Air Quality Modeling YouTube Lecture Handouts for Competitive Exams
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Source of Pollution can be point (vent), area (pond) or volume (conveyor)
Mathematical relationship between emissions and air quality that incorporates the transport, dispersion and transformation of compounds emitted into the air.
Input data required for Air Quality Modeling

Source characteristics

Meteorological conditions

Site and surrounding conditions
Types of Air Quality Modelling
Gaussian Plume Model

Steadystate conditions  rate of emission from the point source is constant.

Homogeneous flow  wind speed is constant both in time and with height (wind direction shear is not considered).

Pollutant is conservative and no gravity fallout.

Perfect reflection of the plume at the underlying surface, i.e. no ground absorption.

The turbulent diffusion in the xdirection is neglected relative to advection in the transport direction, which implies that the model should be applied for average wind speeds of more than .

The coordinate system is directed with its xaxis into the direction of the flow, and the v (lateral) and w (vertical) components of the time averaged wind vector are set to zero.

The terrain underlying the plume is flat

All variables are ensemble averaged, which implies longterm averaging with stationary conditions.
In the lateral, or y, direction the profile shape is given by
Whilst in the vertical, or z, direction it is given by
The concentration at any point is given by
Where
steadystate concentration at a point
emissions rate,
Horizontal and vertical spread parameters, m (these are functions of distance, and atmospheric stability)
Average wind speed at stack height,
Horizontal distance from ground level,
Vertical distance from ground level, m
Effective stack height where physical stack height and
When at ground level – impact on crops
It should be noted that the maximum concentration occurs when
If only concentrations at ground level on the centreline of the plume (along the xaxis direction) are required, &
Rate of emission from the source (Q)
Prevailing wind speed (U)
Direction (x)
Height of the centreline of the plume above ground (H)
Limitations
Cannot be applied for pollutants with chemical transformations
Finding 3 Unknowns
Where and are the turbulent wind speed fluctuations (turbulence intensities)
Where “ln” is the natural log, is the release height and is the aerodynamic roughness representing different topographic ground conditions
Plume Rise
This is the path or trajectory of the plume centreline after it leaves the source. Its course depends upon atmospheric conditions and the amount of buoyancy and vertical momentum in the initial plume at the source.

Buoyancy forces causes the plume rise to vary with

Momentum forces cause the plume rise to vary with
Acceleration due to gravity
Stack exit velocity
Exit gas diameter
Stack gas exit temperature
Ambient air temperature
Buoyancy Flux
Momentum Flux
When we consider only momentum (unstable & neutral conditions)
Where Diameter of the source, e.g chimney exit diameter
Initial vertical velocity of the plume
For Calm Conditions
For wind speeds
The plume rise for a jet is computed as follows:
Buoyant plumes
For Unstable and Neutral conditions
Downwind distance to the point of final rise
For Stable conditions
Mayank