23-CS-3 Sustainability, Engineering and the Environment · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 11-CS-3 Sustainability, Engineering and the Environment. Open book; non-communicating 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.
1. Photochemical smog (ground-level ozone):
NO from vehicle exhaust is oxidized to NO₂, which photolyzes in sunlight to release atomic oxygen that forms ground-level ozone. Normally NO destroys that ozone again (NO + O₃ → NO₂ + O₂), but VOCs (also from vehicles) convert NO to NO₂ via peroxy radicals without consuming ozone, so O₃, PAN and aldehydes accumulate as brown photochemical smog over a city and its downwind region on sunny summer days, irritating lungs and damaging crops. Because transportation supplies about 60% of human-made NOₓ, and emits the VOCs as well, traffic is the main driver of urban smog. 2. Acid rain (acid deposition):
NO is oxidized to NO₂, which reacts with water (mainly via hydroxyl radicals, NO₂ + OH → HNO₃) to form nitric acid. The acid is carried hundreds of kilometres and falls as wet or dry deposition below the natural rain pH of about 5.6, acidifying poorly buffered lakes and soils (for example on the Canadian Shield), mobilizing aluminium that is toxic to fish and leaching nutrients from forest soils. Deposited nitrate also over-fertilizes coastal waters (eutrophication).
In the global nitrogen cycle nitrogen exists as: (1) nitrogen gas (N₂), the inert atmospheric reservoir; (2) ammonia/ammonium (NH₃ / NH₄⁺), the reduced form produced by fixation; (3) nitrite and nitrate (NO₂⁻ / NO₃⁻), the oxidized forms produced by nitrification; and (4) organic nitrogen (in proteins, amino acids, and other biomolecules). Nitrogen oxides (NOₓ) and nitrous oxide (N₂O) are further oxidized gaseous forms.
The Sun emits short-wave ultraviolet and visible radiation that passes through the atmosphere and warms the surface; the surface re-radiates long-wave infrared. Greenhouse gases are transparent to the incoming UV/visible but absorb and re-emit the outgoing IR, trapping heat. A higher greenhouse-gas concentration absorbs more outgoing IR, returning more heat to the surface, which must warm to restore radiative balance—so the planet warms.
Note the prefixes: Tg = 10¹² g, Gg = 10⁹ g, Mg = 10⁶ g. Convert to CO₂e (CO₂ = 1, CH₄ = 25, PFC ≈ 10,000 representative):
The ranking does not depend on the representative value chosen: across the whole tabulated PFC range, 1.73×10⁸ g × 7,390 = 1.28×10¹² g and 1.73×10⁸ g × 12,200 = 2.11×10¹² g CO₂e, both above methane's 1.25×10¹² g.
Ranked by warming effect: PFCs (≈1.7×10¹²) > CH₄ (1.25×10¹²) > CO₂ (1.20×10¹²). But by mass emitted, CO₂ is by far the greatest (1.2×10¹² g, versus 5×10¹⁰ g of CH₄ and only 1.73×10⁸ g of PFCs)—the tiny mass of the ultra-potent PFCs still rivals CO₂'s warming, illustrating why a small mass of a high-GWP gas matters as much as a huge mass of CO₂.
Mitigation reduces the cause of climate change, by cutting greenhouse-gas emissions or enhancing sinks, so that less future warming occurs. Example: replacing a coal-fired power plant with wind or solar generation. Electricity is produced without burning fossil carbon, so the CO₂ that would have been emitted (roughly 0.9–1 t CO₂ per MWh for coal) is avoided and the atmospheric concentration rises more slowly. Other examples are building energy efficiency, electric vehicles and carbon capture and storage.
Adaptation reduces vulnerability to the climate change that is already happening or is unavoidable. It accepts the changed climate and limits the damage. Example: raising dikes and sea walls, or setting higher flood-construction levels for new buildings in a coastal city, protects property and people from higher sea levels and storm surges. Other examples are upsizing storm sewers for heavier rainfall, FireSmart vegetation management and drought-tolerant crops.
The key difference is that mitigation attacks the driver and brings global benefits over decades, whereas adaptation manages the consequences and brings local benefits now. Both are needed, because warming already committed cannot be avoided by mitigation alone.
