عنوان مقاله [English]
Emissions of road traffic status as one of the main sources of pollution in urban areas, especially in the main streets of downtown are known. Considering that most of the traffic in areas where the greatest human trafficking takes place, it is very sensitive and pollution issues will be critical. In small-scale studies of cities, buildings and their placement, height, gender and outer structures can be important in comfort climate debate, climate and pollution designs. In this context, a careful analysis of the physical and natural factors in mitigation or aggravating of pollution from small-scale numerical modeling based on a laboratory model of air flow (CFDs) namely Envi-met were used. The benefits of this method are being quantitative effect of physical and climatic urban factors in microscale temporal and spatial resolution. On the other hand the lack of a consistent and complete data of air pollution, especially in small scale cannot be done in a complete
and comprehensive review but this method to great extent resolves this problem. In the present study, pollution condition at the central part of Tehran between the west to the east along the Azadi and Tehran-pars street with a length of about 18/7 Km by numerical modeling, are discussed. At the beginning and end this region two-terminal (West and East) of the three major passenger terminals of Tehran are located. In addition, a line of Bus Rapid Transportation (BRT) in this path has caused daily movement more than 300 buses. Beginning of this path is at the main entrance of Tehran, so every day, thousands of vehicles are traveling within this area. The information used in this study includes atmospheric elements such as wind direction and speed, humidity, specific humidity and air temperature at two date on July 26, 2011 and November 30, 2010 obtained from Mehrabad synoptic stations. In addition, emissions data related to the pollutant gaseous monoxide (CO) in the same date were taken from statistical methods traffic crossings in sections and squares of path by taken films and statistical calculations. Another set of data are contains urban spatial data, surface covers of streets and square including the type and amount of vegetation, soil type, building height and density and unevenness parameters that was designed in Envi-met model. In Azadi square, the maximum concentration of pollution is around the third receptor. The lowest values is around the first and second receptors in the open spaces nearby vegetation and minor passages that wind flow was more freely between building blocks. Wind on its track passes the open spaces of Azadi
square and causes pollution dispersion while this results in its being held in dense areas of urban structures such as the east side of the field. In other section, Valiasr cross, compressed building shape, high volume of traffic, existence Daneshjo Park in the South-east, the intersection of two main streets (Enghelab and Valiasr) has been influential in changing the concentration monoxide pollutant. CO concentrations at Valiasr cross is more on the streets, especially in the central passageway and is focused nearby number two receptors. Because the north dominant wind in the morning and South flow during the afternoon, as well as the arrival of the longitudinal passages, the pollution is transported to passageway center. The healthiest parts of this site are Daneshjo park space and between buildings away from the center of the Cross. The highest amount of CO pollution has been shown in Tehran-pars, early in the morning. But in the evening due to the increase of temperature, Air stability will disappear, so the temperature gradient causes formation of stronger wind currents and reduces the concentration of pollution. Like two previous sites in Tehranpars, with the distance from the ground level concentration of CO emissions are reduced. Generally, concentrations of pollutants in total area are influenced by fluctuations in the atmospheric condition. In other words, because of amount of inputs, emission and physical condition in all seasons and all times of the day are not fixed, what makes the pollution concentration non-steady varied condition climate and microclimate. For example, in the early morning in autumn season with low values of the temperature, inversion and the stability condition are caused by lack of wind and air. Also with increasing altitude and distance from the center of the earth surface emissions and fewer side effects, wind flows are faster and more regular and reduce effects of pollution.