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Volume 3, Issue 1 (4-2016)
In issues related to air pollution, the thickness of the boundary layer is known as the depth of the mixed layer because the pollution on the ground surface is mixed in this entire layer through turbulence processes. In most cases, the boundary of the area is clearly visible on big industrial cities. The depth of the mixed layer has an important effect in the concentration of air pollution which is dependent on the intensity and duration of solar radiation and wind speed. Usually after 2 to 3 hours from the time of maximum solar radiation, air temperature near the earth's surface reaches its maximum value. At this time convection of heat is formed in the air near the earth surface and transfers the heat from the surface to higher altitudes. These vertical movements will cause atmospheric turbulence and increase in instability. This is when the growth of the mixed layer reaches to its highest level. After sunset, night temperature inversion occurs near the surface. This temperature inversion is due to the rapid cooling of the Earth's surface. In such condition, the cold air layer is near the earth's surface and the warm air layer sits on top of it and air is in a stable condition. As a result, the accumulation of contamination, if there are sources of pollutants, will increase in the earth's near-surface layer. If the conditions remain steady during the day, the mixed layer will not have much growth and as a result, contamination in the shallow layer near the surface of the Earth reduces solar radiation.
Each year, thousands of gaseous pollutants and particulate matter are emitted in the metropolitan area of Tehran and due to the geographical and climatic conditions of Tehran, temperature inversion phenomenon is not something unexpected. By formation of the inversion layer, these pollutants will remain near the earth's surface for a long time which in turn will be the cause of a lot of heart and respiratory problems. Therefore, identifying the characteristics of this layer on polluted days is of particular importance to the health of the residents of this city.
In this research, the study area is Tehran which is in the foothills of the southern Alborz and between longitudes 51 ° 2 'to 51 degrees 36' east and latitude 35 degrees 34 minutes and 35 degrees 50 minutes northern. The height of the northernmost point of this city is 1800 and up to 1200 meters in the center and 1050 meters in the south.
To conduct this research, inversion data including temperature, wind, atmospheric pressure and humidity and vertical navigation radiosonde data at the Mehrabad weather station from January to 29 December 2013, were taken from the Meteorological Organization of country. Then the statistics of daily vertical scroll of atmosphere above the Mehrabad synoptic station was received from the University of Wyoming. Also, the hourly data of air pollutants including gaseous pollutants CO, N2O, O3, SO2 and particulate matter (PM10) were prepared from the air quality control center (AQCC) for the stations Aghdasiyeh, Geophysics, Poonak, Rey and District 11.
After receiving information about the vertical scroll of the atmosphere in Mehrabad station, in order to have a closer examination of the vertical profiles of potential temperature changes in the lower atmosphere, using daily data from the radiosonde to obtain potential temperature changes in height were measured. Then, in order to identify the days with high pollution levels (the unhealthy condition for sensitive groups) and days with good conditions, so that all stations under study were the same, based on a standard index of air pollution Table 1 was developed. In the end, 4 days with critical inversion of potential temperature, including two polluted days (February 6th and August 16th) and two clean days (9 February and 5 June) were detected. Then according to the proposed method of Hefter, the approximate height of the boundary layer was calculated for these 4 days.
In this study, it was observed that the boundary layer height in contaminated cold season of the year reached 1,200 meters in the morning hours while in the afternoon in the cold samples, it grew to 1900 meters. In the warmer months based on the height of critical inversion layer in the selected days it reached more than 6,000 meters. In pure samples of warm and cold seasons, the boundary layer height had significant growth to the extent that in the cold sample of the year it reached to 2,100 meters in the morning and 2,600 meters in the afternoon. On June 5, which is intended to represent the clean and pure heating season, boundary layer height was of 5300 meters in the morning hours which shows a 4,000-meters increase in comparison to its polluted counterpart. The point to be noted is that since the active track of potential temperature can be considered as a measure of air stability, in the critical inversion, for the case of polluted samples of morning hours that were irradiated with inversion, active track of the potential temperature was very high in them. Thus on days with radiated inversion (polluted days) we can say that border of boundary layer was based on the inverted layer. Also the methods used in these types of inversions are more efficient for the determining height of the boundary layer.
