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  • S. Magidi / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.178-181 Determining the atmospheric stability classes for Mazoe in Northern Zimbabwe S. Magidi (Department of Electronic Engineering, Harare Institute of Technology Ganges road, Box BE 277 Belvedere, Harare, Zimbabwe)ABSTRACT The paper presents the method that was dispersion estimates from this method is not goodused in determining the atmospheric stability but may be used in preliminary site survey. Theclasses for a place called Mazoe Citrus situated unreliability is mainly due to human error inin Northern Zimbabwe for two consecutive determining the cloud cover [4]. The method isyears, 2011 and 2012. The stability classes are an summarized in Table 1.during the day, strong refersimportant tool to be used in the environmental to the incoming solar radiation when the solarimpact assessment for an area before an elevation is greater than 60°, while moderate refersindustrial power plant is set up. The study has to incoming solar radiation when the solar elevationshown that conditions favoring neutral stability lies between 35° and 60°. If the solar elevation isare prevalent and that there is moderate to below 35°, then the incoming solar radiation isstrong winds with slight insolation and a cloud referred to as slight. The letters (A-F) refers tocover of more than 50% for 60% of the time different combinations of these conditions as shown in the tables.Key words: stability class, effluent,insolation, temperature Table 1: the simplified method classification criteria I. INTRODUCTION DAY NIGHT The tendency of the atmosphere to resist Incoming solar radiation (cloud)or enhance vertical motion and thus turbulence is Wind Strong Moderate Slight >4/8 < 3/8termed stability. It is related to both the change of speedtemperature with height, called the lapse rate and (m/s)the wind speed. A neutral atmosphere neither <2 A A-B B - -enhances nor inhibits mechanical turbulence. An 2-3 A-B B C E Funstable atmosphere enhances turbulence, whereas 3-5 B B-C C D Ea stable atmosphere inhibits mechanical turbulence. 5-6 C C-D D D DThe turbulence of the atmosphere is by far the most >6 C D D D Dimportant parameter affecting the dilution of apollutant [1]. The effluent dilution increases with IV. THE MODIFIED METHODincrease in atmospheric instability. The most This is the method that have been used incommonly used atmospheric stability classification this work and it uses wind speed, solar radiation,is that of Pasquill originally developed in 1961[2], net radiation, ambient air temperature and windand later modified by Gifford in the same year, and direction to determine the atmospheric stabilityreferred to as the Pasquill-Gifford (P-G) stability classes. This method has been chosen because it isclass. fairly reliable following the reliability of instruments used to collect the data. The method isII. METHODS OF DETERMINING shown in table 2 and 3 below. U is the wind speed THE ATMOSPHERIC STABILITY (m/s) at a height of 10 m above the ground; RD is CLASSES the solar radiation (Langley/hr). RN is the net There are many methods of determining radiation during the night.the atmospheric stability classes [3] but the mostcommonly used ones are; the simplified method,the modified method and the temperature lapse ratemethod. However the study concerns the first twomethods and the temperature lapse rate was notused.III. THE SIMPLIFIED METHOD The method uses the wind speed,insolation, and cloud cover to determine theatmospheric stability classes. The reliability of 178 | P a g e
  • S. Magidi / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.178-181Table 2: The modified method criteria (During the data logger and the P2 command (Voltageday) measurement command in the CR23X), the output U Stability class (Day) voltage was measured every two seconds. The voltage obtained for each two second interval was RD 50>RD 25>RD≥12.5 12.5 converted to W/m2 by linear regression equation ≥50 ≥25 >RD obtained through calibration. U<2 A A-B B D 2≤U<3 A-B B C D 2 WIND SPEED AND DIRECTION 3≤U<4 B B-C C D Pollutants are dissipated in the atmosphere 4≤U<6 C C-D D D by both horizontal and vertical movements of wind. 6≤U C D D D In general, the greater the wind velocity, the greaterNote: 1Lahgley = 1 Cal. Cm-2 = 4.187 J.cm-2 the dilution. As the wind