18-Env-A5 Air Quality and Pollution Control Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2014 — 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 sulphur-bearing hydrocarbon fuel with stoichiometric oxygen (supplied as stoichiometric air):
| 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, supplied 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, $n_S$ | 15.95 mol |
| Total O2 (and air) required | 1040.9 mol O2 (4956.9 mol air) |
| Total flue gas produced | 5281.9 mol |
| SO2 concentration in flue gas | ≈ 3019 ppmv (0.30 vol.%) |
Secondary air pollutants from fossil-fuel combustion. Combustion emits primary pollutants directly (SO2, NOx, CO, unburned hydrocarbons, primary PM), but several of the most damaging species form only afterward, in the atmosphere, from those primary emissions — hence "secondary." SO2 and NOx are oxidized by hydroxyl radicals in the gas phase and by dissolved oxidants inside cloud droplets, forming sulfate and nitrate aerosol and dilute sulfuric/nitric acid, which drives acid deposition and a large share of regional fine-particulate (PM2.5) mass. Separately, NOx and unburned volatile hydrocarbons react photochemically in sunlight through the NOx–VOC–O3 cycle to produce ground-level ozone and photochemical smog (discussed further in Question 6(iii)). Both pathways mean the primary stack emission is not the whole air-quality story: secondary chemistry redistributes the impact downwind in space and transforms the pollutant's chemical identity, so an emission limit written only at the stack can understate the true downwind burden.
Biological: house dust mite allergen. Dust mites thrive in warm, humid indoor environments (bedding, upholstery, carpet) and their faecal particles and body fragments become airborne allergens. Chronic exposure sensitizes occupants and triggers allergic rhinitis and asthma exacerbation, particularly in children. Engineering solutions: (1) mechanical ventilation with humidity control (HRV/dehumidification) to keep indoor relative humidity below roughly 50%, since dust mites cannot sustain populations in dry conditions, and (2) HEPA-filtered central air handling combined with impermeable mattress/pillow encasements to reduce the allergen reservoir and its resuspension into breathing air.
Chemical: formaldehyde (a VOC). Off-gassing from composite-wood furniture, cabinetry and some insulation and adhesives causes eye, nose and throat irritation at low concentrations and is classified as a human carcinogen under chronic, higher exposure. Engineering solutions: (1) source control — specifying low-formaldehyde-emission composite wood products (e.g., ULEF/CARB Phase 2-rated) during construction or renovation, and (2) increasing the outdoor-air ventilation rate (higher air changes per hour) to dilute and flush off-gassed formaldehyde below occupational/residential guideline concentrations.
Health impacts. (1) Carcinogenicity — inorganic arsenic is an IARC Group 1 (confirmed) human carcinogen; chronic inhalation or ingestion is associated with elevated risk of skin, lung and bladder cancer, because arsenic interferes with DNA repair and cellular signalling at the molecular level. (2) Chronic non-cancer toxicity (arsenicosis) — long-term lower-level exposure produces characteristic skin lesions (hyperpigmentation and hyperkeratosis), together with peripheral vascular disease and peripheral neuropathy, since arsenic accumulates in keratin-rich tissue and disrupts cellular energy metabolism.
Ecological impacts. (1) Soil accumulation and phytotoxicity — airborne arsenic deposits onto soil near the source and persists indefinitely (it does not biodegrade), progressively raising soil arsenic concentrations, suppressing plant growth at high concentration and contaminating food crops grown nearby. (2) Aquatic bioaccumulation — arsenic deposited to surface water or leached from contaminated soil accumulates in sediment and is taken up by benthic organisms and fish; inorganic arsenic in particular is more toxic than the organic forms typically biomagnified in higher trophic levels, producing chronic reproductive and developmental toxicity in aquatic wildlife.