Volume 4, Issue 3 (9-2017)
Building heating, air pollution and the use of inappropriate materials in the flooring streets (like asphalt streets due to dark colors in energy-absorption) are effective in phenomenon of urban heat islands that makes unfavorable environment for citizens. Paying attention to the urban surfaces like sidewalk, streets and rooftops has a great role in decreasing effect of this phenomenon. Due to growing urbanization and subsequently cities development, urban heat islands have taken a growing trend in big cities.
In general, the urban heat-island is a result of urbanity features, air pollution, human warmth, presence of impervious surfaces in the city, thermal properties of materials and geometry of urban areas. Heat island phenomenon is a result of many factors that are summarized below: (1) urban Geometry (morphometry) (2) thermal properties of materials which increase the sensible heat storage in the urban texture (3) released human heat as a result of fuel combustion and animal metabolism (4) urban greenhouse gases, leading to an increase in long wave radiation, atmospheric contamination and therefore warmer atmosphere (5) reduction of evaporation levels in cities, which means that energy will be released more as tangible rather than latent heat (6) reduction of turbulence and heat transfer through the streets.
This study aimed to simulate and calculate the maximum amount of heat island (UHI max) according to the conditions of urban geometry in the region of Kucheh bagh in Tabriz that is a pioneer study in Iran.
The study area is located in Kuche bagh region at the intersection of the streets of Ghods and Farvardin in the city of Tabriz.
The Oke’s numerical-theoretical equation was used for this study. First, the geometry of the target area using the radius of 15 meters from the axis of the road was divided into separate blocks. The ratio of street width (W) and height of buildings (H) was calculated in GIS software and at the end, the intensity of UHImax was calculated and simulated using Oke equation.
The urban geometry including building height and street width is calculated using Equation 1.
The theoretical- numerical basis of this equation shows that simulation of H/W ratio is an appropriate ways to describe urban geometry. Increasing the value of this ratio could lead to an increase in urban heat-island through modeling. This model has many advantages compared to other methods used to estimate the urban heat island. So, the selected parameter to calculate urban geometry and the model used to estimate the maximum intensity of heat island is the ratio of H / W and OKE model, respectively. In addition, the average height of buildings located within a radius of 15 meters and an average width of passages were calculated from the equation 2 and 3, respectively.
After calculating the geometry of the study area, the results showed that the blocks E, G and D in terms of height of the buildings have a heterogeneous distribution, but the distribution of blocks C, I and J is illustrative of their standard configuration. Although the blocks E, F and J in terms of street width are less diverse compared to other blocks, but in terms of height of buildings (8.6, 7 and 5 meters) have a different pattern that maximum values of their UHI are 8.3, 7.5 and 6.3 degrees, respectively. Three blocks B, H and I, in addition to their similarity according to street width and height of the buildings, in terms of the ratio of H / W and heat island intensity with the values of 9.6, 9.8 and 10.2 degrees are homogeneous.
It was also found that the greatest difference between the H / W ratio is related to block A (0.54) and block H (1.98); this difference has caused that greatest difference between the maximum intensity of UHI would calculated between the two blocks equal to 5.2 degree.
Misconfiguration causes that energy leaving from city surface deal with the problem due to narrow passages and high buildings. Therefore, consideration appropriate width of passages and streets and height of buildings are necessary to ease heat leaving and reduce intensity of UHI.
These simulations showed that high buildings and narrow streets intensify the heat islands. While in the presence of short buildings and wide streets, the UHI max is lowered. When the ratio H / W in the studied urban area is between 0.54 to 0.81, UHI max remains between 5 to 6.6 C˚, when this ratio increases to 1.01 to 1.98, UHI max will be between 7.5 and 10.2 C˚. The result also revealed that block A and H with 5 and 10.2 C˚ have the minimum and maximum value of UHI intensity, respectively. So can be concluded that block A and H have the most standard and non-standard urban configuration in the region. The estimates from regression model showed that the street width (91.6%) is more effective than the height of the buildings (6.6%) in changing UHI max.
