Question 2 of 7: Meteorology, Atmospheric Stability and Stack Plume Behaviour

Problem 2: Meteorology, Atmospheric Stability and Stack Plume Behaviour (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (i) — stability classes. Stable conditions typically occur at night under clear skies and low wind speed: strong radiative cooling of the ground creates a surface temperature inversion (temperature rising with height), which suppresses vertical air motion; pollutants disperse poorly and can travel long distances near the ground with little dilution. Extremely unstable (superadiabatic) conditions occur on clear days with strong solar heating and light wind: the ground heats rapidly, producing a steep lapse rate and strong convective (thermal) turbulence; vertical dispersion is very effective, but the resulting large eddies can intermittently bring high concentrations to the ground close to the source. Neutral conditions occur under overcast skies and/or moderate-to-strong wind, when the environmental lapse rate is close to the dry adiabatic lapse rate ($-9.8\ ^{\circ}\text{C/km}$): vertical mixing is moderate and fairly uniform, giving steady, predictable dispersion.

Part (ii) — lapse rate and stability. The stability of an air mass is set by comparing its environmental (actual) lapse rate to the dry adiabatic lapse rate: a rising parcel that cools faster than its surroundings (environmental lapse rate less than adiabatic, or an inversion) sinks back — a stable atmosphere; a parcel that cools more slowly than its surroundings (environmental lapse rate greater than adiabatic) keeps rising — an unstable atmosphere. This comparison directly sets the depth of the atmospheric mixing layer available to dilute pollutants: a deep unstable layer dilutes emissions over a large volume, while a shallow, capped stable layer (e.g. beneath an elevated inversion) traps pollutants near the surface and is the dominant cause of severe, multi-day smog episodes.

Part (iii) — two plume behaviours. Looping occurs under strongly unstable (superadiabatic), sunny, light-wind daytime conditions: large convective eddies carry the plume in an undulating, looping path, occasionally bringing high but short-duration concentrations to ground level close to the stack. Fumigation occurs in the one to two hours after sunrise, as a shallow surface-based unstable (convective) layer grows upward into a plume that was emitted overnight into a stable layer aloft; when the growing unstable layer reaches the plume's height, the plume is rapidly mixed straight down to the ground, producing the highest short-term ground-level concentrations of any plume type — a critical design condition for stack-height selection.