Atmospheric Stability Questions and Answers

Answer: A

In very unstable conditions large thermals carry the plume up and down in a looping pattern, bringing high concentrations to the ground intermittently close to the stack. Fanning occurs in stable conditions, coning in neutral ones and lofting where a stable layer lies beneath an unstable one. The plume shape is a direct visual indicator of the stability class. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

A stable layer suppresses vertical mixing entirely, so the plume spreads horizontally into a thin fan while remaining at its release height. Ground level concentrations near the source are very low. The hazard comes later, when morning heating erodes the inversion from below and brings the whole plume down in a fumigation event.

Answer: C

When morning heating erodes a nocturnal inversion up to the level of a plume that has been fanning through the night, the accumulated pollutant is mixed rapidly to the ground over a wide area. Fumigation episodes produce the highest short term concentrations in classical dispersion theory. Lofting is the safest configuration, since the stable layer beneath keeps the plume aloft. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: D

Releasing above the mixing height allows the plume to disperse in the free atmosphere and to loft rather than fumigate, which was the rationale for the tall stacks built in the mid twentieth century. Momentum and buoyancy of the exhaust add to the effective height, so a narrow hot fast release rises further. The strategy transferred the problem to longer range transport, which is why it was later criticised. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

Raising the release height reduced local concentrations but allowed sulphur and nitrogen compounds to travel hundreds of kilometres before deposition, producing acid rain in Scandinavia from British and German sources. Emissions were unchanged in quantity. The episode is a standard illustration that dispersion is not the same as removal. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Buoyant and momentum driven rise adds substantially to the physical stack height, often doubling it, and formulae by Briggs are used to estimate the addition. The rise depends on the exhaust temperature and velocity and on the ambient stability and wind. Neglecting it would badly overestimate ground level concentrations.

Answer: C

Strong wind bends the plume over rapidly and dilutes its buoyancy, limiting the rise, which is why the highest ground level concentrations in neutral conditions occur at moderate to strong winds. Calm conditions allow maximum rise. High exhaust temperature increases the buoyancy and therefore the rise, and instability enhances the rise rather than suppressing it. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: D

The capping inversion at the top of the boundary layer acts as a lid, and once the plume has spread through the full mixing depth further dilution occurs only horizontally. The mixing height therefore appears directly in the concentration formulae. The tropopause is far too high to be relevant and cloud scavenging removes rather than confines the material. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: A

Surface heating deepens the mixed layer from a few hundred metres at dawn to one or two kilometres by mid-afternoon, and it collapses to a shallow stable layer within an hour of sunset. Pollutant concentrations follow the inverse pattern. The morning transition, when a growing mixed layer meets a plume aloft, is when fumigation occurs. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

Aerosol is concentrated within the mixed layer and drops sharply above it, so a backscatter profile shows the mixing depth directly and continuously. Ceilometers installed for cloud base measurement double as mixing height sensors. Twice daily radiosondes cannot resolve a cycle that changes on a timescale of an hour.