Question 1 of 7: Sources and Classification of Atmospheric Pollutants

Question 1: Sources and Classification 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) SO₂ Concentration in Flue Gas and Secondary Pollutant Formation

Given. A fuel is burnt with the stoichiometric amount of oxygen, supplied as air:

Find. The SO₂ concentration in the flue gas (ppmv and % by volume).

Approach. Find the moles of sulphur from the 3%-by-mass fuel composition, balance the stoichiometric combustion equation for the hydrocarbon, add the O₂ consumed oxidizing S to SO₂, bring in N₂ from the air, then divide moles SO₂ by total moles of flue gas.

1. Molar mass and sulphur content of the fuel. $M_{fuel} = 7(12.011)+13(1.008) = 97.18\ \text{g/mol}$. For 20 mol: $m_{fuel} = 1943.6\ \text{g}$, so $m_S = 0.03(1943.6) = 58.3\ \text{g}$, i.e. $$n_S = \frac{58.3}{32.07} = \boxed{1.818\ \text{mol S}}.$$
2. Balance the hydrocarbon combustion. $\text{C}_7\text{H}_{13} + 10.25\,\text{O}_2 \rightarrow 7\,\text{CO}_2 + 6.5\,\text{H}_2\text{O}$ (from $x+y/4 = 7+13/4=10.25$). For 20 mol fuel: $\text{O}_{2,fuel} = 205.0\ \text{mol}$, $\text{CO}_2 = 140.0\ \text{mol}$, $\text{H}_2\text{O}=130.0\ \text{mol}$.
3. Add sulphur oxidation and bring in nitrogen. $\text{S}+\text{O}_2\rightarrow\text{SO}_2$ needs $1.818$ mol O₂, so $\text{O}_{2,total} = 205.0+1.818 = 206.8\ \text{mol}$. At stoichiometric air (21% O₂/79% N₂), $$N_2 = 206.8\times\frac{79}{21} = 778.0\ \text{mol}.$$
4. Total flue gas and SO₂ fraction. $n_{flue} = \text{CO}_2+\text{H}_2\text{O}+\text{SO}_2+N_2 = 140.0+130.0+1.818+778.0 = 1049.8\ \text{mol}$, so $$y_{SO_2} = \frac{1.818}{1049.8} = \boxed{1732\ \text{ppmv}\ (0.173\%\ \text{v/v})}.$$

Secondary air pollutants are not emitted directly from the stack; they form downwind in the atmosphere when primary combustion products undergo further reaction, most often photochemically driven. SO₂ oxidizes slowly (catalyzed by OH radicals, or heterogeneously on particle surfaces and in cloud droplets) to SO₃, which hydrates instantly to sulfate aerosol (H₂SO₄/(NH₄)₂SO₄ mist) — the principal driver of acid deposition and regional fine-particle haze. NO_x released alongside SO₂, together with co-emitted volatile organic compounds, reacts photochemically to form ground-level ozone (O₃) and peroxyacetyl nitrate (PAN), the defining pollutants of photochemical smog. Atmospheric pollutants are classified along two independent axes relevant here: primary vs. secondary (emitted directly from the source vs. formed in the atmosphere by transformation of primary emissions), and by criteria/health-based classification (SO_x, NO_x, CO, O₃, PM, Pb under the Canadian Ambient Air Quality Standards) vs. air toxics (e.g. benzene, mercury) regulated individually because they pose disproportionate risk at low concentration. Because secondary-pollutant formation needs residence time and sunlight, its worst impact is frequently tens to hundreds of kilometres downwind of the stack rather than at the source itself.

(ii) Indoor Air Pollutants — Biological and Chemical

A representative biological indoor pollutant is airborne mould (fungal bioaerosols), which proliferates on damp building materials (drywall, insulation, HVAC condensate pans) and releases spores and volatile microbial organic compounds into occupied space. Chronic exposure triggers allergic sensitization, asthma exacerbation, and upper-respiratory irritation, and in immunocompromised occupants can progress to invasive fungal infection. A representative chemical indoor pollutant is formaldehyde, off-gassed continuously from pressed-wood products, adhesives, and new furnishings; it causes eye and respiratory irritation and headache at moderate concentration and is classified by IARC as a human carcinogen under chronic exposure.

Two engineering remedies address each pollutant at its source and its pathway. For mould: (1) moisture control — continuous mechanical ventilation with heat/energy recovery (HRV/ERV) to keep relative humidity below the ~60% threshold for fungal growth, combined with vapour barriers and prompt repair of building-envelope leaks that create the damp substrate mould needs; (2) source removal and remediation once contamination occurs (containment, HEPA-filtered negative-air removal of affected material) rather than masking with surface disinfectant alone. For formaldehyde: (1) source control — specifying low-VOC or no-added-urea-formaldehyde composite wood products and allowing a bake-out/off-gassing period before occupancy; (2) dilution ventilation — the same HRV/ERV strategy used for moisture control simultaneously lowers steady-state VOC concentration by increasing the air-exchange rate, since indoor concentration at equilibrium scales inversely with ventilation rate for a fixed emission source.

(iii) Outdoor Mercury Release from a Mobile Source

Ocean-going vessels burning heavy (bunker) fuel oil are a recognized mobile source of mercury: fuel oil carries trace Hg impurities that volatilize during combustion and are exhausted with the ship's stack gas, and marine engines are unlike land-based mobile sources (light vehicles) in that they are largely unregulated for trace-metal emissions and operate continuously in coastal shipping lanes and port areas. Health impacts: (1) elemental Hg vapour and Hg(II) deposited near ports is methylated by anaerobic sediment bacteria to methylmercury, which bioaccumulates and biomagnifies up the aquatic food chain; human consumption of contaminated fish is the dominant human exposure pathway, causing neurodevelopmental deficits in the fetus and young children and neurological/cardiovascular effects in adults; (2) occupational and near-source inhalation of elemental Hg vapour causes tremor, memory impairment, and renal toxicity. Ecological impacts: (1) biomagnification through the aquatic food web elevates Hg body burden in top predators (piscivorous birds such as loons, marine mammals), impairing reproduction and neurological function; (2) atmospheric long-range transport and deposition of Hg to remote, otherwise pristine watersheds (a defining feature of Hg's global biogeochemical cycle) contaminates fisheries far from any local source, driving fish-consumption advisories in lakes with no nearby industry.