Investigation and comparison of aerosol optical indices over two urban areas in Iran

Atmospheric aerosols, including solid and liquid particles suspended in the atmosphere, are a mixture of particles in the air, of different sizes, shapes, compositions, and chemical, physical, and thermodynamic properties. They affect the earth’s radiative budget and climate directly by absorbing and scattering the radiation, and indirectly by acting as cloud condensation nuclei. Aerosols have both direct and indirect effects on the climate by scattering and absorbing solar and terrestrial radiation as well as modifying the distribution of clouds and their radiative properties. They have been concerned in health effects and visibility reduction mostly in urban and regional areas. Aerosol types which contribute to the scattering include organic particles, water-soluble inorganic species and dust. In urban areas, the principle particle species that absorbs radiation is black carbon, that is produced from incomplete combustion processes mainly from diesel engines. Natural aerosols are generally larger in size than the secondary aerosols produced from gaseous precursors and combustion, and their chemical composition depends on their sources. However, aerosols produced from natural and antropogenic sources are mixed together and thereby each aerosol particle is a composite of different chemical constituents.
Atmospheric aerosol optical and physical properties are two of the major uncertainties in global climate change which are also responsible for many impressive atmospheric effects. Therefore, retrieval of the aerosol optical parameters is an important issue for the atmospheric research communities. Investigations of aerosol characteristics and their optical properties will lead to a better understanding of both the regional and local behavior of aerosols over a region.
Aerosol optical indices such as aerosols optical depth, Angstrom exponent, single scaterring albedo, asymmetry parameter are the most important characteristics of aerosols that are influenced by the physical properties and concentration of particles. These properties also play an important role in the Earth’s climate and radiation budget. Aerosols optical depth is a key factor to measure the degree of atmospheric pollution and to study the climate response to aerosol radiative forcing. Its value shows the aerosol density, while Angstrom exponent is an intensive parameter that depends on the aerosol size distribution and increases with decreasing particle size. In other words, Angstrom exponent is the slope of the logarithm of aerosol optical depth versus the logarithm of wavelength. It is commonly used to characterize the wavelength dependence of aerosols optical depth and provides some nformation on the aerosols size distribution. When scattering is dominated by fine particles, Angstrom exponent has large values(i.e., around 2); it approaches to 0 when scattering is dominated by coarse particles.
Remote sensing of aerosols from satellite-based sensors turn into an important instrument to monitor and quantify the aerosol optical properties over the globe. Study of aerosol optical properties provides a detailed knowledge of both the regional and local behavior of aerosols as well as their influence on the Earth’s climate, radiative forcing, visibility and photochemistry. Although considerable development has been taken in understanding aerosol properties, they are poorly quantified because of the lack of adequate information on temporal and spatial variability of aerosols. In this paper, using the satellite data from the Ozone Monitoring Instrument (OMI) aerosols optical depth and Angstrom exponent are investigated over two megacities in Iran, Tehran and Mashhad, during the period from January 2010 to December 2013. OMI was launched in July 2004 on NASA’s EOS-Aura satellite, also part of the A-train constellation. The reasons of choosing these urban areas are mainly the existence of a large number of populations and substantial sources of emissions from natural and anthropogenic emissions. Previous studies show that the increasing emissions of aerosols during the past decades in these two area have affected their local climate. Here daily, monthly and seasonal variations of aerosol properties in terms of optical depth and Angstrom exponent are analyzed to provide a detailed insight into the variation of aerosols loading and their possible causes.
Results concerning the seasonal frequency distribution of aerosols optical depth (AOD) at 500 nm indicate that values of this index in Tehran are higher than Mashhad in all seasons. It shows the existence of higher aerosol density causing the higher atmospheric turbidity over Tehran than Mashhad. During the study period, the daily amount of AOD over Tehran is ranged from 0.2 to 1.6, while over Mashhad the daily AOD is ranged from 0.1 to 0.9. High values of aerosol optical depth are obtained during the spring and summer seasons, respectively, and low values are seen during the winter in the both cities. There are also significant variations of Angstrom exponent over the two cities. Based on the results, the dominant mode of aerosols over Tehran is a mixture of fine and coarse particles, but fine particles are dominant over Mashhad. Therefore, it can be deduced that turbidity in Tehran is subject to a complex mixture of aerosol types, including anthropogenic aerosols and dust, while anthropogenic aerosols are dominant over Mashhad. To further understand the seasonal variations of aerosols, AOD was studied at different wavelengths. Results show the seasonal dependency of AOD values that are mainly related to various emission sources.
In order to investigate the origins of aerosols and transports of the air masses toward the understudy regions, back trajectory analyses based on the NOAA HYSPLIT (National Oceanic and Atmospheric Administration Hybrid Single Particle Langrangian Integrated Trajectory) model, was performed. For six days, as the representatives of polluted and clean days, air mass back trajectories were computed using HYSPLIT model. Results indicate the existence of different patterns of particles transport over the two cities. It is seen that the sources of aerosols over Tehran are both from local emissions and from the long range dust transport, while aerosols over Mashhad are more likely from local sources.