baluchistan Province. There are 11 meteorological stations at this region including Sarakhs, Mashhad, Torbat-e-Heydarieh, Torbat-e-Jam, Taybad, Khaaf, Gonabad, Qaen, Birjand, Sarbishe and Nehbandan.
3. Material and Methods
The Aerosol indices were obtained from TOMS sensors with a spatial resolution (1^°×〖1.25〗^°) and OMI sensors with a spatial resolution (〖0.25〗^° 〖×0.25〗^°) from http://disc.sci.gsfc.nasa.gov/giovanni. The monthly, seasonal and yearly mean values of the data sets were calculated from all available data. To study atmospheric aerosols, UVAI, AOD and AAOD indices were used. For this purpose, TOMS sensor data from the satellite Nimbus 7 satellite in 1978 and 1993 and in 1996 and 2005 and the Earth probe OMI on Aura satellite were used from 2004 to 2014.
4. Results and Discussion
The results indicated that the amount of particulate matter obtained using UVAI/OMI is highest in all cities in June, July and April and lowest in December, November, and October. The AAOD related to each city was quite different from other cities and there was no specific month which showed the highest or the lowest concentration of absorbing particles; it seems that the UVAI index does not follow a specific pattern while the UV index has the highest amount in Mashhad. Therefore, UV seems to be associated with air pollution. AOD index also had the highest value in the city of Mashhad in May and the lowest value occurred in Tayabad and then in Khaaf in December. Maximum UVAI/TOMS index occurred in June. The OMI sensor has the same monthly pattern as the TOMS data. Tayabad had the highest aerosol (UVAI) and Mashhad had the lowest. From 2004 to 2014, the amount of aerosol increased in the region. The concept of AAOD is close to UV-absorbing aerosols such as smoke, dust and minerals in all cities increased in the period of 11 years.
5. Conclusion
According to the results, spatial and temporal variabilities of indices are more associated with climate processes and then topography. Low attitude areas have the highest UVAI aerosol value while mountainous areas have the lowest amount of UVAI index.