بررسي تغييرات ارتفاع و ضخامت لايه مرزي در شرايط گرد و غباري شهر اهواز

Studding the Condition of Atmospheric Boundary Layer Height of Khuzestan in the Dusty Days

ارتفاع لايه مرزي، يكي از مهمترين مولفه هاي تعيين كننده، وسعت آميزش آلاينده‌ها و كيفيت هواي لايه نزديك به سطح زمين مي‌باشد. هدف اساسي اين تحقيق مطالعه تغييرات و ارتفاع و ضخامت لايه مرزي استان خوزستان در شرايط گردوغبار شديد ميباشد. در اين تحقيق طي يك دوره 7 روزه (27ژانويه 2015 تا 1 فوريه 2015) كه يك رخداد حاد گردوغبار در استان خوزستان رخ داد ارتفاع لايه مرزي در همان روزها از داده هاي دقت بالاي پايگاه ميان برد اروپايي(ECMWF) اخذ گرديد، داده هاي مربوط به غلظت گردوغبار نيز از سنجش هاي ميداني سازمان محيط زيست بدست آمد در نهايت با تحليل سينوپتيك و تحليل فراواني ارتفاع لايه مرزي و غلظت گردوغبار شرايط لايه مرزي مورد بررسي قرار ميگيرد. نتايج نشان داد ارتباط مستقيم و معني دراي بين شدت گرد وغبار و ارتفاع و ضخامت لايه مرزي در استان خوزستان وجود دارد كه با در نظر گيري كم فشاري كه در منطقه مورد مطالعه در همان روزها در سطوح مياني جو مستقر شده و باعث شده منطقه مورد مطالعه در جلوي فرود حاصله از اين سامانه كم فشاري قرار گرفته بگيرد توجيه ميشود. بر خلاف آلودگي هوا كه غالبا منشا محلي دارند و در شرايطي ايجاد ميشوند كه شرايط پايداري شديد حاكم بوده و كم بودن ارتفاع لايه مرزي مانع تلاطم و گسترش آلاينده شده و تمركز آن را در نزديكي سطح زمين ايجاد ميكند، گردو غبار خوزستان در شرايطي ايجاد و تشديد ميشود كه شرايط ناپايداري برقرار بود اين رفتار گردوغبار به اين دليل است كه برخلاف آلودگي هوا، گردوغبار منشاء خارجي داشته و توسط سامانه هاي كم فشار و شرايط سينوپتيك ناپايدار كه موجب افزايش ارتفاع لايه مرزي ميشوند، وارد منطقه ميشود.

One of the most important components of the extent of pollutants mixing and air quality at near the Earth's surface is the height of boundary layer. Many variables involved in determining the height of the boundary layer of atmosphere. Although all of the troposphere (the lower ~10km of the atmosphere) is affected by surface conditions, most of it has a relatively slow response time. The lower part of the troposphere that is affected on a shorter time scale is commonly defined as the Planetary Boundary Layer (PBL). The depth of the mixed layer has a significant effect on the concentration of air pollution, which itself is dependent on the intensity and duration of solar radiation and wind speed. According to Stull, one can describe the planetary boundary layer as “that part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcing with a timescale of about an hour or less.” Surface temperature has a strong relationship with height of the PBL. As the surface cycles between daytime radiation and nighttime cooling the amount of convection taking place changes. When the temperature gradient is steep, more convection takes place to dissipate thermal energy in the most efficient manner. In other words, the greater the temperature difference between the surface and the lower troposphere, the higher convective eddies must reach to alleviate the gradient. Relating this to Stull’s definition of turbulence, it can be concluded that the height of the PBL varies with surface temperature. In fact, the spatial range of the PBL can vary from less than one hundred meters to several kilometers. The strong relationship between convective turbulence and height of the PBL is sometimes used to define the boundary layer and call it the Convective Boundary Layer (CBL). Analogous to the Stull’s definition but focusing on turbulence, Lloyd et all describe CBL as “a layer of air typically of order 1km in depth, well mixed by turbulence maintained by buoyancy due to heating at the ground. It is bounded above by stably stratified, no turbulent air and grows through the day. In this study we aimed to analysis the status of ABL in 3 dust period days in Khuzestan province of Iran. The Data were used in this study includes: The Daily data of dust concentration during 27Jan to 1 Feb 2015, the daily height of ABL also were used. The daily data of ABL were given from ECMWF with 1/8 degree spatial resolution. We used the Pearson correlation and synoptic analysis to assessment the condition of boundary layer at the mentioned days. For analysis the characteristic of ABL the climatic data of Wyoming University were used to assessment the thermodynamics of atmosphere. The spatial distribution of ABL height at the dusty day also were used for 12 UTC. The results indicated there is the direct relationship between the ABL height and the concentration of dust in the mentioned days. So that in the days that the concentration of dust reaches maximum we fund that the height of ABL reaches maximum simultaneously and vis versa. The spatial distribution of ABL height shown that the height of ABL in the 29Jan reaches maximum that the maximum concentration of dust related to this day. an‎d also the minimum concentration of dust observed in 27Jan and 1Feb that the in this day the height of ABL was minimum. The synoptic analysis also reveals that locating the low pressure system at the 500hp level that the Khuzestan province has been locating at the front of this system lead to transport the dust to study area. In this study we reveal that the height of ABL in the dust days of Khuzestan has a totally revers behavior in compare to the air pollution days in Tehran. In the pollution days in Tehran the lowing of ABL height and inversion lead to intensify the concentration of pollution while in the dust days of Khuzestan the height of ABL were increased in compared with non-dusty days.