Question 1 of 7: Sources, Classification and Health/Ecological Impacts of Atmospheric Pollutants

Question 1: Sources, Classification and Health/Ecological Impacts of Atmospheric Pollutants (20 marks)

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.

(i) Primary vs. Secondary Pollutants in Coal Combustion

Primary pollutants are emitted directly from an identifiable source in the same chemical form found in ambient air. Secondary pollutants are not emitted at all — they form afterward, in the atmosphere, through chemical or photochemical reactions among primary pollutants and background gases, usually driven by sunlight and requiring a period of atmospheric residence time. A coal-fired boiler illustrates both. The coal contains organically bound sulfur and nitrogen; at combustion temperature the sulfur oxidizes directly to SO₂ and both the fuel-bound and thermally-fixed nitrogen oxidize to NO_x — both are released straight from the stack as primary pollutants, along with fly-ash particulate. Downwind, the SO₂ slowly oxidizes further (catalyzed by hydroxyl radicals, ambient moisture and trace metals on fly-ash surfaces) to SO₃ and then sulfate aerosol/H₂SO₄ mist, while NO_x reacting with trace hydrocarbons under sunlight generates ground-level ozone and peroxyacetyl nitrate (PAN) — these sulfate aerosol and ozone/PAN products are secondary pollutants, distinguished by forming well after and often well downwind of the stack rather than at the point of combustion.

(ii) Chemical and Biological Indoor Air Pollutants

Chemical source — formaldehyde. Formaldehyde off-gasses from pressed-wood products, urea-formaldehyde foam insulation and some furnishings/adhesives. Health impact: eye, nose and throat irritation at typical indoor levels, and it is classified as a probable human carcinogen at higher chronic exposure. Engineering remedies: (1) source control — specify low-emission (e.g. CARB Phase 2 / ULEF-rated) engineered wood products during construction or renovation, or seal existing exposed surfaces with a low-VOC sealant to cut the off-gassing rate; (2) increase mechanical ventilation — a heat/energy-recovery ventilator supplying outdoor air at or above ASHRAE 62.1 rates continuously dilutes and purges accumulated formaldehyde rather than allowing it to build up in a tightly sealed building envelope.

Biological source — mould. Mould (e.g. Aspergillus, Penicillium, Stachybotrys) colonizes damp building materials after a leak or chronic condensation. Health impact: allergic and asthma-like respiratory symptoms, and hypersensitivity pneumonitis in sensitized occupants. Engineering remedies: (1) moisture control at the envelope — correct vapour-barrier placement, flashing and exterior drainage, and prompt repair of plumbing leaks, keeping building material moisture content below the roughly 20% threshold mould growth needs; (2) humidity control — maintain indoor relative humidity below about 60% through HVAC dehumidification, since mould growth is strongly RH-driven independent of any single wetting event.

(iii) Indoor Release of Argon

Argon is chemically inert (a noble gas), so it presents no toxicological hazard of its own; its danger is purely physical, through displacement of oxygen. Argon is roughly 1.4× denser than air, so a leak (e.g. from a cryogenic storage dewar, or a GTAW/TIG welding shielding-gas line in a poorly ventilated bay) tends to pool at floor level rather than mixing away. Two related health impacts: (1) asphyxiation — displacement of O₂ below roughly 19.5% causes hypoxia, progressing from dizziness and impaired judgment to loss of consciousness and, at very low O₂ fractions, death; because argon is odourless and non-irritating, this occurs with essentially no sensory warning; (2) incapacitation preventing self-rescue — sudden unconsciousness in a confined low point (pit, tank, welding bay) can trap the victim and creates a serious secondary hazard for an untrained rescuer entering the same atmosphere without breathing apparatus. Engineering solution: install a fixed, continuously-monitoring oxygen-deficiency sensor at low level (argon being denser than air) in any enclosed space where argon is stored or used, interlocked to both a local/remote alarm and automatic mechanical exhaust ventilation, combined with a confined-space entry permit system requiring atmospheric testing before entry — the same control philosophy already codified for other asphyxiant gases (CO₂, N₂) under provincial OH&S confined-space regulations (e.g. WorkSafeBC OHS Regulation, Part 9).