After reading this article you will learn about the application of diffusion climatology in the EIA of industries.
According to B. Padmanabhamurty, environmental impact is defined as any change in the environment that is caused by an activity or a factor.
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The change may be physical, chemical, biological, social or economic. The environmental change may be caused directly or may result secondarily after a series of events. It is therefore, essential to determine the type and intensity of impact resulting from any activity before it is being implemented.
Environmental Impact Assessment (ELA) is a macro-scale study to delineate various beneficial and deleterious effects of the proposed activity on the environment. EIA is of immense use in anticipating adverse effects on the environment and consequently enables in selecting environmentally compatible sites.
The impact on the air quality in one of the major consequences of any industry. The construction and operation of an industry cause emissions of gaseous and particulate air pollutants.
Following steps are to be taken in order to predict the air quality and EIA due to any proposed industry:
(i) Abatement strategies
(ii) Atmospheric emissions
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(iii) Air quality standards
(iv) Background air quality
(v) Meteorological data
(vi) Carrying capacity of the atmosphere
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(vii) Micro-scale impact
(viii) Emission Inventory and
(ix) Mesoscale impact.
Diffusion climatology deals with the aggregate of the meteorological events such as occurrence of various stability classes, mixing heights, ventilation coefficients and air pollution potential. These parameters determine the carrying capacity/assimilative capacity of the atmosphere.
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From these studies, the emission schedule for a particular location can be determined, which will be useful for the planners. Here some aspects of diffusion climatology like stabilities, mixing heights, ventilation coefficients, pollution potential (carrying capacity) of Delhi’s atmosphere which is a part of EIA study are determined.
Materials and Methods:
The representative months, winter (January), Pre-Monsoon (April) Monsoon (August) and Post-Monsoon (October) respectively for the five year period (1983-87) is considered for the present investigation. Required meteorological data was collected from India Meteorological Department and analysed.
(i) Atmospheric Stability:
With the routine meteorological observations, atmospheric stability is estimated by Pasquill’s method modified by Turner and an additional stability class followed by Holzworth.
The stability classed in this system are:
Extremely unstable (A);
Moderately unstable (B);
Slightly unstable (C);
Neutral (D);
Slightly table (E);
Moderately stable (F); and
Extremely table (G).
(ii) Diurnal Variation of Mean Mixing Heights:
Mixing height is the vertical extent above the ground through which vigorous mixing occurs due to mechanical and buoyancy forces. Holzworth’s method incorporating heat island intensities (Padmanabhamurty) and Bahl to determine the mixing height is followed:
(iii) Diurnal Variation of Mean Ventilation Coefficients:
Ventilation coefficient is a product of mixing height and mean wind speed through the mixed layer. The mean wind speed through the mixed layer form OOZ Radiosonde observation is calculated and multiplied with the mixing heights obtained for each hour. In these calculations, precipitation days are avoided.
(iv) Air Pollution Potential:
According to Gross (1977), the criteria for forecasting high pollution potential are that the morning mixing heights should be ≤ 500 m and transport wind speed 4 ms-1 and afternoon ventilation coefficients should be 6000 m2 s-1 and transport wind 4 m s-1.
Results and Discussion:
The percentage frequencies of occurrence of seven stability classes during every synoptic hour in the four months are presented in the Tables 1 to 4.
It is observed that unstable conditions during night time and stable conditions during day time have not occurred in all the four months. It is in agreement with that of Holser, and Padmanabhamurty and Tangirala.
Highly unstable conditions have occurred only in the afternoon with high frequencies in all the months. Highly stable conditions have occurred only during night especially after 2330 1ST. Among the four months, October and January experience high frequencies of “G” stability during the night which is conducive for accumulation of pollutants. August experiences minimum frequency of “G” stability.
During day time, “A” and “B” stability which are favourable for better dispersion of pollutants are observed with high frequencies in April and October, while “C” and “D” are prominent in January and August.
Diurnal variation of mean mixing heights is presented in Tables 5 to 8. Of the four months, in the mornings, lowest mixing heights are observed in January and highest in August. In the afternoon, highest mixing heights are observed in April and the lowest in August.
Mixing height ranged between 184 m and 1640 m in January; 274 m and 3616 m in April; 426 m and 1501 m in August; and 282 m and 2460 m in October during the course of the day.
Diurnal variation of mean ventilation coefficients is presented in Tables 9 to 12. As in the case of mixing heights, the lowest morning ventilation coefficients are observed in January and the highest in August. The highest afternoon ventilation coefficients are in April and the lowest in August.
Applying the criteria of gross, it is found that morning periods have high pollution potential in all seasons and potential decreases gradually with time by afternoon. It is also found that no pollution exists in the afternoon periods.
Good vertical diffusion of pollutants occurs during day time in April and October owing to higher frequencies of unstable conditions and higher values of mixing heights and ventilation coefficients.
On the other hand, poor dispersal of pollutions occurs during night in all the months expect in August. Especially in August, high pollution potential is non-existent. From these results, the emission schedule is prepared and presented in Table 13.
