Meteorology and Dispersion Questions and Answers

Answer: D

Correct answer: Reducing the exit velocity

Reducing the exit velocity would not reduce ground level concentration - it raises it. The exit velocity supplies the momentum contribution to plume rise, so cutting it lowers the effective stack height, and since the maximum varies roughly as \( 1/H^2 \) the ground level concentration goes up. A very low exit velocity can also let the plume be drawn into the downwash behind the stack.

The other three all help. A taller stack raises \( H \) directly, a hotter exhaust increases the buoyancy that drives plume rise, and a windier day dilutes the emission through more air per second.

Answer: A

Correct answer: The ground level concentration

The ground level concentration is the model's output, the quantity being predicted. Everything else listed must be supplied first: the emission rate \( Q \) from the source, the wind speed \( u \) from measurement, and the dispersion coefficients \( \sigma_y \) and \( \sigma_z \) from the Pasquill-Gifford curves at the stability class and distance in question.

The distinction is worth keeping clear when a model disagrees with monitoring data, since the discrepancy must lie in one of the inputs or in the model's assumptions, never in the output itself.

Answer: B

Correct answer: Unwise: it reduces plume rise

The proposal is unwise. Exit velocity supplies the momentum contribution to plume rise, so reducing it lowers the effective stack height, and because the maximum ground level concentration varies roughly as \( 1/H^2 \) the penalty is more than proportional. The fan power saved is bought at the cost of dispersion.

There is a second hazard: below roughly \( 1.5 \) times the wind speed, the plume can be caught in the low pressure wake behind the stack and dragged down its lee side, so that a tall stack behaves like a much shorter one. The problem is not confined to stable conditions, and a tall stack does not exempt it.

Answer: C

Correct answer: It varies enormously through the day

This is the true statement. The ventilation coefficient is mixing height times mean wind speed, and both factors follow a strong diurnal cycle: the mixing height may be a hundred metres or so at dawn and well over a thousand by mid-afternoon, while the wind usually strengthens with the daytime mixing. Swings of a factor of forty between dawn and afternoon are routine.

The coefficient describes the atmosphere's capacity to disperse and does not involve the emission rate at all. A high value indicates good dispersion, not poor, and it is anything but constant.

Answer: D

Correct answer: Looping plume behaviour

Looping would not be expected on a clear calm night. It requires strongly unstable, superadiabatic conditions produced by solar heating of the ground, so it is a daytime form.

The other three are exactly what a clear calm night produces. The ground radiates heat to the cloudless sky and cools, forming a radiation inversion; that inversion caps the layer and gives a low mixing height; and the strong stability suppresses vertical spread, so a plume takes the thin flat form of fanning.

Answer: A

Correct answer: It varies from day to day with the weather

This is the false statement. The dry adiabatic lapse rate is a constant of about \( 9.8 \) degrees per kilometre, following from thermodynamics - the rate at which an unsaturated parcel cools as it rises and expands against falling pressure. It does not change with the weather.

The other three are true: the value is about \( 9.8 \) degrees per kilometre, it describes a parcel raised without heat exchange, and it is compared with the environmental rate to assess stability. It is the environmental lapse rate that varies from hour to hour, and the whole stability judgement rests on comparing that variable profile against this fixed benchmark.

Answer: B

Correct answer: The pollutant's chemical formula

The chemical formula is the least relevant. The Gaussian plume model treats the pollutant as a passive, conserved tracer, so the concentration field depends on the emission rate and the meteorology but not on what the molecule is. Two different gases emitted at the same rate under the same conditions give the same predicted concentration.

The other three all enter directly: wind speed appears in the denominator, effective stack height in the vertical exponential term, and stability class selects the dispersion coefficients. Chemistry matters for reaction, deposition and health effects - but those are additions to the basic dispersion calculation, not part of it.

Answer: C

Correct answer: A breaking inversion brings a plume down

This is the true statement. Fumigation is precisely the rapid downward mixing that occurs when a nocturnal inversion breaks up and the growing unstable layer reaches a plume travelling in the stable air above.

It therefore occurs in the morning, in the hour or so after sunrise, not in the late evening - the evening is when the inversion forms. Far from dispersing pollution well, it produces some of the highest short-term ground level concentrations recorded, because material accumulated overnight arrives at once. Removal of a plume by rain is wet scavenging, a quite separate process.

Answer: D

Correct answer: The plume must grow before reaching the ground

The plume leaves at an effective height \( H \) and travels horizontally while turbulence spreads it. None of it reaches the surface until the vertical spread \( \sigma_z \) has grown to a size comparable with \( H \), which takes a finite distance. The ground level concentration therefore rises from nearly zero, peaks where that condition is met, and falls again as continued dilution outweighs the increasing share of plume reaching the ground.

The wind does not carry all the material away - it carries it downwind at the effective height, where it is still spreading vertically. The stack absorbs nothing, and the ground level concentration at the base is very small rather than exactly zero. How far downwind the peak lies depends on stability: close in under unstable conditions, when the plume grows quickly, and far out under stable ones.