JOURNAL OF INTERNATIONAL ACADEMIC RESEARCH FOR MULTIDISCIPLINARY Impact Factor 1.393, ISSN: , Volume 2, Issue 4, May 2014

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1 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 A STUDY OF INVERSIONS AND ISOTHERMALS OF AIR POLLUTION DISPERSION DR.V.LAKSHMANARAO DR. K. SAI LAKSHMI P. SATISH Assistant Professor(c), Dept. of Meteorology & Oceanography, Andhra University, Visakhapatnam, India Professor, Dept. of Engineering Chemistry, Department of Basic Sciences, SVP. Engineering college, Visakhapatnam, India Research Scholar, Dept. of Meteorology & Oceanography, Andhra University, Visakhapatnam, India ABSTRACT Inversions and isothermals are most important atmospheric conditions as for as dispersion of pollutants concerned. These conditions inhibit the dispersal of pollutants depending upon their intensity, thickness of the layer. The events of isothermal conditions over Visakhapatnam are very less in frequency. Over the station ground based inversions were less in frequency (13%) when compared to elevated inversions (1%).The highest percentage of ground based inversions were observed in summer season (%) followed by winter (18%). In case of elevated inversions even though highest percentages of inversions were present in winter season, most of the occurrences were confined at higher levels (85 8 hpa and 8 75 ).It implies that the influence of inversions on air pollution dispersion is not that much significant. Where as in case of elevated inversions are relatively lees percentage (3%) was registered in summer season. However most of the inversions (3%) were present in the lower level of 959 hpa which essentially indicates greater inversion influence in dispersing the pollutants. In monsoon and post monsoon seasons, the occurrences of inversions were very less in frequency. During winter season the inversion base is above 75 m, where as in summer season most of the inversion bases were confined below 5 m. Both in the monsoon and post monsoon seasons there was no consistency in the inversion base. In all the four seasons the inversion thickness is confined within the range of 575 m. higher percent of inversion intensities during the winter month were in the class C.In case of summer season even though higher percent of inversion intensities were in the class C. Significant percentage was also noticed in the class 4 C and 46 C.In the monsoon months the intensities were distributed irregularly in all the categories. During post monsoon season higher percentages of intensities were in the range C. KEYWORDS: Isothermal, Dispersal, Inversions, Pollutants. 659

2 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 I. INTRODUCTION Air pollution climatology is concerned with the aggregate of weather which affects the atmospheric concentrations of pollutants. It is based on documented records of meteorological elements and air quality measured at particular places during particular time periods. Inversions and isothermals are most important atmospheric conditions as for as dispersion of pollutants concerned. These conditions inhibit the dispersal of pollutants depending upon their intensity, thickness and base of layer. In general, temperature decreases with altitude (temperature lapse rate) in the atmosphere. If the temperatures increase with height, it is known as temperature inversion. In such conditions, the air tends to stagnate and atmosphere is said to be stable. The level from which the increase of temperature in vertical starts is known as the base of inversion and the cessation level of increase of temperature is known as top of inversion. The temperature difference between the top and base of the inversion is known as the intensity of inversion. The height difference between the top and the bottom of the inversion is known as the thickness of the inversion. The inversion is further divided into two broad categories depending upon the level of the base of the inversion. If the base is right at the surface it is known as the surface based inversion. If the base is some altitude above the ground then it is known as elevated inversion. Isothermals are a condition in the atmosphere where temperature remains constant with height. The surface based and elevated isotherms are defined in the same way as inversions. As these conditions inhibit the dispersal of pollutants, these are unfavorable from air pollution point of view. So a study of occurrence of these conditions at a given locality is of utmost importance in air pollution studies either in setting of new industry or warming the concerned for emission monitoring or in the development of air quality model. The importance of inversions and isotherms in air pollution studies was first recognized by Vittal Murthy et al (1977). A detailed analysis was carried out by Sadhuram (198). II MATERIALS AND MOTHODS In this paper, important parameters in air pollution studies such as inversions and Isothermals are presented. Air pollution potential has been discussed. The stability, the most important meteorological parameter that influences mixing, diffusion and dissipation of pollutants has been emphasized. The level at which the increase of temperature in vertical starts is known as the base of the inversion and the cessation level of increase of temperature 66

