Meteorology and Dispersion Questions and Answers

Answer: B

Correct answer: Statement I is incorrect but Statement II is correct

Statement I is incorrect. The maximum ground level concentration from an elevated source occurs at a finite distance downwind, not beneath the stack, where the concentration is in fact very low.

Statement II is correct and explains why: the plume leaves at an effective height and must grow vertically before any of it reaches the surface. The ground level concentration therefore rises with distance, peaks where the vertical spread has become comparable with the effective stack height, and then declines as further dilution takes over. The peak is close in under unstable conditions and far downwind under stable ones.

Answer: C

Correct answer: 1, 2, 3, 4

The calculation proceeds in the order in which each quantity becomes available. First the Pasquill stability class is assigned from wind speed and insolation or cloud cover (1), because it governs everything that follows. That class then selects the dispersion coefficient curves, from which \( \sigma_y \) and \( \sigma_z \) are read at the downwind distance of interest (2).

Next the effective stack height is computed from the physical height and the plume rise (3), giving the \( H \) that enters the vertical term. Finally the concentration is evaluated, including the reflection term for the ground (4). The class must come first; nothing else can be looked up until it is fixed.

Answer: D

Correct answer: 1, 2, 3, 4

The sequence follows the night. After sunset the ground loses heat by radiation to the sky and cools (1), and because it cools faster than the air, the layer immediately above it becomes colder than the air higher up, so a shallow inversion forms and deepens through the night (2).

After sunrise the ground absorbs solar radiation and begins to warm (3), and the warmed surface heats the air in contact with it, so the inversion is eroded from below and the mixing height rises (4). The erosion always proceeds upward from the ground, which is why fumigation reaches plumes in order of increasing height.

Answer: A

Looping belongs to strong convection, coning to neutral conditions, lofting to a surface inversion with unstable air above, and fanning to a deep stable layer. The plume's appearance is a direct readout of the stability profile.

Answer: B

Correct answer: 1, 2, 3, 4

The chain runs from the synoptic cause to the observed effect. A stagnant high pressure system establishes over the region (1); within it air descends and, being compressed, warms adiabatically to form a cap over the mixed layer (2).

That cap lowers the mixing height, and the weak pressure gradients of a stagnant high leave the wind light (3), so the ventilation coefficient collapses. With emissions continuing into a small, poorly ventilated volume, concentrations build over successive days (4). The persistence of the high is what makes such episodes last, unlike a nocturnal inversion that breaks each morning.

Answer: C

Correct answer: 1, 2, 3, 4

The contributions act in the order the gas experiences them. Momentum from the exit velocity gives the initial upward thrust as the gas leaves the stack (1). As the jet slows, buoyancy from the temperature difference between the hot gas and the ambient air takes over and carries the plume higher (2).

Entrainment of ambient air gradually cools the plume until it reaches the surrounding temperature and buoyancy is exhausted, so the plume levels off (3) - this is the top of the plume rise, and the height at which it occurs defines the effective stack height. From there the plume is simply carried downwind at that height (4).

Answer: D

Correct answer: Only three

Three pairs are right. Strongly unstable conditions give looping, as large convective eddies carry the plume up and down; neutral conditions give the smooth cone of coning; and strongly stable conditions give the flat ribbon of fanning.

The fourth is wrong. A surface inversion with unstable air above gives lofting: the plume disperses upward into the unstable layer while the inversion below keeps it from reaching the ground, which is the most favourable condition of all. Fumigation is the reverse arrangement - unstable air below and a stable layer above - which brings the plume down to the surface.

Answer: A

Correct answer: Only three

Three pairs are right. The dispersion coefficients \( \sigma_y \) and \( \sigma_z \) are read from curves indexed by stability class and downwind distance; the ventilation coefficient is the mixing height times the mean wind speed; and the effective stack height is the physical height plus the plume rise.

The fourth is wrong. The Pasquill class depends on wind speed together with insolation by day or cloud cover by night - properties of the atmosphere, not of the source. Emission rate and stack diameter are source characteristics and enter the calculation elsewhere, through \( Q \) and through the plume rise.

Answer: B

Correct answer: Only three

Three pairs are right. A radiation inversion is caused by nocturnal cooling of the ground; a subsidence inversion by air descending and warming adiabatically in a high pressure system; and a frontal inversion by warm air overrunning cold air at a front.

The fourth pair is wrong because it assigns the subsidence mechanism to a radiation inversion. The two differ in more than cause: a radiation inversion is shallow, based at the ground and gone within hours of sunrise, whereas a subsidence inversion is elevated, regional and may persist for days - which is why the severe episodes are associated with subsidence.

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.