عنوان مقاله [English]
Precipitation is one of the most efficient mechanisms for washing the atmospheric pollutants, specifically particulate ones out. The process has two very important consequences (one good and one bad). The good one is that it clears atmosphere from many substances that have adverse impacts on many aspects of life whereas the bad one is that their reaction with rain water leads to the formation of some dangerous and destructive substances that have great impacts on both, the natural environment as well as manmade constructions. Precipitation chemistry plays an important role in understanding the air quality in a study area, because the concentrations and distribution of chemical components in rain depend on a variety of emission sources. Realizing that, a lot of effort has been made in order to identify the nature and type of those particulates, especially in urban environment. Study upon the atmospheric pollutants and their nature in Tehran atmosphere goes back to 1991, when Ismaeili Sari, initially attempted to identify the nature of atmospheric pollutants over Tehran by sampling rain waters from synoptic and climatology stations (also, he did the same research for one station in Tehran. The results for two researches have shown and also compared with present study in the section of results and discussion. Tehran city located in central part of the Alborz Mountain Range and characterized by its complex topography and diverse climate in an area of about 700 Km2. In this article, two Mehrabad and Aghdasiyeh synoptic stations were selected for rain water sampling. Aghdasieh synoptic station started to work in 1988 and is located in the northeast of Tehran, the area with highest annual average of precipitation among other areas of the city. According to Iranian Meteorological Organization records, the annual average of precipitation in this area is 420mm. Mehrabad synoptic station is the oldest meteorological station of the city that started to work in 1951 and is located in Mehrabad airport at the west of Tehran. Recent years observations indicate that some unprecedented behavior in the city’s precipitation regime has occurred. The city fast growth of population, area, industrial activities and the number of motor vehicles alongside with substantial changes in the constructions from horizontal to vertical, has Considered responsible to that observed changes. Accordingly, to do that 30 rainwater samples from two Mehrabad and Aghdasieh synoptic stations (15 samples from each station) collected during the autumn and winter of 2012 and spring and summer of 2013 and 2015. During the examination of the chemical properties of the collected samples rain water total hardness, total dissolved solids (TDS), PH, total alkalinity and electrical conductivity (EC). Measured PH showed that autumn and winter precipitation were acidic while the springs' were mostly alkaline. Dust highest concentration in spring and summer may be the main reason for rain water alkalinity and higher TDS, EC, and hardness relative to the winter samples. In order to analyze the causes of acid and alkaline precipitation were measured Concentrations of gases and aerosols (SO2,NOx, PM10 and PM2.5) Air Quality Control Company of Tehran and the friction velocity values obtained from the output of the numerical model WRF also taken. The results show that rainfall in autumn and winter seasons were acidic because of the increased concentration of gases in the atmosphere (SO2 and NOx) and inversion. In late winter and early spring due to increasing alkaline dust in the atmosphere precipitation were alkaline. Acidity of rain waters was higher in Mehrabad samples (with minimum amount of 4.6).The pH ranged from 4.6 to 8.1 with a mean of 6.3. Results show that in Aghdasieh the lowest measured pH was 4.72 and the highest was 7.95. The highest calculated Alkalinity was 33.58ppm. The same studies for Mehrabad indicate that the lowest and highest measured pH were 4.61 and 8.12, respectively. Again, the highest calculated Alkalinity was 40ppm. EC, TDS and Total Hardness were slightly higher at Mehrabad than Aghdasieh. They were 135μs/cm, 110ppm, and 57.95ppm in Aghdasieh and 195μs/cm, 130ppm, and 76.01ppm in Mehrabad, respectively. The results have shown the friction velocity, TDS, EC, and alkalinity of the soil erosion occurring before the spring rains. System type, the large-scale circulation, mixed layer depth, local pollution sources, the inversion temperature, wind speed and direction and the geographical features of the region, the important factors affecting the pH precipitation in Tehran considered.