Atmospheric Stability Questions and Answers

Answer: C

Classical Gaussian models use empirical dispersion coefficients tabulated against the Pasquill stability class and the downwind distance, which is why the classification exists at all. Modern models use the Obukhov length and the friction velocity directly, which makes the last option the contemporary rather than the classical answer. Both approaches parameterise the same physical spreading, the newer one with a firmer theoretical basis and the older one with the merit of requiring only routine observations.

Answer: D

The model assumes the concentration is normally distributed about the plume centreline in both the crosswind and the vertical, with standard deviations that grow with downwind distance. Reflection at the ground and at the mixing height is handled by image sources. Real plumes are skewed in convective conditions, which is a recognised limitation the newer models address. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

The model assumes the wind is constant in direction and speed and the turbulence uniform, which fails over complex terrain, near coastlines and during the morning and evening transitions. Ground level sources are handled by placing the source at zero height. Chemistry is a genuine additional limitation for reactive species but the meteorological assumptions bite first. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

Releasing large numbers of computational particles into a modelled wind field and tracking them individually handles arbitrary terrain and time varying flow, at much greater computational cost. Computational fluid dynamics resolves the flow around individual buildings. Adjusting Gaussian coefficients cannot represent flow separation or channelling, which are precisely the effects complex terrain produces. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

Emergency response for volcanic eruptions, nuclear accidents and chemical releases relies on Lagrangian models driven by forecast winds, since the plume must be tracked over hours to days across large distances. The Eyjafjallajokull eruption made this capability widely known. Long term average exposure is usually estimated with simpler and cheaper approaches. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

Errors in wind direction translate directly into errors in the position of the plume, and because the wind varies with height an error in the plume altitude compounds the problem. This is why ash forecasts are issued as probability fields rather than as single trajectories. Temperature and humidity matter for the plume rise and for wet removal but contribute far less to the positional uncertainty. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

Stable marine air crossing a heated land surface develops a growing unstable internal boundary layer, and a plume released above it fans until the layer grows to intercept it and then fumigates suddenly. Coastal fumigation is a recognised hazard for shoreline industrial sites. The marine air is modified rapidly rather than continuing unchanged.

Answer: B

A stack whose plume is intercepted by the internal boundary layer a few kilometres inland will fumigate the ground there, so the release height must be chosen with the expected growth of that layer in mind. Detailed modelling of the coastal transition is standard for such sites. A short stack would release directly into the layer and give high concentrations immediately. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

Particles serving as condensation nuclei are incorporated into cloud droplets and removed when the droplets precipitate, which is rainout, while falling drops also collect particles beneath the cloud, which is washout. Together these are far more effective than dry deposition for fine particles. Sedimentation is negligible for particles below about a micrometre, since their fall speeds are measured in millimetres per hour, which is why they persist for a week or more in the absence of precipitation.

Answer: D

Particles in the accumulation mode are too large to diffuse rapidly to surfaces and too small to sediment or to be efficiently intercepted, so they persist for a week or more and travel intercontinentally. Ultrafine particles coagulate quickly and coarse particles settle out within hours. The resulting gap in the removal efficiency is why the accumulation mode dominates the aerosol mass at long range.