Plume Model, Role of Meteorological Parameters for the Transportation

Plume model and diffusion of the pollutants

The atmospheric temperature profile affects the dispersion of the pollutants from a smokestack. Plume refers to the path and the extent in an atmosphere of the gases effluent release from a source. There are six types of plume behavior which determine the diffusion level of the pollutants.

1: Coning

Conning occurs when the atmosphere is neutrally stable. Its shape is symmetrical and the cone-like. It occurs with vertical temperature gradient which is sub-adiabatic.

2: Looping

Looping occurs when the atmosphere is very unstable and there is rapid vertical air movement i.e both up and down. The high degree of the turbulence helps in the dispersing the pollutants rapidly. Here vertical temperature gradient is super-adiabatic.

3: Fanning

Fanning occurs when a stable atmosphere greatly restricts the dispersion of the plume in the vertical direction it occurs when the temperature gradient is positive so it spreads only horizontally.

4: Lofting

Lofting occurs when an inversion exists only below the plume and the plume is inhibited from mixing downward and mixing in the upward direction is uninhibited. Such loading helps to keep the pollution high above the ground.

5: Fumigation

Fumigation occurs when the plume is under an inversion layer, emission move downward much more easily than upward.

6: Trapping

Trapping occurs when the plume is caught between the inversion and can only diffuse within a limited vertical height.

The lofting plume is most favorable with respect to the minimizing air pollution. The fumigation and the trapping plume are very critical from the ground level point of view in pollutants concentration.

Role of meteorological parameters for the transport and diffusion of pollutants

Influence of the meteorological parameters is the greatest parameters during the transport and the diffusion of the air pollutants. The motions of the atmosphere which may be highly variable are responsible for the transport and the diffusion of the air pollutant. Winds carry the pollutants away from the single source and produce the leeward plume that experiences a progressive decrease in the intensity of the pollutants. Precipitation reduces the air pollutants by washout effects. Pollutants in the air are removed by the rain diluted by the wind and unstable air.

The meteorological parameters that affect the atmospheric pollutants can be grouped into two categories are primary parameters and the secondary parameters. The wind speed direction and the atmospheric turbulence are under the primary meteorological parameters, whereas the temperature precipitation humidity solar radiation pressure visibility are the secondary parameters. The primary parameters are responsible for the dispersion and dilution of the pollutants whereas the secondary parameters affect the primary parameters.

1: Wind direction

Wind direction represents the direction of the travel of the pollutants.

2: Wind speed

Wind speed affects the pollutants in two ways, first wind speed determines the travel time from a source to given receptor and the second effect of dilution in a downwind direction. The concentration of the air pollutants is inversely proportional to the square of the speed of the wind.

3: Turbulence

Turbulence is induced by the stability of atmosphere when the earth surface is heated by the solar radiation the lower layer of the atmosphere becomes unstable and the thermal turbulence becomes greater under the condition of the light winds. In the clear night with the light wind, heat is radiated from the earth surface resulting in the cooling of the atmosphere near the earth surface. Under this condition, the turbulence is minimum.

Similarly, temperature, pressure, humidity, pressure, insolation, precipitation, visibility are secondary parameters which influence the air pollutants.

Topographic effects of air pollution

Air pollution, meteorology deals with the transport and the diffusion of air pollutant released from stake i.e chimney, flash fire and from the automobiles, wind is only a means of transport for any kind of the pollutants. Besides this stability of the atmosphere, daylight conditions and the season also influences it. The topography of a region plays the great role for the transportation of the pollutants. Oceans, mountains valleys and the building will influence the air motion and consequently the transport of the pollutants. If we examined the topography of it. However, it is assumed that at the center of the Kathmandu valley there is a big connective phenomenon called the chimney from where the tremendous amount of the polluted air dissipated away from it. The effect of the topography is explained in the detail below:

1: Ocean wind:

Surface topography can affect the sea breeze either in the shore or off shore because of the unequal heating of the land and the water body. During that time, the sea breeze is developed and reaching the peak in the afternoon. During day time, land absorbs more heat than that of the sea i.e ocean and warm the air that moves over the land and cool air replaces that warm air. Similar and opposite current moves from land to the warm ocean producing the land breeze during night.

If the power plants are located on the sea coast the stake influence will be dispersed over the land during the day time by the sea breeze and may be subjected to fumigation dispersion of pollutants towards ground level. An opposite behavior persists during the night.

2: Mountain valley wind:

During daytime, air flow over the base and the mountains slope is heated up by the solar radiation begins to rise and flow over a valley and the mountain slope. After the sunset, the earth surface losses the solar radiation resulting in the cooling of the earth surface and air surroundings gives rise to the change in the density of the air. At the night the air along the valley slope become dense and cooler air moves downward into the valleys. This cold and dense air occupies the valley warm air occupies the high altitudes of the valley so the stable condition prevails that leads to the formation of the inversion i.e trapping of the pollutants at a height and they cannot move by the stable condition. During the day time, the pollutants will move or dispersed up to the valley. Only then they return to the valley at the night since the wind shifts towards the lower end of the valley, this circulation of the wind may give rise to build up of the pollutants concentration up to the dangerous level.

3: Obstacles like buildings:

The factors like the characteristics of the land surroundings a stake of the location of the non-uniform height of the buildings relative to the stake height influences the behavior of plume and formation of the mechanical turbulence. Two different types of the conditions are presented below:

A: Location of stake behind the hill or building:

Location of the stake or chimney behind a hill may allow the effluent to the downwards but the plume may rise above the stock level. If the stake gas velocity (Vs) is equal to or less than the ambient air velocity (Va) the pollutants may be varied downward on the backside the stake. The backwash of flow is behind the structure. Backflow can offer within the cavity, the pollutants entered into this region tend to remain there. If flow reverses in its direction near the ground level separation can occur on the backside of the buildings.

B: Location of stake before the hill or buildings;

When a stake is placed before the upward of the hill and buildings it will cause an effect on the plume behaviors. If the stake height is more it will help to disperse and transmits the pollutants.

In case if the stake is not sufficiently high the plume enters the cavity. High pollutants concentration accumulates along the back side of the buildings hence the changes have to be visualized about the influenced of the topography in the fixing the stake height. For safety, the stake height near height near the building should be 2 to 2.5 times the height of the surrounding objects.

References:

.S.C., Santee. Environmental Science. India, New Center: New Center Book Agency (P) Ltd, 2004.

Lal, D.S. Climatology, Sharda Pustak Bhawan, Allahabad.(2010)