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عنوان مقاله [English]
Visibility is defined as the maximum distance at which the selected target can be recognized. Atmospheric pollutants (aerosols and gases) may cause a visibility reduction by absorbing or scattering of visible light. Both gases and particles scatter and absorb radiation and contribute to the light extinction coefficient of the atmosphere.Atmospheric extinction in the vicinity of airports is of importance to the aviation.Â Moreover, although the light extinction causes no adverse health effects, it usually indicates the presence of pollutants harmful to health. These pollutants are found to be associated with health problems such as increases in daily mortality, and asthma. Therefore, light extinction is not just a visibility problem but is also a visual indicator of adverse air quality.
Â Â Â Atmospheric extinction coefficient is directly related to the visual air quality and represents the optical characteristics of the pollutants along an optical path that contributes to visibility impairment. Thus, the extinction coefficient is an optical parameter related to air quality.
Â Â Since the variation in human-eye observation could be highly due to different personal characteristics, the quantitative measures of atmospheric visibility are increasingly being used as indirectestimates of the air pollution especially where direct measurementsare not available. Several instruments, such as telephotometers, nephelometers and transmisometers have been developed to monitor visibility. However, in recent years the digital techniques are rapidly applied in visibility monitoring. Digital cameras can be used to measure the distance of a target for computation of visibility, i.e. the digital image data can be characterized by illumination and reflectance components and translated to specific brightness values.
Â Â Â This study was based on a new and more accurate method for measuring the atmospheric extinction coefficient for the city of Tehran using a digital image processing technique. A series of images obtained by a digital camera were archived from May 2010 to August 2011 and some selective cases were studied in details. Moreover, the visual ranges given from the Institute of Geophysics synoptic station were also used for the same period. The meteorological visual range was the distance at which a person could discern a dark object against the horizon sky. The Koschrneider equation was applied to convert the visual range into the light extinction.Â First, the average image intensities of the selected images were calculated to obtain the inherent and apparent contrast ratios. Then, using these quantities, the atmospheric extinction coefficient was determined for each image.
Â Â Our results showed that there was a good agreement between the atmospheric extinction coefficients estimated from both methods, although the extinction coefficients estimated from the visual ranges were more than those of the digital image processing method. It is likely that the smaller values of the visual distances are due to the fact that they are reported by human observers at the synoptic stations. In addition, measurements of the air pollution concentration were used to investigate the influence of air pollutants on the extinction coefficients in the selected cases. Our results indicate that the light scattering by small particles formed from gaseous air pollutants, is a major contributor to light extinction rather than by large particles.