18-Env-A5 Air Quality and Pollution Control Engineering · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; open book; Casio or Sharp approved calculator only. Four (4) of the five (5) questions constitute a complete paper (the first four answers as they appear are marked, maximum 100 marks); all five are solved below for completeness. Each question is worth 25 marks with section marks shown in brackets per the paper's own printed Marking Scheme.
Reference texts. Cooper & Alley, Air Pollution Control: A Design Approach (4th ed.); Wark, Warner & Davis, Air Pollution: Its Origin and Control (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Canadian Environmental Protection Act, 1999 (CEPA) and the Canadian Ambient Air Quality Standards (CAAQS) administered by the CCME and Environment and Climate Change Canada.
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.
The Pasquill stability classes are a six-category (A through F) scheme for characterizing the vertical mixing intensity of the lower atmosphere, from A (extremely unstable, vigorous convective turbulence) through D (neutral) to F (moderately stable, suppressed vertical motion). Composition: the class is looked up in the Pasquill–Turner table using two inputs — the 10 m wind speed and, during daytime, the incoming solar radiation intensity (a proxy for surface heating and thermally driven turbulence), or during nighttime, the fractional cloud cover (a proxy for radiative cooling and how strongly a surface inversion develops). Light wind plus strong sun gives class A/B; overcast or moderate-to-strong wind gives class D regardless of time of day; light wind plus a clear night gives class F.
Use in Gaussian dispersion models. Stability class is the key parameter selecting the empirical horizontal and vertical dispersion-coefficient curves, σy(x) and σz(x) (the Pasquill–Gifford curves), that appear directly in the Gaussian plume equation. For example, a coastal LNG terminal doing a worst-case ground-level concentration assessment for a continuous release would run the model at class F (light wind, clear night) because the slowly growing σz under stable conditions keeps the plume narrow and elevated far downwind, which for an elevated release can produce the highest ground-level concentration at a specific downwind distance; the same facility would separately run class A/B to check for high, close-in concentrations from a looping plume under unstable daytime conditions. Both bracket the design case rather than relying on a single stability assumption.
Six named plume behaviour types are commonly recognized: looping, coning, fanning, lofting, fumigation and trapping. Four are named here in answer to the "name 4" instruction — looping, coning, fanning and fumigation — and three (looping, fanning, fumigation) are selected below for a diagram and detailed description, since together they bracket the most distinct and exam-relevant stability regimes (strongly unstable, strongly stable, and the transition between the two).
Looping occurs under strongly unstable (superadiabatic) conditions — typically a sunny afternoon with light wind, Pasquill class A/B — where large convective eddies loop the plume up and down. Close to the stack the loops can bring high, but brief and intermittent, concentrations down to ground level; farther downwind the vigorous turbulence dilutes the plume rapidly, so the time-averaged ground-level concentration falls off quickly with distance despite the short-lived close-in peaks.
Fanning occurs under a strong low-level temperature inversion (very stable, class F) — typically a clear, calm night. Vertical turbulence is almost completely suppressed, so the plume spreads only horizontally, forming a thin, coherent ribbon at stack height. Because it does not mix down, ground-level concentration directly under the plume is very low near the stack; the practical hazard is instead that the narrow ribbon can travel a long distance intact before it is finally diluted, potentially affecting a receptor far downwind, or the base of a hill/building it happens to intersect.
Fumigation is the transition case: a plume released into a stable layer (fanning conditions persist aloft, e.g. overnight) becomes trapped under a growing convective layer as the morning sun breaks up the surface inversion from below. The plume, elevated and undiluted from the night before, is caught by the newly unstable layer beneath it and rapidly mixed all the way to the ground over a short distance, producing the highest short-term ground-level concentrations of any of the plume types — it is the classic worst-case morning event for a facility near sensitive receptors.
A receptor model, specifically the Chemical Mass Balance (CMB) model, works in the opposite direction to a dispersion model. Rather than starting from known emission rates and predicting downwind concentrations, CMB starts from measured ambient concentrations of many chemical species at a monitoring ("receptor") site and a library of source emission "fingerprints" (the fractional chemical composition characteristic of each candidate source type, e.g. diesel exhaust, wood smoke, road dust, secondary sulfate). It solves, by least-squares, for the linear combination of source contributions that best reproduces the observed ambient chemical profile. This makes it especially useful for source apportionment in an airshed with many overlapping, poorly characterized emitters (e.g. urban PM2.5), where building a reliable forward emissions inventory for a dispersion model would be impractical.
