11-CS-3 Engineering Management · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2014 — 11-CS-3 Sustainability, Engineering and the Environment. Closed book; approved calculator permitted. Any four questions constitute a complete paper; all questions are of equal value (25 marks each).
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.
Nitrogen fixation: atmospheric N₂ → ammonia/reactive nitrogen (reactants N₂ + H₂ or N₂ + O₂; products NH₃ or NO). Nitrification: bacteria oxidize ammonium to nitrite then nitrate (reactant NH₄⁺ + O₂; products NO₂⁻ then NO₃⁻). Denitrification: bacteria reduce nitrate back to nitrogen gas under anaerobic conditions (reactant NO₃⁻; products N₂ (and some N₂O) + returns nitrogen to the atmosphere). Together these steps cycle nitrogen between the atmosphere, soil, and organisms.
Eutrophication is the over-enrichment of a water body with nutrients (chiefly nitrogen and phosphorus) that stimulates excessive growth of algae and aquatic plants; the subsequent decay of this biomass consumes dissolved oxygen, depleting it and killing fish and other aquatic life.
The major anthropogenic source of NO is high-temperature combustion, especially in motor-vehicle engines (and power plants). NO drives ground-level ozone formation:
NO oxidizes to NO₂, which photolyzes in sunlight to release atomic oxygen that combines with O₂ to form ozone (with VOCs allowing ozone to accumulate). Ground-level ozone is a pollutant because it is a strong oxidant that irritates and damages the respiratory system and vegetation where people are exposed, whereas stratospheric ozone is beneficial because it absorbs harmful solar ultraviolet radiation—"good up high, bad nearby."
Besides ozone, peroxyacetyl nitrate (PAN) is a secondary pollutant formed in photochemical smog when NO₂ reacts with organic (peroxyacetyl) radicals derived from VOCs; PAN is a potent eye irritant and phytotoxin. (Equally acceptable: secondary nitric-acid aerosol/acid deposition, formed when NO₂ is oxidized to HNO₃—$3\text{NO}_2 + \text{H}_2\text{O} \rightarrow 2\text{HNO}_3 + \text{NO}$—since the acid is not emitted directly but formed in the atmosphere from NO.)
The Sun (surface ~5800 K) emits short-wave radiation—mostly visible and near-infrared, with some ultraviolet (most of the UV is absorbed by stratospheric ozone). The visible and near-infrared pass largely through the atmosphere and warm the Earth's surface. The much cooler surface (~288 K) re-radiates energy at longer wavelengths as thermal (long-wave) infrared radiation, peaking near 10 µm. Greenhouse gases (CO₂, CH₄, water vapour) are largely transparent to the incoming visible/UV but strongly absorb and re-emit the outgoing infrared, trapping heat in the lower atmosphere. Increasing greenhouse-gas concentrations enhances this trapping, warming the planet.
Mitigation reduces the cause—cutting greenhouse-gas emissions or enhancing sinks (e.g. switching to renewable energy or carbon capture). Adaptation adjusts to the effects already underway (e.g. building sea walls or planting drought-resistant crops). Mitigation attacks the source; adaptation manages the consequences.
CO₂ equivalent (CO₂e) expresses the warming effect of a quantity of any greenhouse gas as the amount of CO₂ that would cause the same warming, by multiplying the mass by the gas's global warming potential (GWP). Ranked by GWP (per unit mass), from highest to lowest: SF₆ (≈23,500) > HFCs (hundreds to ~14,000) > N₂O (≈265) > CH₄ (≈28) > CO₂ (1). Despite having the lowest GWP, CO₂ currently adds the most CO₂e from human activity, because it is emitted in vastly greater quantities than the others—its enormous emission volume outweighs its low per-molecule potency.
