GAW Report No. 205 - IGAC Project

CHAPTER 3 - ASIATo establish an effective control strategy for achieving the AQS for PM 2.5 , comprehensivesource apportionment of PM 2.5 is necessary. Organic aerosols account for a large fraction of thetotal fine-mode aerosol mass concentration in the Kanto area. However, the observed levels oforganic aerosols are not well reproduced by 3-D chemical transport models [e.g., Matsui et al.,2009]. Further studies on the formation of organic aerosols are clearly needed.3.14 TEHRAN, IRANIntroduction and geographical settingTehran, the capital of the Islamic Republic of Iran, has a population of approximately 10million people. Located at (35° 42’N, 51° 25’E) with an area of 2300 km 2 , the city is situated in asemi-enclosed basin just south of the Alborz Mountain chain (with average height of 2000 mASL;Figure 60). Tehran has suffered from poor air quality since the oil boom decade of the 1970s andover the last fifteen years rapid population growth has made matters even worse. On some days,the pollution loading of the atmosphere is so high that the impressive Alborz Ranges becomeinvisible from most vantage points. Tehran’s Clean Air Committee stated recently that 10,000people die every year due to air pollution related cardio-pulmonary disease.Figure 60 - Map of TehranTehran’s location is unusual; unlike most major cities it is not near a river or even close tothe sea. Due to high elevation (approximately 1140 m), aridity, and latitude, Tehran experiencesfour seasons. Climate can be extremely hot in the summer (with midday temperatures rangingbetween 30 to 40°C) and cold in winter, when night temperatures can fall well below the freezingpoint (Figure 61). Local precipitation is absent for 6 months of the year in low lying areas.Originating in the Mediterranean, synoptic scale low pressure systems propagate over the region inspring and autumn, while in winter the southward extension of the Siberian high pressure systemcan advect cold air over the Iranian Plateau. The average annual rainfall is approximately 230 mm,with most precipitation falling in autumn and winter months. The large scale easterly flow thatdominates the area in the summer is thought to be associated with a circulation pattern named ‘thewinds of 120 days’ caused by a thermal low over Pakistan [Zawar-Reza, 2008]. Outside of the basinwhere Tehran is situated westerly winds prevail except in summer when the flow tends to beeasterly. The airflow over most of Tehran is influenced by the sloping topography as discussedbelow.127