بررسي آلاينده اُزن سطحي در شهر تهران طي دوره سال‌هاي (2008-2011)

Investigation of surface ozone over Tehran for 2008-2011

در اين تحقيق، تغييرات زماني مكاني آلاينده اُزن سطح زمين و عوامل هواشناسي موثر بر آن در شهر تهران طي سالهاي 2008 تا 2011 بررسي شده است. بررسي مقادير روزانه شاخص كيفيت هوا نسبت به آلاينده اُزن سطحي در ايستگاه اقدسيه نشان داده است كه بيشترين درصد روزهاي ناسالم در ماههاي تير و مرداد (93%) قرار دارد. اوج روزانه اُزن در ساعتهاي 15-17 رخ داده و متاثر از عوامل گوناگوني، بهويژه دماي هوا و تابش خورشيدي است. غلظت اُزن در ساعت 11 قبلازظهر، همبستگي مثبت زيادي با غلظت ميانگين روزانه آن دارد. يكي از جالبترين نتايج اين است اغلب كاهش وارونگيهاي دمايي شبانه و كاهش پايداري جوّي مي‌تواند باعث افزايش غلظت اُزن سطحي شبانه شود. بررسي بيشينههاي آلاينده اُزن سطحي بهاري روشن ساخته است كه برخي سامانههاي كم ارتفاع وَردسپهر مياني همراه با پديده‌هاي وَردْايست تاشونده كه منطقه ايران را در عرضهاي شمالي درياي خزر تحتتاثير قرار ميدهند، ميتوانند از راه نزول هواي خشك سرشار از اُزن پوشنسپهري به وَردسپهر، باعث افزايش مقدار آلاينده اُزن سطح زمين شوند. بررسي بيشينههاي اُزن سطحي تابستاني و مقايسه آن با شرايط هواشناسي جوّ بالا نشان داده است كه اغلب اين بيشينهها در اثر حضور واچرخندها در وَردسپهر مياني منطقه ايران ايجاد مي‌شوند. شرايط هواشناسي آفتابي، خشك و آرام در طي روز ناشي از اين سامانه‌ها باعث افزايش توليد اُزن طي واكنشهاي فوتوشيميايي مي‌شود. بررسي تاثير بارش بر مقدار آلاينده اُزن سطحي روشن ساخته است كه در روز بارشي غلظت همه آلايندهها ازجمله اُزن كاهش مي‌يابد ولي در يك تا دو روز پس از بارش درحاليكه هنوز اكثر آلايندها غلظت بهنسبت كمي دارند، غلظت اُزن افزايش مييابد؛ كه اين نشانه شفافتر بودن جوّ براي تابش خورشيدي و ترغيب بيشتر فرايند فوتوشيميايي تشكيل اُزن است.

Near surface ozone in urban areas can be potentially hazardous for the city dwellers. As a secondary pollutant it can also be an indicator for other air pollutants as nitrogen oxides, NOx and phenol acetate nitrate (PAN). In this paper time and spatial variations of near surface ozone and the effective meteorological parameters influencing it concentrations, have been investigated for Tehran for the period 2008-2011. The hourly ozone data of four stations namely Aghdasieh (north east), Geophysics (central), Poonak (north west) and Ray (south) for this period were acquired from the Air Quality Control company of Tehran, and the hourly temperature difference between the near surface and 8 m height were also from the Resalat Mast west of the city. The meteorological data were acquired from the Geophysics station and the data of the vertical distributions of humidity were from Wyoming University data base that provides Skew-T data of the troposphere. The synoptic maps were also acquired from NOAA. Ozone air pollution index for some air pollution stations as Aghdasieh (north east of the city) show that the period with highest concentration during the year are warm summer months in which 93% of the days were unhealthy. The daily peak of ozone concentration is found at 15-17 hours local time, depending on various meteorological parameters especially air temperature and solar radiation. There is also a strong correlation between the ozone concentration at 11AM local time and its daily mean. Usually the mean daily ozone concentration is about 66% of that of the values at 11AM local time. The annual peak of the ozone concentration is also in summer as expected. It is also found that as the nocturnal temperature inversion in the surface layer is reduced or the overall stability of the atmosphere is reduced the near surface concentration of ozone increased. This is attributed to the easier down mixing of the ozone in the residual layer (formed previous day) towards the surface. Tehran is surrounded by high mountains in the north and to some extent in the east affecting local near surface circulation. Such flows may advect air with different air pollutants especially from the emission areas (mainly center and south of the city) towards north or east, as towards Agdasieh, the station with an ozone monitoring facility. Spring time surface ozone variations also show that some mid-latitude low pressure weather system can cause strong tropospheric mixing, including tropopause folding, hence down transport of stratospheric ozone rich air towards the surface. While meteorological conditions associated with maxima of near surface ozone in summers are those with anticyclonic systems in the mid-troposphere of this area. Such conditions are associated with clear skies and strong solar radiations leading to strong photochemical activities that are essential for ozone formation in the atmosphere. It is also found that precipitation strongly reduces the air pollutants including ozone. It is interesting that following such perceptive events as the sky clears, ozone rate of concentration increase is faster than those of other pollutants. More clear sky following such events leads to stronger solar radiation near the earth surface, leading to stronger built up of ozone near surface.