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Remote sensing of aerosol optical depth (AOD) could be used to assess particulate matter levels at the ground; however, such measurements often need further validation. AOD is a measure of aerosol loading in the atmosphere, which means a higher AOD shows a higher aerosol loading and thus lower visibility. Aerosol concentration, determined by AOD, affects air quality in urban and semi-urban environments.
Visibility is defined as the distance at which a large dark object can just be discerned against a light sky. Visibility degradation is the most obvious manifestations of pollution, which is mainly due to absorption and scattering effects of aerosols in the atmosphere. Thus, visibility can also reflect the local air quality. In this paper an overall study for assessing aerosol optical thickness over Zanjan and Tehran area, representatives of semi urban and urban areas in Iran, are presented.
In this study, aerosol optical depth retrieved by; 1) measurements of the MODerate resolution Imaging Spectro-radiometer (MODIS) on board NASA’s Terra and Aqua satellites, 2) sun-photometer  measurements form a Cimel sun-photometer located in Zanjan area and 3) visibility measurements taken from Geophysics Institute of Tehran University and Zanjan synoptic stations. The data were collected from Decembere 2009 to September 2010 for both stations.
Our objectives in this study are, to validate the MODIS AOD measurements, to find the possibilities of establishing near-real time AOD retrievals based on visibility data and to determine the atmospheric pollution of the study area, on a local scale.
The sun-photometer was used for the validation of both MODIS and visibility data at Zanjan area. Validation of MODIS AOD retrievals is presented for the first time in Iran. The results obtained by the direct comparison between MODIS and sun-photometer AOD data exhibited a significant correlation (r= 0.87). We have also compared the AOD retrieved by sun-photometer to visibility data in Zanjan area to verify the formula which is used to estimate AOD from visibility data. A significant correlation (r=0.62) between sun-photometer AOD and that estimated from visibility measurements was found.
A relationship between MODIS AOD and AOD estimated from visibility values was also examined for both locations. Correlation coefficient was found 0.66 for Zanjan and 0.23 for Tehtan area. It is concluded that MODIS AOD data provide more accurate information on the aerosol content in Zanjan than for Tehran area.
The major finding derived from this study suggests that MODIS AOD data provide relatively accurate information of the aerosol loading, in terms of aerosol optical depth. While in the absence of such data, visibility measurements could be used as a secondary source of aerosol load information in such a semi urban area. The advantage of using visibility data is that such data are available every hour on each day from meteorological stations and provide useful information on a near-real time basis, whenever data are available.