مدل سازي ترموديناميكي و واكاوي همديد آلودگي هواي شهر تهران (ذرات معلق PM10)

Synoptic Analysis and Thermodynamic Modeling for Air Pollution of PM10 in Tehran

شرايط اقليمي از جمله وارونگي‌هاي دمايي و استقرار سامانه‌هاي پر فشار همراه با هواي پايدار به‌ويژه در دوره سرد سال، و همچنين بادهاي محلي و همديد مقياس باعث افزايش تراكم و حجم آلاينده‌هاي هوا در لايه‌هاي مجاور سطح زمين مي‌شوند. هدف مطالعه واكاوي علل هواشناختي تشديد آلودگي هواي تهران با مطالعه اثر الگوهاي همديد غالب جو مبتني بر مدل سازي اقليمي است. با شناسايي سامانه‌هاي تاثيرگذار بر شهر تهران و بررسي نقشه‌هاي همديد نشان داد كه استقرار الگوهاي واچرخندي روي شهر بيش از ديگر سامانه‌هاي جو در تشديد آلودگي هواي تهران مؤثر هستند، بطوريكه علاوه بر ايجاد پايداري و هواي آرام، از عوامل مهم رخداد پديده وارونگي دمايي هستند. در ضمن در دوره سرد سال كاهش ضخامت لايه آميخته ناشي از انقباض و برودت هوا بطور غالب در شرايط حاكميت شرايط پرفشار رخ مي­دهد. براي محاسبه عمق لايه وارونگي از نمودار ترموديناميكي (SKEWT) داده­هاي جو بالاي ايستگاه مهرآباد استفاده گرديد. سپس با استفاده از مدل ميان مقياس آلودگي هوا (TAPM) به واكاوي عددي پراكنش مكاني و زماني آلودگي براساس سرعت و جهت باد، امگا و نيمرخ­هاي قائم عناصر جوي حاصل از مدل پرداخته شد. نتايج بدست آمده از واكاوي خروجي­هاي مدل نشان داد كه سرعت و جهت باد در موقعيت مكاني تهران، مهمترين عامل اقليمي تاثيرگذار در پراكنش آلودگي هوا است. به طوري‌كه با كاهش و يا افزايش سرعت باد و تغيير جهت آن شرايط آلودگي به شدت تغيير مي­كند. بادهايي كه به جز باد غالب غربي در ايستگاه مهرآباد در مناطق و ايستگاه­هاي ديگر تهران عموما تحت تاثير عوامل محلي داراي بادهاي با شدت پايين و جهات متفاوت است. بنابراين بادهاي داخل شهري هم قدرت كافي براي تهويه هواي شهر ندارند و در نتيجه تنها در جابجايي مكاني هسته­هاي آلودگي در طول شبانه­روز نقش دارند.

Introduction Stable Climatic conditions such as thermal inversions and highpressure systems  established in stable weather, particularly in cold periods of a year increase the density and air pollutants mass in the layers adjacent to the ground surface. Spatial and synoptic scale winds are effective on the pollution aggregation as well. Tehran is considered as one of the most polluted cities in the world and on average one day out of each 3 days, is polluted by one or several main pollutants during the year. Air stability in both autumn and winter seasons provide favorable conditions for thermal inversion and thus leading to pollution aggregation in the ground surface layer and breathing layer of people. The objective of this study is to analyze climatic causes of air pollution and its intensity in Tehran through studying the effects of synoptic scale atmospheric systems based on climate modeling.   Materials and Methods At First, the synoptic systems affecting Tehran were identified. Surveying the synoptic charts we looked for establishment of anticyclonic events in the city more than other atmospheric systems, that are effective on air pollution intensification.  In addition to establishment of stability and still air, the antisyclonic events are important factors for creation of thermal inversions. Furthermore, during the cold season, mainly in highpressure conditions thickness of mixed layer reduces due to air contraction and coldness. To detect the inversions, thermodynamic diagrams (SKEWT) produced from upper air data of Mehrabad Station were used. At the next stage, using the TAPM numerical air pollution model, () spatial and temporal distribution of polluted episods were analyzed based on the model outputs for surface wind speed and direction, and vertical profile of atmospheric elements.   Results The air quality is controlled by variations of temperature and wind elements and the air pollution sources. Both of the weather elements may be considered as the most effective climactic factor affecting the temporal and spatial distribution of Tehran air pollution, so that the horizontal and vertical variations create different phenomena such as horizontal and ascending flows. Out of 4 identified types of atmospheric systems that affect air pollution intensity, two have the maximum frequency and effect on spatial and temporal distribution of the polluters, namely northwest anticyclone and Siberian highpressure. Between the two elements, the northwest anticyclone is more effective on the spatial distribution and the Siberian highpressure is more effective for the temporal continuity of air pollution episods.   Conclusion Indeed, in this study by comparing synoptic and modeling analysis methods, it was concluded that the results for the descriptive analysis of the systems are justifiable for numerical analysis of overall air pollution in the city and climatic synoptic analysis is an appropriate method. In order to examine air pollution intensity locally in a district or city, modeling facilities are important because status of elements affecting air pollution such as wind as an important and effective climatic factor on urban polluter, can be analyzed and investigated in detail.