speed at a given location varies with time, it is necessary to express it as anTable 3: The modified method criteria (During the average over a given time interval. The wind speednight) and direction were measured using a wind sentryU Stability class (NIGHT) set mounted 1.9 m above the ground. The anemometer was used to measure the wind speed. RN>-1.8 -1.8≥RN>- -3.6≥RN 3.6 3 THE CR23X MICROLOGGER This is a portable, rugged and powerfulU<2 D - - data acquisition system. All the sensors mentioned2≤U<3 D E F above were wired to the data logger accordingly3≤U<4 D D E and programmed in such a way that the data for4≤U<6 D D D every five minutes interval was collected. The6≤U D D D operating voltage of the data logger was kept between 12 V and 16 V from the AC mains drawnV. MATERIALS AND METHODS from nearby pump house and sampling interval was The required environmental parameters were 15 seconds averaged over a 5- minute’s period.measured using the appropriate sensors connectedto a data logger. VI. RESULTS Determination of the atmospheric stability1 SOLAR RADIATION classes was done using a developed computer The solar radiation depends upon the program. Generally, it was observed that thelocation and has pronounced effects on the type and stability classes peaks during mid day due to largerrate of chemical reaction in the atmosphere. It net radiation. The typical daytime stability classesaffects the ambient air temperature and hence the (A, B and C), start increasing significantly aroundair convection and mixing. Therefore its 0700 hrs in the morning and stop decreasingdetermination is very important for the sake of air significantly at around 1700 hrs. The D, E and Fpollution modeling. A CM11 pyronometer (Kipp stability classes which are typical night classes alsoand Zonen, Delft, Netherlands), installed 2.1 m penetrate into daytime. Class A is very unstable andabove the ground was used to measure the global corresponds to hot, calm days which lead to thesolar radiation , while the net radiation was greatest amount of dispersion. A plume of smoke ismeasured using a Q-7.1 net radiometer (radiation broken up and spread widely. The neutral class Denergy balance systems, Seattle, USA), mounted at can occur during the day or night and corresponds0.8 m above the ground. The pyronometer contains to windy days or to the transform times of dawna sensor installed on a horizontal surface that and dusk. This is the most frequently occurringmeasures the intensity of solar radiation. The sensor stability class. The stable classes (E and F) onlyis protected and kept in a dry atmosphere by a glass occur at night. Class F is very stable anddome that should always be wiped clean. The Net corresponds to nights with low winds. A plumeall wave radiation is then measured by recording experiencing “F” stability will suffer very littlethe difference in output between the sensor facing dispersion. Overally, it was discovered that theupward and downward. The CM 11 pyronometer is hourly accumulation of the stability classes dependssuitable for measuring global sun plus sky upon the season.radiation. This also is a measurement of energy fluxdensity for both direct and diffuse radiation passingthrough a horizontal plane of known unit area. Thepyronometer was set on a level surface free fromany obstruction to either direct or diffuse radiation.It has got a photodiode, shunted by a resistor,which produces a voltage output. Using the CR23X 179 | P a g e
  • S. Magidi / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.178-181Table 4: monthly distribution of the stability Table 4: Monthly distribution of atmosphericclasses for the year 2011 stability classes for the year 2012.MON FREQUENCY OF STABILITY STABI MONTHLY STABILITY CLASSTH CLASS % LITY FREQUENCY % A B C D E F CLASSJAN 16.5 17.6 1.4 53.6 0 10.7 FE M AP M SE O NO 32 08 78 29 53 B AR R AY P CT VFEB 15.9 18.1 2.5 50.0 0.1 13.2 A 0 20. 19. 16. 14 13. 12. 23 55 35 00 49 44 2 0 9 .1 6 2MAR 12.2 19.7 2.0 46.7 0.6 18.5 B 0 20. 18. 18. 23 24. 20. 31 58 16 74 72 48 0 9 5 .3 2 6APR 12.7 20.5 1.5 32.6 0.5 31.9 C 10 0.9 0.6 2.0 2. 2.4 5.9 78 56 28 43 56 44 .6 4 9 0MAY 11.0 19.2 1.8 44.6 0.1 23.1 D 89 58. 61. 62. 60 59. 61. 