3 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 known as the top of the inversion layer. But, in case of isothermals, the level at which the temperature starts remains constant with height is taken as the base of the isothermal and the level at which the temperature starts changing is known as the top of the isothermal. The temperature difference between the top and base of the inversion or isothermal gives the general stability of the layer. The height difference between the top and bottom of the layer is the thickness of the layer. The base, thickness and top of inversions as well as isothermals are calculated using the formula Z = R Tv log (P1/P) (1) Where R is universal gas constant, Tv represents virtual temperature and P1, P represents pressure of the first and the second layers respectively. Intensity of inversion: According to Robinson (195) the intensity of an atmospheric inversion defines the overall effect of inversion on the diffusion of air pollutants in and under the layer and can be expressed as a function of several variables; I = f [(dө/dz), ө, Z] () In this expression the potential temperature gradient (dө/dz) through the inversion layer is a measure of the stability of inversion. This stability factor may be approximated by ( ө/ z) through the inversion layer from base to top. The thickness of the inversion is an important factor in atmospheric diffusion and air pollution problems. The potential temperature difference through the inversion, ө, is a measure of the thickness. The height of the inversion base, will determine the volume of air by which pollution may be diluted. The lower the inversion base, the greater will be its effect on ground level pollution. It is desirable to have both the inversion intensity and thickness as the inversion is very important factor in air pollution. It is directly proportional to ө and ө/ z and inversely proportional to the height of the inversion base, Z. This relationship may be expressed as I = [( ө) / Z Z ] (3) In practice it has been found advisable to add a constant, 3 to the denominator, which avoids the sensitive nature of the denominator for low values of Z and Z. The equation is I = [( ө ) / ( 3+ Z Z ) ] (4) Where I is the intensity of inversion / isothermal, ө is the potential temperature gradient in C, Z is the base of the inversion /isothermal in m, Z is the thickness of the inversion in m. 661

4 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 The final expression in equation (4) has been used for calculating inversion intensity. The constant value (3) might be considered as a leakage correction term since there will always be some diffusion through the inversion. The constant also serves to keep the value of I from becoming factiously larger when the inversion is weak and shallow with all the terms in the expression being quite small, so that the terms in the expression may have the same order of magnitude, z, and z are in units of hundred meters. In order that the very shallow radiation inversion may not seriously affect, the value of 15 m is taken for z. In many case where the actual z is less than this, such as with a surface inversion, z is set equal to 15 m and the values of ө and z calculated with respect to the point where z equals to 15 m.it is readily apparent that this combination of variables is not the only one which could be used as a measure of the inversion intensity. However, it is one, which has been adequate in the present air pollution study. For the purpose of present study the same formula has been used for both the inversion and isothermal layers and also for elevated inversions. Modification of the formula: In order to overcome the problem of computation in case of ground inversions based inversions, where the inversion base height becomes zero, Robinson (195) suggested an arbitrary value 15 m to the inversion base. But, in practice the value 15 m is not always suitable for all the days and for all the places. It is because of the temporal and spatial variations of mixing depth. When the heat generated within the urban area is considerably large and the lapse rates are near adiabatic, the assumed value of 15 m is not at all reasonable. In such a condition, actual values are to be incorporated, in order case large values of inversion intensities are possible. Hence, in the present the author has considered the urban morning mixing height of the particular day as there is a possibility of maximum mixing of pollutants only up to the height of morning height depth at any case. Therefore, in case of ground based inversions the computed urban morning mixing of the day was suggested for the inversion base. All the precipitations days were excluded in computing the intensity of inversions because there will be no such significant pollutant concentrations on the day as they were washed out. Holtzworth (1974) classified the inversion intensity into different categories and the criteria was,, 4, 46, 68 and above 8.In case of inversion intensity,for example, o C should be placed in the to C category.using the above said classification all the 5 years data is processed. The data has been processed in terms of temperature inversion and temperature lapse conditions in the lower 3 km of the atmosphere. Summaries of inversion 66

