Meteorology and Dispersion Questions and Answers

Answer: C

Correct answer: Wind speed

Ground level concentration is inversely proportional to wind speed. The wind speed \( u \) appears once in the denominator of the Gaussian plume equation, \( C = \dfrac{Q}{2\pi u \sigma_y \sigma_z} \exp(\cdot) \), because a faster wind carries the same emission rate \( Q \) through a greater length of air in the same time and so dilutes it in proportion.

Concentration is directly proportional to the emission rate, so that option has the relation the wrong way round. The dependence on \( u \) is first power, not the square, and stack diameter enters only indirectly through the plume rise.

Answer: D

Correct answer: Pollutant reflected up from the ground

The reflection term treats the ground as a perfect reflector. In the mathematics it is an image source placed as far below the surface as the real source is above it; physically it represents pollutant that reaches the ground and is turned back up into the plume instead of being absorbed or deposited.

Including it raises the calculated ground level concentration, which makes the estimate conservative - appropriate when the deposition velocity is small or unknown. It has nothing to do with radiation, with wind shear, or with deposition, which if it occurred would remove material and lower the concentration.

Answer: A

Correct answer: The physical stack height plus the plume rise

The effective stack height \( H \) is the physical stack height \( h_s \) plus the plume rise \( \Delta h \): the height at which the plume levels off and begins to travel horizontally. It is this height, not the stack top, that enters the Gaussian plume equation.

The distinction matters because maximum ground level concentration varies roughly as \( 1/H^2 \), so a hot buoyant plume from a modest stack may disperse better than a cold one from a taller stack. Plume rise comes from the momentum of the exit velocity and, more importantly, the buoyancy of the hot gas.

Answer: B

Correct answer: Buoyancy from the temperature difference

Plume rise from a hot stack is driven chiefly by buoyancy. The exhaust gas is much warmer than the surrounding air, so it is less dense and accelerates upward, continuing to rise until it has mixed with enough ambient air to reach the same temperature and level off.

Momentum from the exit velocity contributes as the gas leaves the stack, but for a hot combustion source the buoyant contribution dominates and carries the plume far higher. The pollutant's molecular weight is irrelevant at trace concentrations, and stack diameter and wind speed modify the rise rather than causing it - a stronger wind in fact reduces it by mixing the plume out faster.

Answer: C

Correct answer: Strongly unstable conditions

Looping is the plume of a strongly unstable, superadiabatic atmosphere. Large convective eddies of a scale comparable with the plume itself carry whole sections of it up and down, so the plume traces a wavy, looping path downwind.

Dilution overall is rapid, but the loops can bring undiluted plume to the ground close to the stack, producing brief high concentrations there. Neutral conditions give the smooth cone of coning, and strongly stable conditions the flat horizontal ribbon of fanning.

Answer: D

Correct answer: A strong inversion suppresses vertical motion

Fanning occurs in a strong inversion. Vertical motion is suppressed, so the plume spreads horizontally into a thin ribbon that meanders with the wind but hardly grows in depth, and viewed from above it fans out while remaining flat in section.

Because the plume stays aloft, ground level concentrations near the source are very low - until the inversion breaks, when fumigation can bring the accumulated material down abruptly. Strongly unstable conditions produce looping instead, and neither wind speed nor stack height defines the fanning form.

Answer: A

Correct answer: A breaking inversion mixes a plume to the ground

Fumigation occurs when a nocturnal inversion breaks up in the morning. Solar heating erodes the inversion from below, and as the growing unstable layer reaches a plume that has been travelling in the stable air above, the trapped material is mixed rapidly down to the surface.

The result is a short period - typically half an hour to an hour - of very high ground level concentration, because pollutant accumulated overnight arrives all at once. A plume trapped aloft indefinitely is the fanning case, and rain scavenging is wet deposition, an entirely different removal process.

Answer: B

Correct answer: Wind frequency by direction and speed

A wind rose is a polar diagram showing how often the wind blows from each direction, with the spokes usually divided into bands by speed. The length of each spoke gives the frequency from that sector, so the prevailing directions are immediately visible.

It is fundamental to siting decisions: the sector downwind of a source most often is the one exposed most often, and monitoring stations and new development are placed accordingly. It carries no information about temperature profiles, stability, or how concentration varies with distance.

Answer: C

A displaced parcel then remains at the same temperature as its surroundings and neither rises further nor sinks back. Overcast and windy conditions produce it, and it is the reference case against which the others are defined.

Answer: D

Correct answer: At a finite distance downwind

The maximum ground level concentration from an elevated source occurs some distance downwind, not at the stack. The plume leaves at an effective height \( H \) and must first grow vertically before any of it reaches the surface, so the ground level concentration rises from nearly zero, peaks where the vertical spread \( \sigma_z \) has become comparable with \( H \), and then falls again as continued dilution takes over.

The distance to the peak depends on stability: it is close to the stack in unstable conditions, when the plume grows quickly, and far downwind in stable conditions. Directly beneath the stack the concentration is very low.