22 23 82 24 34 18 .4 9 4 5 .6 9 5JUN 6.52 22.0 3.3 42.6 0.4 25.0 E 0 0 0 0 0 0 0 8 83 33 39 17 00 F 0 0 0 0 0 0 0JUL 4.43 22.0 5.7 39.3 1.0 27.2 5 43 82 82 75 82 Comparing with the year 2011, the sequence ofAUG 6.85 22.0 6.3 24.5 0.6 39.5 stability class distribution on a yearly basis is such 5 43 17 97 72 16 that D>B>A>C. the classes E and F did not occurSEP 7.91 23.0 5.9 14.7 1.5 46.8 for the whole of the 2012 year. However, it can be 7 56 72 22 28 06 seen that the stability class D dominates for theOCT 5.27 15.2 4.3 14.3 5.0 55.8 whole period under consideration, with the highest 8 78 06 06 00 33 frequency distribution being 90% in February ofNOV 7.22 19.0 7.6 27.2 2.0 36.8 2012, and the minimum is 155 in September of 2 28 39 22 83 06 2011. For the year 2012, the monthly occurrence ofDEC 11.4 15.9 3.2 31.9 0.8 36.5 stability class D is almost constant from March to 25 45 26 89 06 59 December, with a value that is about 60% of all the stability class distribution. This is in accordanceFrom the frequency distribution shown, we note with Pasquill classification criteria. This impliesthat the stability classes on a yearly basis is such that the atmosphere was mainly neutral for the yearthat D>F>B>A>C>E. We see that class A have 2012. Classes A and B are almost equallyhigh frequencies at the beginning of the year and distributed, translating to the fact that for at mostattains its lowest value in July. As for atmospheric 20% of the time, the atmosphere is extremelystability class B, there is no remarkable change unstable to moderately stable. The distribution ofthroughout the year. Its frequency almost remains these classes has not significantly changed for thethe same. For class D, high values above 50 % are whole of the period under study. Class C,observed in January. However, its probability of interpreted as being slightly unstable is the secondoccurrence decreases with month until it attains its least distributed occupying at most 10% of the timelowest value in September through October, from as observed in February of 2012. It is mostlywhich again it start to increase. Stability class E observed from 1100 hrs in the morning to 15 00 hrsalways has low values whereas class F has low in the afternoon. This is the time of maximum solarvalues in January from which it starts to increase radiation on the site. At most two counts of class Cattaining maximum value in October. It seems that are observed. Class E is hereby observed occupyingclass F is the opposite of class D. a nearly constant percentage of time which is 20 % of the whole period from January 2011 to December 2012, with very slight variation. Class f which represents a moderately stable atmosphere shows a remarkable variation. In the year 2011, it occupied the atmosphere with competitively high frequencies, but in the year 2007 it did not occur at all. VII. CONCLUSION From the analysis done, it can be seen that the atmosphere for the climate of Mazoe Citrus area site, is in most of the time neutral. The conditions favoring neutral stability are prevalent. There is moderate to strong winds with slight insolation and 180 | P a g e View slide
  • S. Magidi / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.178-181a cloud cover of more than 50% for 60% of thetime. That the atmosphere is predominantly neutralimplies that it does not enhance or resist verticalmotion. Any effluent released into the atmospherehas a tendency to form a plume which is shapedlike a cone. Therefore if the source of the effluent ishighly elevated, it will not contribute to muchpollution on the ground. But if the effluent source isnear the ground, it might encourage the effluent tosit on the ground.REFERENCES [1] Canepa. E, Dallorto. L, Ratto C.F, Plume rise description in the code SAFE AIR. International journal of environment and Pollution, 14 (6), 2000, 235-245. [2] Essa, K, Mubarak S, Elsaid F, Effects of plume rise and wind speed on extreme values of air pollutants concentration. Meteorology and Atmospheric Physics; Sprigler 93(3), 2006, 247-253. [3] Finnd K. Y, Lamb B leclers M.Y, Lovejoy S, Multifractional analysis of line source plume concentration in surface layer flows. Journal of applied Meteorology 40, 2001, 229-245. [4] Georgopoulos P.G, sanfield J.H, Instanteneous concentration fluctuations in point source plumes, AlChE journal 32(10), 1986, 1042-1654. [5] Gifford, F.A, Atmospheric transport and diffusion over cities; Nuclear safety 13, 1972, 391-402. [6] Slade D.H, Meteorology and Atomic Energy,(National information services,1968) 181 | P a g e View slide