the impact of urban air pollutants on atmospheric visibility in tehran, 2008

atmospheric visibility has been defined as the greatest distance at which an observer can see a black object viewed against the horizon sky, which in quantitative terminology is known as visual range. visibility, in the absence of special meteorological events (e.g. rain and fog), is an excellent indicator of air quality. visibility impairment results from light scattering and absorption by atmospheric particles and gases that can originate from natural or anthropogenic sources. it is an important factor in everyday life mainly in aviation industry and surface traffic. air pollution in big cities, which is a serious environmental problem, especially in developing countries, may cause remarkable visibility reduction. much of emphasis in the recent atmospheric visibility studies has been to establish the factors contributing to visibility reduction. since the factors used to determine visibility impairment, including absorption and scattering of incoming light, depend on time and location, then it should be studied on local scale.  in this paper, the effect of different air pollutants on horizontal visibility is presented in the south-west of tehran for 2008. tehran is a highly industrializes and densely populated city in our region that is well-known for its air pollution problem. the data used in this study are based on midday measurements of meteorological quantities such as horizontal visibility distance, relative humidity, wind speed, present weather code, dew point and wet bulb temperature performed at mehrabad synoptic station. moreover, intensive measurements of particulate matter (pm10) and gaseous materials (e.g., co, no2, so2, and o3), carried out in tehran-azadi-square station, were used for further analysis. the monthly and annual changes in atmospheric visibility and air pollutant concentrations including so2, no2, and pm10, as well as their relationships with each other are studied. in order to focus mainly on the changes in visual air quality, the cases of visibility impairment that were concurrent with reports of fog, mist, precipitation or relative humidity of %90 or above were filtered from the visibility data using the present weather code that is a part of wmo synoptic coding system. then both the yearly and monthly correlations between visibility and air pollutant concentrations were examined. the results from regular measurements of air pollutant concentrations indicate that air pollution in tehran is severe in comparison with other cities around the world. the results of regression analysis also show that the most correlated pollutants with visibility are co and no2 followed by pm10. the fairly significant correlation between reduced visibility and no2 concentration implies that the impact of primary emissions of no2 and enhanced secondary pollutants, formed via photochemical processes in the atmosphere, that could not be ignored. the monthly analysis of visibility shows that the cold season is the most affected one by air pollutants and the significant anti-correlation is found between visibility and so2 in this season. more detailed analysis presents the significant role of relative humidity on the correlation of visibility and pollutants, especially on so2.