Question 3 of 6: Particulate Measurement and PM 2.5 /PM 10

Problem 3: Particulate Measurement and PM_2.5/PM₁₀ (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) — PM_2.5 and PM₁₀. PM₁₀ is particulate matter with an aerodynamic diameter ≤10 µm — it includes both the coarse fraction (2.5–10 µm, mechanically generated: road dust, construction, wind-blown soil, sea salt) and everything finer. Characteristics: dominated by mechanically generated, often mineral or biological material that settles relatively quickly (hours) and does not travel far from its source. Health: largely filtered by the nose and upper airway, so effects concentrate on upper-respiratory irritation, eye irritation and aggravation of existing bronchitis. Aesthetic: causes visible surface soiling, dustfall on cars/windowsills, and reduced visibility near the source. PM_2.5 ("fine" PM, ≤2.5 µm) is dominated by combustion-derived and secondary aerosol (soot, sulphate, nitrate, secondary organic aerosol) that can remain airborne for days and travel hundreds of kilometres. Health: penetrates past the upper airway deep into the alveoli and bloodstream, driving the more serious cardiovascular and deep-lung outcomes (heart attack, stroke, chronic bronchitis, premature mortality) that dominate air-quality health statistics. Aesthetic: because its diameter is comparable to the wavelength of visible light, it scatters light very efficiently and is the primary cause of regional visibility-reducing haze even when no surface dustfall is noticeable.

Part (ii) — a PM measurement method with diagram. The high-volume gravimetric sampler (the reference/regulatory method) draws ambient air through a size-selective inlet (cutting at PM₁₀ or PM_2.5), then through a pre-weighed filter for a fixed sampling period (typically 24 hours); the filter is re-weighed after conditioning, and the mass gain divided by the total sampled air volume (from the flow meter/timer) gives the average PM concentration.

Merits: (1) it is the simple, low-cost EPA/ECCC reference method against which every other technique is calibrated, and directly measures mass rather than an indirect proxy; (2) size-selective inlets are interchangeable, so the same instrument measures PM₁₀ or PM_2.5 by swapping the inlet head. Disadvantages: (1) it only gives one integrated average concentration over the whole sampling period (e.g. 24 h) — there is no real-time or sub-daily data; (2) it is labour-intensive (filter conditioning, handling, weighing in a controlled-humidity balance room) and prone to weighing error from filter handling artifacts.

Part (iii) — condensable and respirable PM fractions. The condensable fraction is material that is emitted as a vapour at stack temperature but condenses to sub-micron liquid or solid particles once it cools and mixes with ambient air — e.g. sulphuric acid mist condensing from SO₃ in a coal-plant plume, or semi-volatile organic compounds condensing from a wood-combustion or asphalt-plant exhaust; because it is invisible at the stack (still vapour), it is easily under-counted by a filter-only (front-half only) stack test that does not include a condensable-PM impinger train. The respirable fraction is the sub-set of particles small enough (roughly ≤4 µm, corresponding to PM_2.5 and finer) to penetrate past the upper airway into the gas-exchange region of the lung — e.g. diesel soot, fine fly ash, and secondary sulphate/nitrate aerosol; its sources are the same fine-combustion and secondary-chemistry sources that dominate PM_2.5, and it is the fraction regulators target with the health-based PM_2.5 standard.