Volume 5, Issue 4 (3-2019)
Volume 8, Issue 2 (9-2021)
The daily cycle of radiant heating from sunrise and sunset leads to the daily cycle of tangible and hidden heat fluxes between the earth's surface and the atmosphere. These fluxes, which cannot directly reach the whole atmosphere, are confined to the shallow layer near the surface, called the boundary layer of the atmosphere. . The processes that take place in this layer are important in various aspects such as the dynamics of fluxes and atmospheric systems, surface radiation, the hydrological cycle, and air pollution research. The thickness of the boundary layer of the atmosphere varies with time and place, and its size varies from a few hundred meters to several kilometers on land under different conditions. This thickness depends on various factors such as the type of atmospheric systems and their structure, surface fluxes, steep vertical arrangement and wind direction and surface cover. The depth of the boundary layer can be calculated by different methods. This depth, which indicates the thickness of the turbulence zone near the surface, is usually called the depth of the mixed layer or the depth of the mixture. The methods used to determine the boundary layer of the atmosphere or the depth of the mixed layer are commonly used to investigate air pollution. Estimating the depth of the mixed layer is one of the most important parameters in the pollutant diffusion model. Therefore, the purpose of this study is to investigate the causes of monthly fluctuations in the height of the western border layer of the country with respect to the barley station above Kermanshah.
Materials and methods
Data on inversions of Kermanshah meteorological station during February and August 2012; Obtained from the Meteorological Organization of the country. Also, the data related to the vertical barley survey in this station, which were collected by radio sound, were used and the statistics of daily vertical barley survey above the Kermanshah synoptic station were obtained from the climatic database of the University of Wyoming. After obtaining information about vertical barley survey in Kermanshah station, Skew-T diagram, indicators and profile information of atmospheric conditions were drawn to recognize the dynamic and thermodynamic status of the atmosphere during the selected days in RAOB software environment. Then, in order to study the lower atmosphere more accurately, the changes in the vertical index of potential temperature, using daily radiosound data, the curves of potential temperature changes in terms of altitude were plotted. Then, using Huffer's computational method, days with critical inversion at potential temperature were found. Then, using geopotential height, wind and vertical ascent (omega) data, the synoptic causes of boundary layer depth fluctuations (mixed) and the effective factors were investigated.
Results and discussion
The main purpose of this study is to implement Hafter's proposed model to investigate the monthly fluctuations of the height of the boundary layer of Kermanshah station. The results of using Hafter method in estimating the depth of the mixed layer of the city and its daily changes for Kermanshah station in August and February 2012. In this regard, the effective factors in minimizing and maximizing the mixed layer in every two months (August and February), including: the synoptic situation in the study area on selected days, heat transfer, humidity, vertical arrangement and wind speed were investigated.
Conclusion
The results showed that in August, the depth of the layer during the month was between 3680 to 10292 meters. In this month, temperature subsidence, type of synoptic systems and vertical wind arrangement have directly played a significant role in the growth or weakening of the layer. Considering the comparison of the role of effective factors in maximizing and minimizing the depth of the boundary layer in August, it can be concluded that all factors have a positive role in maximizing the depth of the mixed layer; while the vertical wind arrangement plays an essential role in minimizing the layer depth in this month. In February, the depth of the mixed layer was about 2273 to 7017 meters and significant fluctuations in the values of the depth of the mixed layer were observed during the month. In this month, temperature subsidence, vertical wind arrangement and synoptic systems have been effective in changing the depth of the mixed layer. Comparing the results obtained from both months, it can be said that the amount of surface flux is higher in summer than in winter; thus, the average depth of the mixed layer in August has almost doubled to February. In general, it can be concluded that the depth fluctuations of the mixed layer in winter due to the passage of different systems and the occurrence of atmospheric instabilities, have more changes than in summer.
Volume 10, Issue 3 (9-2023)
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