Meteorological conditions of acute air pollution episodes for 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 Municipality and from synoptic station of the institute of Geophysics. Initially some statistical methods were used to find the episodes for acute air pollution episodes, and then their meteorological conditions were considered.
The most abundant levels of the relative air pollution concentration for most pollutant were in the range of 0.5-1.0 and 1-1.5 especially for PM10. 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. Annual monthly mean concentration of CO also show a peak in winter time in which the adverse effects of meteorological condition, in synoptically calm condition are enhanced. 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 patterns creating such acute episodes lasting between 4 to 7 days. The bubble like pattern is also associated with pressure and temperature rises near the surface.