11-CS-3 Engineering Management · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2013 — 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). In sunlight and the presence of volatile organic compounds, NOₓ drives ozone formation via the photolysis of nitrogen dioxide:
The NO from combustion is oxidized to NO₂, which photolyzes; the atomic oxygen released forms ozone, which accumulates as a respiratory-irritant smog because VOCs interrupt the reaction that would otherwise destroy it.
2. Acid rain. Nitrogen oxides oxidize to nitric acid, which returns as acidic precipitation:
The nitric acid acidifies rain, lakes, and soils, harming fish, forests, and structures.
Optimize the existing system: (1) three-way catalytic converters, which reduce exhaust NOₓ back to N₂; and (2) exhaust-gas recirculation and combustion tuning to lower peak flame temperature and thus thermal NOₓ. Re-engineer the system: (3) electric or hybrid vehicles, or cleaner fuels (natural gas, hydrogen), removing tailpipe NOₓ at source. Redefine the problem: (4) reduce private-vehicle travel through public transit, dense mixed-use planning, cycling, and telecommuting—cutting vehicle-kilometres rather than treating each vehicle.
Humans directly perturb the carbon cycle in two main ways. First, combustion of fossil fuels (coal, oil, and natural gas) transfers carbon that had been locked in geological storage into the atmosphere as CO₂; this adds on the order of 30–35 billion tonnes of CO₂ per year (roughly 9 Gt of carbon). Second, land-use change, chiefly deforestation, removes vegetation that stored carbon and releases it through burning and decay while reducing future uptake; this adds on the order of 3–5 billion tonnes of CO₂ per year (about 1–1.5 Gt of carbon). Together these anthropogenic flows exceed the capacity of natural sinks (oceans and biosphere) to absorb them, so atmospheric CO₂ rises. (Exact figures vary by year and source; the fossil-fuel term is by far the larger.)
The Earth is warmed by the greenhouse effect: incoming short-wave solar radiation passes through the atmosphere and warms the surface, which re-radiates energy as long-wave infrared. Greenhouse gases such as CO₂ are largely transparent to the incoming sunlight but absorb and re-emit the outgoing infrared, trapping heat in the lower atmosphere and raising surface temperature. By adding CO₂ (and other greenhouse gases), human activity enhances this natural effect, increasing the amount of infrared trapped and driving global warming.
Methane (CH₄)—from agriculture (livestock enteric fermentation and rice paddies), landfills, and natural-gas leakage. Nitrous oxide (N₂O)—from nitrogen-fertilizer use in agriculture and from combustion. (Chlorofluorocarbons from refrigerants are a further example.)
