Using Satellite Observations and Ground-based Data to Assess Spatial-Temporal Changes of Aerosols over Iran

Understanding the nature, temporal and spatial dynamics of dust aerosols is important in terms of their impact on the climate system (such as droughts, precipitation patterns and fluctuations, and climate variations) and several respective maladies.

Iran has been subjected to high frequent and heavy dust storms especially during recent years. This major climate incident originates in the Middle East and western neighbors of Iran including Iraq, Syria, and Saudi Arabia. Dust activity begins in March-April and reaches a peak in August. The strong winds blow from west to north of Iran and bring immense desert dust and it causes respiratory problems and reduce visibility to less than 1 kilometer and in some cases it lasts for near one week over large areas of western Iran.

This study incorporates Aerosol Optical Depth (AOD) retrieved by both the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Multi-angle Imaging SpectroRadiometer (MISR) along with the Aerosol Robotic Network (AERONET) to evaluate aerosols AOD changes over the Iran during the 2001-2011.

MODIS Daily Level 2 aerosol data (MOD04_L2, containing data collected from the Terra platform; and MYD04_L2, containing data collected from the Aqua platform) were downloaded from the Earth Observing System Data Gateway at the Goddard Space Flight Center. We extracted AOT at 550 nm (MODISparameter name: Optical_Depth_Land_And_Ocean). Overland, the MODIS AOD fraction of fine mode particles (MODIS parameter name: Optical_Depth_Ratio_Small_Land) is a rough estimate based on the ratio of MODIS path radiances at 660 nm and 470 nm. We calculated the MODIS partial AOD by including only the AOD fraction due to fine mode particles. MISR AOD and other aerosol information are calculated based on a rich set of aerosol mixture models and reported at 17.6 km resolution. MISR Level_2 aerosol data (version 15) were downloaded from Data Center at NASA GIOVANNI (Goddard Earth Sciences Data and Information Services Center Interactive Online Visualization And aNalysis Infrastructure). We extracted AOD at 555 nm (MISR parameter name: MIL3DAE.004). Since the accuracy of the AERONET aerosol optical depth measurements is ~0.01 for the wavelength≥0.44 μm and the uncertainty in measured sky radiances due to calibration error is ~5%, AERONET data have been widely used as a standard for validating satellite aerosol retrievals and here we benefited from this point. AERONET Level_2 data of AERONET Zanjan site (since December 2009) were downloaded from AERONET data archive. These daily mean AODs (500 nm) from AERONET data were interpolated to wavelength of 550 nm using Angstrom exponent (α440-870 nm) provided by AERONET in order to compare with MODIS and MISR AODs (AOD_550nm/AOD_500nm =(550/500)^(-∝)).

The preliminary results of MODIS and MISR monthly AODs were compared for 2009 over the Zanjan sky. There is a high correlation between them and the correlation coefficient is 0.8. It confirms the results of other similar studies in which the MODIS/MISR AODs correlation factor is more than 0.96.

Although there is a high correlation factor between MODIS and MISR AODs, but the correlation factor between MODIS and AERONET AODs (0.64) is less than the correlation factor between MISR and AERONET AODs (0.79). Another factor which can show better the difference of MISR and MODIS AODs results is the Root Mean Square Error (RMSE). Indeed based on the results, MODIS data shows a greater amount of RMSE (RMSE=0.6) compared with MISR data (RMSE=0.032). Therefore present analysis indicate that the accuracy of MISR data is more appropriate than the MODIS one and the MISR data was used to evaluate the temporal changes of Iran's AOD during the last decade.

Preliminary result of this study also indicates that AOD has significantly increased in most of the regions across country. Western provinces have participated with a larger magnitude in this change due to increase in number and intensity of dust storms originating from Iraq and Syria while northern provinces do not show a significant change in AOD value over the study period. In total, the average AOD over the country is 0.25 and monthly aerosol pattern indicate that AOD value increases at the beginning of spring with increase in temperature and it reaches its maximum in summer months and minimum in autumn. Dust outbreaks caused by strong trade winds can occur throughout the year in the Middle East region. However, summer dust outbreaks have a greater capacity to lift sand and dust due to convective instability induced by high surface temperatures. During next 6 months, further research will be performed to confirm these preliminary results.

By presenting the new picture of spatial temporal distribution of aerosols over the country, we try to help environmental managers and decision makers in taking right measures to reduce or control the amount of entering dust aerosols from neighboring countries in a regional collaboration, installing dust storm pre-warning systems in vulnerable areas, human health situation and its relationship with dust storms, and assessing environmental impacts corresponding to the new aerosol pattern.