بررسي تغييرات شبانه‌روزي و فصلي باد و دماي هوا و آلاينده‌هاي CO و PM10 در لايه‌ي سطحي جو شهر تهران

Study of Daily and Seasonal Variations of Wind, Temperature, CO and PM10 in the Atmospheric Surface Layer over Tehran

براساس داده هاي بادسنج فوق صوتي سال 2007، بيشتر زمان ها تندي باد بينm/s 0.5 تا 2 و دامنه چرخه سالانه آن كوچك است كه شرايط حاد آلودگي هوا را در تهران فراهم مي كند. همچنين CO و PM10 تغييرات فصلي از خود نشان مي دهند كه به شرايط هواشناختي و منابع آلاينده ها وابسته است. بررسي چرخه سالانه CO و PM10 نشان مي دهد كه غلظت CO در روزهاي پاييز تا زمستان افزايش دارند. غلظت PM10 در روزهاي زمستان تا بهار مقادير پاييني دارد. افزايش غلظت آلاينده ها در زمستان عمدتا ناشي از كمبود سامانه هاي همديدي فعال و وارونگي دمايي سطحي، بر اساس پارامتر پايداري N2، كه در پاييز و زمستان نسبت به بهار و تابستان بيشتر است.) است. در چرخه سالانه براساس ميانگين هاي شبانه روزي، ضريب همبستگي CO و PM10، 0.4 و در زمستان، 0.7 است كه نشانگر ارتباط قوي منابع اين دو در اين فصل است. در بهار، خودروها، گرد و غبار ناشي از سطح و يا از منابع دورتر، منشا PM10 هستند، اما در پاييز، منابع عمدتاً خودروها و وسايل گرمايشي هستند. دو بيشينه در نمودار تغييرات CO در ابتداي صبح و شب رخ داده است كه تقريباً با كمينه هاي تندي باد همزمان و وابسته به تغييرات فصلي نيز هستند. طي شب، شارش هاي كوه دشت (سرد) و نشست هوا ناشي از سامانه هاي پرفشار سبب ايجاد وارونگي دما بر روي منطقه مي شوند كه افزايش غلظت آلاينده ها را در پي دارد.نمودارهاي سه بعدي مؤلفه هاي افقي سرعت باد، دماي هوا و آلاينده ها نشان مي دهند كه مؤلفه نصف النهاري باد، نقش برجسته تري در انتقال CO كه مستقل از دما است، به عهده دارد كه با توجه به وضعيت توپوگرافي منطقه، مي تواند نشانگر محلي بودن منابع آن باشد. در حالي كه هردو مؤلفه سرعت باد در انتقال PM10 نقش دارند. همچنين بيشينه هاي PM10 در فصل سرد با باد كم همزمان و در فصل گرم مستقل از تندي باد هستند.

Nowadays air pollution in large cities such as Tehran have dramatic effects on public health, hence study of the way air pollutions varies with meteorological parameters appears to be important. One important aspect of sustainability of large cities such as Tehran, is controlling the emissions of pollutants as the meteorological (climatic) conditions are becoming more acute in terms of air pollution and temperature rise. In this paper some recent records of near surface meteorological parameters as well as some pollutants records are examine to observe how they change daily, monthly and annually and how they are correlated. Considering the variations of winds and temperature (extracted from a 2D sonic anemometer at 10 m at the Institute of Geophysics, Tehran University in the northern part of central Tehran, with one minute intervals) and hurly data of CO and PM10 concentrations for the same station for 2007, their relations were investigated. Also using upper air meteorological data (at 00.00 and 12.00 UTC) from Mehrabad Airport station, the stability of the atmosphere during this period was analysed. Here the buoyancy frequencies that are measure of stability of air column were calculated. For averaging of winds two methods based on the real wind vectors and wind unit vectors were used. By correlations between the pollutants concentrations and meteorological parameters, their relationships were considered. Based on the probability distributions of winds for 2007, it was found that most of the time wind speeds were in the range of 0.5 and 2 m/s. Hence most of the time due to this weak wind there was a condition of air pollution accumulations over the city and only local winds could move the polluted air over the area. Annual cycle of variations of mean surface winds had small litude that appears to be due to high mountain ranges that surround the city from north and east. The annual cycle of CO variations showed a peak in autumn and winter while PM10 amounts showed a trough in winter and spring. The higher values of CO in winter seems to be due to the surface temperature inversions and improper burnings of the fuel of vehicles as well as the domestic heating systems. This was indicated in the correlations between temperature and CO concentration. In annual cycle the correlation between CO and PM10 concentrations was about 0.4 which increased to 0.7 for spring time. This may indicate that in this season the sources of these two are similar and one of them may be used to estimate the others is the sources are not changed. There are two maxima in the daily variations of CO which coincides with minima of wind in morning and evening transition times. In this study it was found that due to calm meteorological conditions (often od local origin, called mountain breezes) over the city air pollution problem is a serious problem requiring more emission control. Also trend factors as the pollutant sources (traffic) and the depth of the atmospheric surface layer are important. It is particularly noticeable that during the midday as the depth of the mixed layer increases, the air pollution concentration is reduced substantially. At night surface drainage flow from north of the city and surface radiation cooling creates near surface inversions that can limit mixing and ventilation of the polluted air from the area leading to higher values of gaseous pollutant over the city. Also lager stability in the air over the city at higher levels in autumn and winter is due to subsidence inversions as a result of the prevailing meteorological conditions of high pressure systems over this area in these months. Such conditions seem to have increased the creation of more acute conditions for air pollution over the city. For a more resilient city in terms of air pollution, some mitigation need to be undertaken in the face of climate change effects that are deteriorating the atmosphere of the city.