Controls on air pollution over a semi-enclosed basin, Tehran: A synoptic climatological approach

In this study, a new array of meteorological elements and a synoptic climatologic classification to produce a baseline climate and to derive meteorological factors that are effective on air pollution in Tehran, which is located in a semi-enclosed basin was developed. The data set includes daily sea-level pressure and 500mb geopotential height fields from NCEP-NCAR and Tehran upper air sounding data at 00:00UTC for six months (July- December) of every year in the period 2001-2006. The classification is done through the principal component analysis (PCA) of data from the upper-air station. The results show that three meteorological factors are effective on variations of the Carbon Monoxide (CO) concentrations in the complicated terrain of Tehran basin. Thickness variations of different pressure layers (especially 850mb-700mb), Total totals index (TT) and surface wind speed are the most important dependent variables of these factors. Examination of the Carbon Monoxide (CO) pollution levels in Tehran, where emission sources are high shows that the synoptic classification identifies the patterns that are conducive to high pollution and those that are conducive to low pollutants build up in the basin. The synoptic climatology of events associated with five categories of CO concentrations show the role of synoptic forcings on decreasing CO concentrations. On the other hand, the role of high and thermal low pressure patterns in producing sever polluted episodes in the basin is also shown. These events occurred exclusively under special synoptic situations with high pressure developing on the airflow entrance of the basin and a thermal low pressure on the leeward side of the mountainous region. The coupled pressure system inducing horizontal pressure gradient produces a low level southerly cross wind toward the mountain barrier.