18-Env-A5 Air Quality and Pollution Control Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2014 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; closed book with a candidate-prepared 8.5×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five (5) questions constitute a complete paper (the first five answers as they appear are marked); all seven are solved below for completeness. Each question is worth 20 marks with section marks shown in brackets.
Reference texts. Cooper & Alley, Air Pollution Control: A Design Approach (4th ed.); Wark, Warner & Davis, Air Pollution: Its Origin and Control (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Canadian Environmental Protection Act, 1999 (CEPA) and the Canadian Ambient Air Quality Standards (CAAQS) administered by Environment and Climate Change Canada.
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.
A chemiluminescence NOx analyzer measures nitric oxide (NO) directly by reacting the sampled gas with ozone generated internally by the instrument: $\text{NO}+\text{O}_3\rightarrow \text{NO}_2^{*}+\text{O}_2$. The excited-state NO2* molecule relaxes to its ground state by emitting a photon in the near-infrared, and a photomultiplier tube (PMT) housed in a light-tight reaction chamber measures the resulting light intensity, which is directly proportional to the NO concentration in the sample. Because ambient NOx contains both NO and NO2, and only NO reacts with ozone in this way, total NOx is measured by first routing a parallel sample stream through a converter (heated molybdenum or, in newer instruments, a UV photolytic converter) that reduces NO2→NO, so that channel reports NO+NO2 (=NOx); NO2 is then obtained by difference, $[\text{NO}_2] = [\text{NOx}] - [\text{NO}]$. This dual-channel differencing approach, rather than a direct NO2 reaction, is what makes chemiluminescence analyzers the reference-method instrument for continuous ambient and source NOx monitoring.
Cyclones separate particles by centrifugal inertia, so particle density and size are the controlling properties — collection efficiency rises sharply with particle mass (density × diameter3), and a cyclone is a poor choice for fine, low-density particulate regardless of loading. Fabric filters rely on a filter cake building up on the fabric (surface filtration), so particle size, cohesion and adhesion matter most — particles must cohere to each other and adhere to the fabric/cake to build an effective cake without excessive pressure-drop growth, and very fine, poorly-cohesive dusts can "bleed through" a new bag before a cake forms. Electrostatic precipitators (ESPs) depend on the particle acquiring and holding an electrical charge, so particle surface (electrical) resistivity is the dominant property — resistivity in the optimal 104–1010 Ω·cm range collects well, while very-low-resistivity particles lose their charge on contact with the collection plate and re-entrain, and very-high-resistivity particles (e.g., low-sulphur fly ash) build up a back-corona that limits achievable current and efficiency.
Two key sources. (1) Direct (primary) combustion emissions — diesel vehicle exhaust, residential wood/biomass burning and industrial combustion emit soot and condensable organic PM2.5 directly. (2) Secondary formation — SO2, NOx and VOC precursors emitted from combustion and industrial sources react in the atmosphere (as in Question 1(i)) to form sulfate, nitrate and secondary organic aerosol, which together make up a large fraction of ambient PM2.5 mass even far from any primary source.
Two aesthetic effects. (1) Visibility/haze reduction — PM2.5 particles are close to the wavelength of visible light and scatter it efficiently (per Mie scattering theory), reducing visual range and producing regional haze. (2) Soiling of buildings and surfaces — fine particulate deposits on painted surfaces, monuments and building facades, requiring more frequent cleaning and accelerating material degradation.
Two health effects. (1) Cardiovascular effects — PM2.5 is small enough to penetrate deep into the alveolar region and cross into the bloodstream, triggering systemic inflammation linked to increased risk of heart attack and stroke. (2) Respiratory effects — PM2.5 exacerbates asthma and chronic obstructive pulmonary disease and is associated with reduced lung function development in children, because the fine fraction deposits deeper in the respiratory tract than the coarser PM10 fraction.
