Meteorology and Dispersion Questions and Answers

Answer: C

Correct answer: Only three

Three pairs are right. A higher wind speed lowers concentration, since \( u \) sits in the denominator; a greater effective stack height lowers the maximum, which varies roughly as \( 1/H^2 \); and a shallower mixing height raises concentration by reducing the volume available for dilution.

The fourth is wrong in its detail. More unstable conditions raise concentrations close to the stack, because the plume grows quickly and reaches the ground sooner, but they lower them far downwind, since the same vigorous mixing dilutes the plume rapidly thereafter. Instability moves the peak nearer and makes it higher, not the reverse.

Answer: D

Correct answer: Only three

Three pairs are right. The mixing height is the depth through which pollutants disperse readily; the ventilation coefficient is the mixing height times the mean wind speed; and the environmental lapse rate is the actual measured change of temperature with height.

The fourth is wrong. The dry adiabatic lapse rate is not an observed profile at all but a theoretical rate - about \( 9.8 \) degrees per kilometre - at which an unsaturated parcel cools as it rises and expands. It is the yardstick against which the environmental rate is compared to judge stability, so defining both as the actual change makes the comparison meaningless.

Answer: A

Correct answer: Effective stack height

The effective stack height is a property of the source: it is the physical height of the stack plus the plume rise produced by the exit momentum and buoyancy of that particular exhaust. Change the stack or the gas it emits and the value changes.

The other three describe the atmosphere at the time. The Pasquill class summarises its turbulent state, the mixing height gives the depth available for dilution, and the environmental lapse rate is the observed temperature profile. This division matters in modelling: source terms and meteorological terms enter the calculation separately, and only the source terms are under the operator's control.

Answer: B

Correct answer: A greater effective stack height

A greater effective stack height lowers ground level concentration, and does so strongly - the maximum varies roughly as \( 1/H^2 \), so a 40 per cent increase almost halves it. The plume is released higher, so it must spread through more depth before any reaches the surface, and it is much diluted by the time it does.

The other three all raise concentration. A shallower mixing height reduces the volume available for dilution, a lower wind speed means the same emission is spread through less air per second, and a stronger inversion suppresses the vertical mixing that would otherwise disperse the plume.

Answer: C

Correct answer: Adiabatic inversion

There is no such thing as an adiabatic inversion. Adiabatic describes a process without heat exchange, and the dry adiabatic lapse rate - a fall of about \( 9.8 \) degrees per kilometre - is the opposite of an inversion, in which temperature rises with height.

The other three are the standard types. A radiation inversion forms when the ground cools by radiation on a clear calm night; a subsidence inversion forms aloft where descending air in a high pressure system is compressed and warms; and a frontal inversion forms where warm air overruns colder air at a front.

Answer: D

Correct answer: Ground level concentration

The ground level concentration is what the Gaussian model computes - the output. Everything else in the equation must be supplied before it can be evaluated.

The emission rate \( Q \) comes from the source, the wind speed \( u \) from measurement, and the dispersion coefficients \( \sigma_y \) and \( \sigma_z \) from curves indexed by the stability class and the downwind distance. Keeping the distinction clear matters when a model is being checked: a disagreement between predicted and measured concentration is a question about the inputs, not about the output.

Answer: A

Correct answer: Looping plume behaviour

Looping is a daytime phenomenon. It requires strongly unstable, superadiabatic conditions, which are produced by solar heating of the ground on a sunny day with light winds; the resulting large convective eddies carry sections of the plume up and down.

The other three belong to the night. A radiation inversion forms as the ground cools under a clear sky, fanning is the plume form produced by the strong stability of that inversion, and accumulation beneath a shallow inversion is the consequence - pollutants confined to a thin layer through the night, ready to be brought down by fumigation after sunrise.

Answer: B

Correct answer: A radiation inversion traps, then fumigates

The pattern is the signature of a radiation inversion. On clear calm nights the ground cools by radiation, a shallow inversion forms, and emissions released beneath it are confined to a thin layer and accumulate - giving the first peak. After sunrise the ground warms, the inversion erodes from below, and material that was held aloft is mixed rapidly to the surface, giving the second peak within the hour.

Wind speed is typically lowest, not highest, on such nights, which is part of why the inversion forms. Neither an overnight change in emissions nor instrument drift would produce a peak specifically in the hour after sunrise.

Answer: C

Correct answer: Strongly stable conditions

A thin flat ribbon with almost no vertical spread is fanning, the plume form of a strongly stable atmosphere. Vertical motion is suppressed because a parcel displaced upward becomes colder and denser than its surroundings, so the plume spreads horizontally by meandering but scarcely grows in depth.

Ground level concentrations near the source are very low while this lasts, which can be deceptive - the material is accumulating aloft and may arrive at the surface all at once when the inversion breaks. Neutral conditions give the gently expanding cone of coning, and strongly unstable conditions the vigorous looping form.

Answer: D

Correct answer: Raise effective stack height by 40 per cent

Since the maximum ground level concentration varies roughly as \( 1/H^2 \), raising the effective height by 40 per cent reduces it by a factor of \( 1.4^2 = 1.96 \) - very nearly a halving. That is the change to make.

Halving the exit velocity does the opposite: it reduces the momentum contribution to plume rise and so lowers the effective height. Doubling the stack diameter at fixed volumetric flow also lowers the exit velocity, with the same effect. Moving the stack \( 100 \) m sideways relocates the footprint without changing the peak concentration at all.