5 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 conditions (base height, thickness and T/ Z values) are discussed. Further T/ Z values in no inversion conditions have been computed by taking the layer, surface15 m as suggested by Holtzworth (1974). III RESULTS AND DISCUSSION Table 1 gives the percent of both grounds based and elevated inversions. The upper part of the table gives the frequencies of inversion base height and inversion thickness. The middle part gives the frequencies of T/ Z classes (the criteria of T/ Z were shown in the table) for the entire layer beneath the inversion base. The lower part of the table gives the specification of percentage base height ranges and also T/ Z classes for noninversion days up to 15 m when no inversion occurred below 3 m. From top of the Table.1, it is clear that on an average 15%of surface based inversions are present in the thickness range 515 m range. In case of elevated inversions out of %,11%are confined in the thickness range 515 m and 9% were found in the thickness range 5175 m.with this one can understand that about 4%(ground based and elevated) of inversions were in the lesser thickness range. Hence over Visakhapatnam if an inversion exists it is advisable to supply more heat energy to the plume so that it can overshoot the inversion. This provides considerable mixing of pollutants. It is also important to note that 8% of elevated inversions are in the base range 55 m and about 1% were above a base height of 75 m. This condition is favorable for lesser concentrations of pollutants as more volume of air is available under the inversion layer. From the middle part of the table it is clear about 15%of the inversion days show stability below the inversion layer of class 5 and about 8% in the class 4.Both of these classes are near dry adiabatic and near standard atmosphere, where in general less dispersion is possible. In case of no inversion days higher percentage were noticed in the stability class 4 and 5.These two classes indicate moderate dilution of the pollutants. Inversion Intensities: The mean monthly distribution of inversion intensities for both ground based and elevated for all the seasons were presented in Table.. In winter season, in all the years the higher percentages of inversion intensities were confined only in the lowest range of C in December 9 and 13,about 7% occasions showed inversions in C range. In case of January 9 and 13 about 7% of 663

6 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 occurrences were noticed in the range of C.While the highest value during February was noticed in 11. During summer months the percentage magnitudes were less concentrated in the range of 6 C when compared to winter months. In March a peak was observed in 1 for C range. In case of April, 13 has registered significant percentage significant (6) in the lower intensity limit. In May 11, C range showed peak percentage of 35.When compared to winter and summer months, monsoon and post monsoon months registered very less percentages. However, there were some peaks particularly in the lower limits of intensity. From this we can infer that over the station, the intensities of inversions are low and more or less below 4 C. Table.3 represents the five year mean percent frequency for both ground based and elevated inversions in the individual seasons. In case of elevated inversions the percentage frequency distribution of inversions for different levels were also presented. The highest percentage of 3.6 ground based inversions was noticed during summer season. Winter recorded 18.8% of ground based inversions. Other two seasons recorded insignificant percentage below 1.On an average the overall percent in all the four seasons was Over this coastal station elevated inversions were relatively at higher percent with 1, which is about one and half to the ground based inversions.41% of elevated inversions were occurred during the winter season followed by summer season with 31.5%.This phenomenon is contrary to other inland stations. It is also important to note that higher percentages of elevated inversions during the summer season were present in the pressure layers of 95 to 9 hpa. The percentage occurrence of ground based as well as elevated inversions in all the individual months and years were presented (Table.4).In case of elevated inversions, the percentages of elevated inversions and the corresponding pressure layers were also specified separately. From the Table.4, shows among all the years of December, 13 recorded the highest percent (16.) of ground based inversions. In case of elevated inversions, December 13 showed the highest of 45% in the pressure level 858 hpa. However the total elevated inversions were more than 8%.The total elevated inversion percentage of five years for December month is about 5%. In all the years of January, 13 recorded the highest ground based inversions with 3.3%.The least ground based inversions were in the year 11 with 7% of occurrences. In 664

