18-Env-A5 Air Quality and Pollution Control Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; closed book with a candidate-prepared 8.5×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five (5) questions constitute a complete paper (the first five answers as they appear are marked); all seven are solved below for completeness. Each question is worth 20 marks with section marks shown in brackets.
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 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.
Given. Combustion of a sulfur-bearing hydrocarbon fuel with stoichiometric oxygen:
| Quantity | Symbol | Value |
|---|---|---|
| Hydrocarbon fuel | — | C7H13 |
| Moles of hydrocarbon | $n_{HC}$ | 100 mol |
| Sulphur content of the total fuel (by mass) | $w_S$ | 5% |
| Oxidant | — | Stoichiometric O2 (as dry air, 21% O2 / 79% N2 by mole) |
Find. The SO2 concentration in the resulting flue gas (ppmv and vol.%).
Approach. Back-calculate the moles of sulphur from the 5%-by-mass fuel specification, balance the stoichiometric combustion of both the hydrocarbon and the sulphur, build up the total flue-gas mole count (CO2 + H2O + SO2 + N2, no excess O2), and take the SO2 mole fraction.
| Quantity | Value |
|---|---|
| Sulphur combusted, $n_S$ | 15.9 mol |
| Total O2 (and air) required | 1040.9 mol O2 (4956.8 mol air) |
| Total flue gas produced | 5281.8 mol |
| SO2 concentration in flue gas | ≈ 3010 ppmv (0.30 vol.%) |
Secondary air pollutants from fossil-fuel combustion. Combustion emits primary pollutants directly (SO2, NOx, CO, unburned hydrocarbons, primary PM), but many of the most damaging species form only afterward, in the atmosphere, from those primary emissions — hence "secondary." SO2 and NOx are oxidized by OH radicals and, in cloud droplets, by dissolved oxidants, to sulfate and nitrate aerosol and to dilute sulfuric/nitric acid, driving acid deposition and a large share of regional PM2.5 mass. Separately, NOx and unburned volatile hydrocarbons react photochemically in sunlight (the NOx–VOC–O3 cycle) to produce ground-level ozone and photochemical smog, discussed further in Question 7(iii). Both pathways mean that reducing only the primary emission at the stack is not the whole air-quality story — secondary chemistry redistributes the impact in space (downwind) and in pollutant identity (SO2 gas becomes sulfate PM).
Biological: mould/mildew. Chronic moisture intrusion (condensation, roof/plumbing leaks, poor ventilation in bathrooms) supports mould growth on building materials; occupants are exposed to airborne spores and mycotoxins, causing allergic rhinitis, asthma exacerbation and, in sensitized individuals, hypersensitivity pneumonitis. Engineering solutions: (1) mechanical ventilation with heat recovery (HRV) sized to keep indoor relative humidity below the ~60% threshold that supports mould growth, and (2) moisture-source control — vapour barriers, bathroom/kitchen exhaust fans ducted to outdoors, and prompt remediation of water intrusion rather than only surface cleaning.
Chemical: formaldehyde (a VOC). Off-gassing from composite-wood furniture, cabinetry and some insulation causes eye/throat/respiratory irritation at low concentrations and is classified as a human carcinogen at higher, chronic exposure. Engineering solutions: (1) source control — specifying low-VOC/low-formaldehyde-emission (e.g., CARB Phase 2 / ULEF-rated) composite wood products during construction or renovation, and (2) increased outdoor-air ventilation rate (higher air-changes-per-hour) to dilute and flush off-gassed VOCs below occupational/residential guideline concentrations.
Health impacts. (1) Neurodevelopmental toxicity in children — lead crosses the blood–brain barrier and, even at low blood-lead levels with no clinical threshold yet identified, is associated with reduced IQ, attention deficits and behavioural problems, because a child's developing nervous system is disproportionately vulnerable. (2) Cardiovascular and renal effects in adults — chronic lead exposure elevates blood pressure and is associated with increased cardiovascular mortality and impaired kidney function, since lead substitutes for calcium in bone and is slowly re-released into the blood over decades.
Ecological impacts. (1) Soil and sediment accumulation — airborne Pb aerosol deposits onto soil near the source and persists for decades (lead does not biodegrade), progressively raising soil lead concentrations along roadways and near industrial stacks and contaminating urban garden produce grown in that soil. (2) Bioaccumulation and food-chain uptake — Pb taken up by plants and soil invertebrates is passed to herbivores and predators, and in aquatic systems Pb-contaminated sediment is taken up by benthic organisms and fish, producing chronic toxicity (reduced reproduction, neurological/behavioural effects) in wildlife well beyond the immediate deposition footprint.
