شرايط هواشناختي جو بالا و وضعيت حاد آلودگي هوا (مطالعه موردي : شهر تهران)

Upper Air Meteorological Conditions ofAcute Air Pollution Episodes (Case Study:Tehran)

In this paper meteorological conditions of some acute air pollution periods for the city of Tehran have been considered. The data used, were obtained from the Air Quality Control Company (AQCC) of Tehran Municipality and from synoptic station of the Institute of Geophysics, university of Tehran. Initially some statistical methods were used to find the episodes for acute air pollution episodes, and then their meteorological conditions were considered. One suitable method for determining acute air pollution episodes is the use of P parameter which is defined as the number of events in which the air pollution concentrations is 1.5 times (and more than) the averaged seasonal mean. A typical case for this study for P value is shown in figure( 1) in which a bubble structure between polar and subtropical jet streams over the area, (see below) has led to large values of showing an acute air pollution episode.According to this study statistically, the most abundant levels of the relative air pollution concentration for 1110st pollutant was in the range of 0.5-1.0 and 1-1.5 especially for PMlO • The daily trends for CO in summer and winter show two peaks, one in early morning and the other early at night. Traffic load, small values of urban mixed layer height and static stability at nights and early morning are factors leading to these two peaks. A monthly averaged CO concentration for the monitoring stations shows that usually two peaks are observed during the year.Fig. (2) shows comparison of the monthly averaged CO for a period of 3 years in Fatemiʹs Station, which is situated in about center of Tehran and Aghdasiyeh Station which is located in the north west of Tehran. The peak for cold season occurs in Nov. or Dec. and the one for warm season occurs in July or August. Usually the peak occurring in cold season is due the persistence of a high pressure system and hence meteorological stable condition which usually occur in cold season in Tehran. However the peak for warm season can be due to nocturnal inversion and lack of precipitation in this period.Large scale atmospheric weather patterns, especially of the two upper level jets (at 200 mb), namely subtropical and polar jet streams, show that when they form a bubble like structure over Iran, i. e. one is at higher latitude and other at lower latitude, very low winds prevail over Iran creating acute air pollution episodes (e. g. the episode leading authorities to reinforce traffic restrictions in Dec. 2005). Also existence of a stagnant ridge over northern Iran leads to such episodes, although with shorter period. Hence we show that an interaction between these two jets can lead to large scale semi - stationary synoptic patterns, creating such acute episodes lasting between 4 to 7 days. For example fig. (3) shows the patterns of two jets forming a ridge and bubble structures at 200 mb level. Table (1) also shows some episodes with acute air pollution in the years from 2004 to 2007, with their respective properties.Usually the case with bubbl e like structure. especially when the atmo sphere is more barotropic in this region, shows a more acute air pollution period which may last longer. Patters other than these especiall y when the jet are jo ined togeth er with least meridi onal comp onent (nearly zonal). windy conditions in the area prevails and hence. we have least air pollut ion. The bubble like pattern is also associated with pressure and temperature rises near the surface. The pressure rise usually occurs a bit earlier than when the bubble pattern is formed . Also there arc positive correlations between the air pollution concentration levels and near surface temperature inversion s (~T . temperature di fferenc e between 22 m and 2 m above ground). During the acute air polluti on episodes. a large scale subsidence of air over the area is observed. Careful and continuous monitoring of large sca le weather patterns as well as near surface meteorological paramet ers as pressure and temperature may be used to give short time forecast for acute air pollution episode s in Teh ran.