Question 3 of 5: Particulate Matter Sources, Measurement and Health Effects

Problem 3: Particulate Matter Sources, Measurement and Health Effects (25 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.

Part (i) — kiln industries, control technology, and significance of particle-size distribution. Two industries that use a kiln: cement manufacturing (rotary kiln calcining limestone/clay clinker) and lime manufacturing (vertical or rotary kiln calcining limestone to quicklime); brick manufacturing is a further example. Kiln off-gas typically carries a broad particle-size distribution — coarse mineral dust entrained from the feed alongside a much finer fume fraction condensed from volatilised alkali salts and trace metals. A practical control train stages a cyclone ahead of a fabric-filter baghouse (or an electrostatic precipitator): the cyclone cheaply removes the coarse fraction (typically >10 µm, where inertial separation is efficient) at low pressure drop, protecting the downstream fine-particle control device from abrasive loading and letting it be sized for the much smaller (and harder-to-collect) submicron fume it alone needs to capture at high efficiency. Particle-size distribution therefore governs both which technology is chosen (cyclones are efficient only for coarse particles; baghouses/ESPs are needed for the fine fraction) and the overall removal efficiency achievable, since collection efficiency for every mechanical/electrostatic device falls off sharply below a few micrometres.

Part (ii) — two PM measurement methods. 1. High-volume gravimetric sampler (Hi-Vol / PM₁₀ or PM₂.₅ reference method). Ambient air is drawn at a known, size-selective flow rate through a pre-weighed filter over a fixed sampling period (typically 24 h); the filter is re-weighed and concentration is computed as mass gain divided by sampled air volume. Merits: it is the regulatory reference method — simple, robust, and directly traceable to a physical mass measurement with no assumed calibration curve. Disadvantages: labour-intensive (manual filter handling, conditioning, and weighing), gives only a time-averaged result with no real-time feedback, and is prone to volatilisation losses (semi-volatile nitrate/organic aerosol) or moisture retention bias.

2. Beta-attenuation monitor (BAM). Particulate is continuously collected on a moving filter tape; a beta-particle source and detector measure the attenuation of beta radiation through the loaded tape, which is proportional to the collected particulate mass, giving a near-real-time (hourly) concentration. Merits: continuous, automated, near-real-time data suitable for public air-quality index reporting and early-warning alerts. Disadvantages: requires a radioactive source (added regulatory/licensing burden), and its calibration against the gravimetric reference method can drift with particle composition, so periodic gravimetric co-location checks are needed.

Part (iii) — PM₂.₅ vs. PM₁₀: two key differences. 1. Depth of respiratory penetration and health effect. PM₁₀ (up to 10 µm) is largely filtered by the nose and upper airway, so its dominant health effect is upper-respiratory irritation; PM₂.₅ (up to 2.5 µm) penetrates past this filtering into the bronchioles and alveoli, where it can cross into the bloodstream, driving the stronger, better-documented links to cardiovascular disease and premature mortality. 2. Source composition and visibility (aesthetics). PM₁₀ is dominated by mechanically-generated coarse dust (road dust, construction, wind erosion, pollen) that settles out relatively quickly near its source and is the primary driver of visible soiling/dustfall aesthetic complaints; PM₂.₅ is dominated by combustion-derived and secondary (photochemically formed) fine particles that stay airborne for days and travel hundreds of kilometres, so its aesthetic signature is regional haze and reduced visibility rather than local dust deposition.