7 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 case of elevated inversions insignificant percentage were noticed especially in the lower pressure layers say 959 hpa and 985 hpa.higher percentage was noticed above 85 hpa in all the years of January. The average elevated inversion percentage frequency was about 5%.The ground based inversion average in January was %, which is almost double the average of December. In the month of February the ground based inversion frequency was about %. In case of elevated inversions the total percentage frequency was 5.February 1 showed maximum frequency for 757 hpa. Ground based inversions were more significant for February 13. In all the three months of winter there was an increasing tendency of ground based inversion from December to February. It is interesting to note that higher percentages of ground based inversions were registered in March (36.%).The average of elevated inversions in March 13 was about 4%, with higher percentage of elevated inversions noticed within the layer of 959 hpa. In all the years of April elevated inversions are in higher percentage and mostly confined in the layer of 959 hpa. Particularly April 1 and April 13 reported significant inversions in this layer, with April 13 registering 8% of elevated inversions. In May, when compared to ground based inversions, elevated inversions were more dominant and were concentrated in the layer 959 hpa. There was an isolated peak of elevated inversions in the year 11 (6%).Most significant elevated inversions were noticed in the layer 959 hpa in this year. It is interesting to note that in the month of March, ground based inversions were dominant, while in the other two months the elevated inversions were dominant. In the monsoon month s general inversion frequency was less but the ground based inversions were relatively dominant in all the months, particularly during July, August and September. The highest average frequency is noticed for June in case of ground based inversion (15.3%).A significant ground based inversions was observed in 9 and 1 for the month of June. In September also a peak was observed for 1. In post monsoon season the ground based inversions in October were dominant while the elevated inversions dominant in November. In case of ground based inversions, some peaks are noticed particularly in the months of October 11 and 1 and November 9. Table.5 presents the particulars of single, double and multiple inversions (both ground based and elevated) percentage frequencies occurred in all the specified months (December, January, February); summer (March, April, May); monsoon (June, July, August and 665

8 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 September) and post monsoon (October and November).Among the three types of inversions the percent frequency of single inversions were high followed by double inversions. Almost in all the months and years the multiple inversions were zero except on a few occasions. Highest average percent of single inversions were present in the winter months of January and December (about 45%). Highest seasonal aggregate percent of single inversion was present during winter with a value of 4% followed by summer with 4%. Monsoon season recorded the average value of 9.6%. In case of double inversions the seasonal frequency in winter was the highest with a value of 9% followed by summer with 5%. Insignificant percent of 1.3 was noticed during monsoon season and % in the post monsoon season. In all seasons multiple inversion percentage was below unit value. The mean percent frequency of single inversions was 5.6%.Out of this 13.5% were ground based and 1.1% were elevated type. The former one is favorable for lofting conditions and the latter one is favorable for fumigation. As double inversion case were less in frequency, the trapping conditions were least possible. Isothermals: Table.6 shows the mean monthly percentage frequency of both ground based and elevated isothermal layers averaged over five years from 9 through 13.The other pertinent parameters like isothermal base, thickness, top, and intensity were also presented. The frequency of both ground based and elevated isothermals were very less in all the five years of the study period. Ground based isothermals with highest percentage (.1) were present in the month of April, and the least (.66) in March, where for elevated isothermals highest percentage (.7) was noticed in the month of January. In case of ground based isothermals, except in the month of October, all other months represented higher intensities of above C; with an average base height is of about 9 m. This clearly indicates that if the effective stack height of about this mean height of 9 m the pollutant concentrations to the ground may be less. The mean thickness of ground based isothermals was about 18 m where as the mean isothermal top was about 6 m, with an average intensity of 1.3 C. The percentage occurrence of elevated isothermals was absolutely zero in all the months of July through December. The mean isothermal thickness was of 45 m with an average base of about 4 m and the top was about 65 m. There was no much variation in the intensities among the months and the average intensity was about.37 C. 666

