Long-term variations of aerosol optical depth and aerosol radiative forcing over Iran based on satellite and AERONET data F. Arkian & S. E. Nicholson Received: 9 May 2017 /Accepted: 26 October 2017 /Published online: 1 December 2017 # Springer International Publishing AG, part of Springer Nature 2017 Abstract In this study, three different sensors of satel- lites including the Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR), and Total Ozone Mapping Spectrometer (TOMS) were used to study spatial and temporal variations of aerosols over ten populated cities in Iran. Also, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used for analyzing the origins of air masses and their trajectory in the area. An increasing trend in aerosol concentration was observed in the most studied cities in Iran during 1979–2016. The cities in the western part of Iran had the highest annual mean of aerosol concentration. The highest aerosol optical depth (AOD) value (0.76 ± 0.51) was recorded in May 2012 over Ahvaz, and the lowest value (0.035 ± 0.27) was recorded in December 2013 over Tabriz. After Ahvaz, the highest AOD value was found over Tehran (annual mean 0.11 ± 0.20). The results show that AOD increases with increasing industrial activities, but the increased fre- quency of aerosols due to land degradation and desert- ification is more powerful in Iran. The trajectory analy- sis by the HYSPLIT model showed that the air masses come from Egypt, Syria, and Lebanon and passed over the Iraq and then reached to Iran during summer. Aero- sol radiative forcing (ARF) has been analyzed for Zan- jan (Aerosol Robotic Network site) during 2010–2013. The ARF at surface and top of the atmosphere was found to be ranging from - 79 to - 10W m -2 (average - 33.45 W m -2 ) and from - 25 to 6 W m -2 (average - 12.80 W m -2 ), respectively. Keywords Aerosol optical depth . MODIS . MISR . TOMS . AERONET . Iran Abbreviations AOD Aerosol optical depth ARF Aerosol radiative forcing HYSPLIT Hybrid Single-Particle Lagrangian Inte- grated Trajectory MISR Multi-angle Imaging SpectroRadiometer MODIS Moderate Resolution Imaging Spectroradiometer TOMS Total Ozone Mapping Spectrometer Introduction Aerosol optical depth (AOD) is an integral measure of the total amount of aerosols contained within a vertical column of air. Atmospheric aerosols are a complex mixture of particulate matter (PM) and liquid/solid par- ticles suspended in the atmosphere. These particles in- clude a combination of organic compounds, inorganic Environ Monit Assess (2018) 190: 1 https://doi.org/10.1007/s10661-017-6336-1 F. Arkian (*) Department of Meteorology, Faculty of Marine Science and Technology, Tehran North Branch, Islamic Azad University, Tehran, Iran e-mail: f.arkian@gmail.com S. E. Nicholson Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, FL 32306, USA