بررسی تغییرات ضریب خاموشی جو بر مبنای دید افقی در چهار فرودگاه پرتردد کشور

دید‌، علاوه بر اینکه به عنوان یکی از مهم‌ترین ویژگی‌های نوری جو محسوب می‌شود، از جنبه‌های مختلف از جمله ترافیک زمینی و هوایی دارای اهمیت است. در مقاله حاضر بانک اطلاعات تاریخی برای نشان دادن تغییرات دید در 4 فرودگاه پرتردد کشور شامل فرودگاه‌های مهرآباد تهران، مشهد، شیراز و اصفهان در دوره مشترک 40 ساله بررسی شده ‌است. ضریب خاموشی با استفاده از رابطه کشمایدر برآورد می‌شود. تغییرات بلندمدت خاموشی در روزهای آفتابی و نسبتاً خشک با کاربست پالایه در شرایط رطوبت نسبی بیش از 70% و ابرناکی بیش از 50% مطالعه شده ‌است. نتایج نشان می‌دهد که روند تغییرات میانگین سالانه ضریب خاموشی جو در بازه زمانی 1970 تا 2010 افزایش نسبتاً ملایمی را در هر چهار ایستگاه فرودگاهی دارد. مقایسه حدهای پایین و بالای دید، تغییر قابل ملاحظه‌ای در ایستگاه‌های مختلف را نشان نمی‌دهد. کم‌ترین مقادیر حدی دید مربوط به ایستگاه مهرآباد تهران است. بررسی تغییرات ماهانه میانگین دید افقی حاکی از آن است که در ماه‌های زمستان کمترین مقادیر وجود داشته و اندازه آن حدود 2 کیلومتر کمتر از ماه‌های دیگر است. ارتباط مستقیم میان رطوبت نسبی و ضریب خاموشی جو احتمالاً ناشی از تبدیل آلاینده‌های اولیه به ثانویه در حضور رطوبت است. نتایج مطالعه حساسیت روند تغییرات بلندمدت خاموشی به رطوبت نسبی و ابرناکی نشان می‌دهد که با اعمال پالایه، تغییر اندکی در مقادیر مطلق داده‌ها ایجاد می‌شود؛ ولی در روند کلی تغییرات خاموشی، به ویژه در ایستگاه‌های تهران و اصفهان، تغییری دیده نمی‌شود که بیانگر کاهش دید در بلندمدت به‌دلیل افزایش غلظت آلاینده‌های جوی است.

Study of longterm variation of extinction coefficient based on horizontal visibility in busiest airports in Iran

Atmospheric visibility is a key factor in everyday life mainly in aviation, industry and surface traffic. It has been defined as the greatest distance at which an observer can just see a black object viewed against the horizon sky which is known as the visual range. Furthermore, light extinction which is mostly due to absorption and scattering effects of aerosols in the atmosphere, can be calculated from visual range using Koschmieder formula. Visibility and extinction have similar units. In a nonpolluted atmosphere, visibility ranges from 145 to 225 km, and in normal atmospheric condition it ranges from 10 to 100 km; yet in polluted areas it can be remarkably low down. Visibility is regularly measured at synoptic meteorological stations all over the world as a standard meteorological parameter. Skilled observers have been measuring the visual range using individual markers at known distance from the meteorological location against the horizon sky. In this paper, airport visibility data for the period of 1970 to 2010 are examined in the four busiest airports in Iran including TehranMehrabad, Mashhad, Shiraz and Isfahan. All data from the four airport stations have been used for analyzing the temporal variations. The analyses are based on daily average measurements, i.e. the average of 9, 12 and 15 UTC data. Midday values are usually used in studies of this kind as they are more representative of regional visibility levels, because early morning radiation fogs and high relative humidity which may reflect only the local environment would regularly have dispersed by midday. The historical trends of extinction coefficient based on visual range for the four aforementioned airports are computed. Trend is determined by a least square regression analysis of midday average extinction. In general, an upward atmospheric extinction trend is seen for all stations. TehranMehrabad airport has the most increasing trend. The extinction was around 0.3 km1 in early 70’s but it increases in the present and reaches up to around 0.4 km1. Airport visibility data that inherently undervalue the true visibility are most appropriately summarized by cumulative percentiles. The Nth cumulative percentile is the visibility that is equal or exceeds N percent of the time. Visibility data lends itself well to the treatment in this manner. Daily visibility observations are investigated during the last four decades at 10th and 90th cumulative percentiles to show the threshold visibility in each airport. The 10th and 90th cumulative percentiles of visibility are used to identify the frequency of ‘good’ and ‘poor’ visibilities, respectively. Results show that there is not a distinctive difference between the 40year poor or good visibilities among all stations. However, Tehran airport has the least quantity in visual range than the other airports. The good visibility has the largest threshold value of around 20 km in Shiraz and Mashhad airports. Monthly comparison of extinction for 40 years, shows that there is an about 2km difference between winter months and the rest of the year. It can be due to the effect of weather or concentration of pollutants in different months. The results of correlation analysis indicate that the difference may be due to the variation in relative humidity value in different months. To minimize the effect of humidity, the days with relative humidity value of above 70 percent and cloudiness of above 4/8 of sky are removed from the visibility trend analysis. Detailed analyses show that the trends of the screened days are nearly parallel to the trends of raw data, but with a slight difference in each airport. Increase in extinction is also observed since 1970 when absolute values of extinction change. The extinction trend is not significantly changed in Tehran and Isfahan airport which may emphasis on the role of air pollution on atmospheric extinction.