9 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 IV SUMMARY AND CONCLUSIONS The clear sky conditions during the night favored the development of inversions. It may be due to significant radioactive cooling during the nights. High pressure systems near the study area. The presence of haze, mist and fog over the station are indications for the formation of inversion layers. It is because haze, mist and fog will develop only when a significant stability is established. The synoptic conditions associated with most of the inversions were due to high pressure systems over the station, characterized by clear skies and low wind speeds. The factors quite often encourage the subsidence on a wide scale. Such conditions do occur frequently during winter months. Over Visakhapatnam it is found that the sea breeze had played a greater role in establishing stable layers like inversions during summer. It is also inferred that the synoptic wind systems may be balanced by the sea breeze leading to stable conditions. Table I Vertical variation of Temperature Percent frequency Over the Visakhapatnam for the period of 9 13 Surface Inversion base height(m) Thickness (m) >15 ΔT/ΔI 5 From inversion Base to surface _ ΔT/ΔZ For same layers As Inv ++ Base O Rounded to whole percentage but value > Zero Indicated by < Indicates Inversion 667

10 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 Table Mean monthly inversions intensities ( C) for all the individual years Description December January February Intensities > Description March April May Intensities >

11 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 Description June July August Intensities > Description September October November Intensities >

12 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 Table 3 Five year mean percentage occurrences of inversions (ground based and elevated) Belated inversion frequencies (hpa) Season Ground base Total Winter Summer Monsoon Post monsoon Overall average Table 4 Percentage occurrence of ground base3d inversions and Elevated inversions with their vertical distribution Total Elevated MM/YY Gr. Bs hpa hpa hpa hpa hpa Dec 9 Dec 1 Dec 11 Dec 1 Dec Jan 9 Jan 1 Jan 11 Jan 1 Jan Data not Feb 9 Feb 1 Feb 11 Feb 1 Feb Mar 9 Mar 1 Mar 11 Mar 1 Mar

13 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 Apr 9 Apr 1 Apr 11 Apr 1 Apr May 9 May 1 May 11 May 1 May June 9 June 1 June 11 June 1 June Jul 9 Jul 1 Jul 11 Jul 1 Jul Aug 9 Aug 1 Aug 11 Aug 1 Aug Sep 9 Sep 1 Sep 11 Sep 1 Sep Oct 9 Oct 1 Oct 11 Oct 1 Oct Nov 9 Nov 1 Nov 11 Nov 1 Nov

14 Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 Table 5: Percentage of Inversions over Visakhapatnam (8 13) Mon th total S D M S D M S D M S D M S D M S D M S D M Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

15 Month Jan JOURNAL OF INTERNATIONAL ACADEMIC RESEARCH FOR MULTIDISCIPLINARY Impact Factor 1.393, ISSN: 3583, Volume, Issue 4, May 14 Table: 6 Five years mean percentage occurrences of isothermals (ground based and elevated) and the pertinent parameters base, top, thickness and intensity ( C) Ground based isothermals Elevated isothermals Perce ntage (%) 1.3 Base(m) Top(m) Thick (m) Intensity.97 Percent age (%).7 Base(m) 153 Top (m) 96 Thick (m) 516 Intensity.34 Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec V References 1 Holtzworth George.C., 1974: Meteorological Episodes of Slowest dilution in contiguous United States National Environmental Research Center Office and Development.,U.S Environmental Protection Agencypp 115, A1,B1B63. Hosler Charles, R., 1961: Low level inversion frequency in the contiguous United States.Mon.Wea.Rev., 89, Sadhuram,Y (198), A Study of Dispersion of Pollutants over complex Terrain with special Reference to Visakhapatnam City, Ph.D.Thesis,Andhra university, Visakhapatnam. 4 Subrahmanyam.V.P.and Sastry.V.M., 1976: Temperature inversions and stable layers in the atmosphere over Visakhapatnam.Vaymandal, 6, 3, Vittal Murty K.P.R., V.M.Sastry and V.P.Subramanyam (1977) Seasonal and Annual Variation of Ground Concentration of Air Pollutants for Visakhapatnam. 673