Identification of the Thresholds of Extreme Values and Synoptic Analysis of PM10 Pollution in the Atmosphere of Ahvaz

Ahvaz can be regarded as one of the most polluted cities in the world in terms of air pollution. Successive years of drought and weather conditions in recent years have resulted in particulate matter (PM10) concentration in Ahvaz. In this study, using probability distribution techniques, an appropriate threshold to identify the PM10 maximum extreme concentrations (MEC) has been detected. Based on log-logistics probability distribution, which has the best fit to the data of PM10 concentration in Ahvaz, the 0.99 percentile threshold which is specified by 1516 μg/m³ is known as the primary PM10 concentrations in Ahvaz air. Based on the mentioned threshold, 24 days in which the PM10 concentration was equal to or more than the threshold were selected for synoptic analysis. Analysis of the circulation of weather types showed that two weather types circulations at 500 hPa level provide the climatic conditions for the occurrence of (MEC) caused by PM10 concentration ≥1516 μg/m³ in Ahvaz in the first type (which is for hot days). Under such condition, the closed high pattern of 500 hPa level is accompanied by the ground low pressures. In the second type (which is for cold and transitional days) the closed high pattern of 500 hPa level is accompanied by the ground high pressures. In addition, this study showed that the (MEC) of PM10 in both models fed with several different sources at different levels and due to being multi-source, storms can create MEC.