Study of Mean Pressure Patterns Based on Different Levels of Occurrence of CO Pollutant in Tehran for the Summer and Autumn Seasons

Introduction
Tehran is one of the growing cities of the world with special features; Alborz Mountains range in the north of the city and arid region in the south. It has rapidly grown in the last decades. It is also situated in a basin area, partially surrounded by mountain ranges which make it prone to acute severe air pollution episodes. Air pollution in large cities due to the increase of population growth by increasing the industrialization progress, has created problems for many residents. Tehran is one of the polluted cities in the world. Very high traffic, high energy consumption and complicated topographic and atmospheric stable systems have led Tehran to be considered as one of the world's polluted cities.
The surface wind field is important for pollution dispersion and cyclonic conditions, producing moderate to strong westerly flows, are most effective in producing non-severe pollution conditions as dispersion and transport of pollutants. Vertical temperature gradients play an important role in the vertical distribution of pollutants. An inversion acts to limit the vertical mixing of pollutants, which allows concentrations to happen. Nocturnal inversions trap emissions released during overnight hours close to the ground. This reduces the possibility of pollutants being diffused vertically through suppression of the atmospheric boundary layer mixing height.

Materials and Methods
In the present study, in order to determine mean pressure patterns based on CO concentration levels of pollutants, initially based on data PSI of Tehran Air Quality Control Company of Tehran Municipality is taken. Gridded sea level pressure (SLP) and 500-hpa geopotential height data from the National Center for Environmental Prediction-National Center for Atmospheric (NCEP-NCAR) 6-year Reanalysis data were used to the synoptic classifications. These data analyzed at 00 UTC were prepared for the 10 pressure patterns based on the levels of carbon monoxide pollutant in Tehran (Table 3). Daily averages for each group of summer and autumn seasons from 2001 to 2006 were obtained for a large region (10°N-60°N, 0°E-80°E).
The PSI data that are available since 1999 show that during 9 years (1999-2007), there have been 883 days of unhealthy air quality conditions in Tehran and it has been very unhealthy (Table 1). In the 6-year statistical period (2001-2006) a greater number of unhealthy days were selected. Considering that the unhealthiest days in Tehran were usually in the months of summer and fall, reportedly nearly 647 unhealthy days have occurred during 6 years (Table 2).

Results and Discussion
According to the featured synoptic patterns ruling on the summer and autumn months in Tehran, these months can provide the most potential air pollution. The results indicate that the mean pressure patterns are as follows:

A –Mean Pressure patterns for summer settlement with the following characteristics reduce potential air pollution in Tehran (air quality index in clean and healthy condition is placed):
1 – Elongate east toward Azorse high pressure, pressure systems strengthen northern Caspian Sea, expanding ridge of high pressure on the south and southwest of the Caspian Sea area and southern Alborz
2 -The strong pressure gradient in the southwestern areas of the Caspian Sea and the west of Tehran
3 – Zonal flow in mid troposphere with strong contour gradient over the area of study
B - Mean Pressure patterns for summer settlement with the following characteristics increase the potential air pollution in Tehran (air quality index in unhealthy and very unhealthy situation is placed):
1 - limited expansion of the eastward Azorse high pressure weaken the high pressure system on the Caspian Sea and decrease the pressure gradient in the southwestern Caspian Sea and over the studied area
2 - Strengthening and expansion of the thermal low pressure system over north east of Iran, east and south east of the Caspian Sea
3 – Strengthening and expansion of the deep ridge of subtropical heights in the middle atmosphere from the south to the north of the Caspian Sea and very unhealthy conditions in the North Aral lake
C - Mean pressure patterns for fall settlement with the following characteristics reduce potential air pollution in Tehran (air quality index in clean and healthy condition is placed):
1 – elongate to westward the Siberian high pressure
2 – Strengthening of the ridge of high pressure with the strong pressure gradient in the southwest of the Caspian area and northern strip of Iran
3 - Prevailing the strong zonal flow or passing trough in the mid troposphere with severe contour gradient over the area under study
D - Mean Pressure patterns for fall settlement with the following characteristics increase the potential air pollution in Tehran (air quality index in unhealthy and very unhealthy situation is placed):
1 – weakening of the ridge of high pressure system over the Caspian Sea and extensively decrease of the pressure gradient on the studied area and the Caspian Sea area
2 – prevailing thermal low pressure system in the northeast of Iran and also southeast and east of the Caspian Sea
3 – strengthening and expansion of the ridge in the middle troposphere from the southwest of Iran to the east of the Caspian sea and Tehran area located on the east of ridge axis where convergence upper flow is generally located

Conclusion
The summer seasonal synoptic patterns compared with the fall seasonal synoptic patterns for each group of Table 3 show that in both seasons synoptic patterns are associated with unhealthy conditions and severe pollution episodes in Tehran and have similar characteristics that may not be quite equal. The most important features of these similarities, are creating the thermal low pressure system in the south-east of the Caspian Sea and expansion of the ridge of high pressure from the south west of the Caspian Sea to some parts of the centre of Iran and southern Alborz mountains that are associated with deep ridge in the mid troposphere on these regions. The high pressure system is weakened on the north Caspian Sea and to strengthen and expand the thermal low pressure over the northeastern of Iran, southeastern and eastern parts of the Caspian Sea and reduced pressure gradient is also accompanied. Also in the mid troposphere, Strengthening and expansion ridge on the Tehran area and higher geographical latitudes provided suitable conditions for the occurrence of severe pollution episodes in Tehran.
In both seasons (summer and fall), if high pressure system strengthens on the Caspian sea and pressure gradient increases in the southwest of the Caspian sea and also with these meteorological conditions coupled with zonal flow associated with strong contour gradients in mid troposphere, the potential of air pollution in Tehran reduces.