Question 3 of 7: Measurement Techniques and Particulate Characteristics

Question 3: Measurement Techniques and Particulate Characteristics (20 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.

(i) Passive vs. Active Measurement Techniques

Three key differences. (1) Mechanism — passive samplers (e.g., diffusion tubes/badges) rely on molecular diffusion of the pollutant onto a sorbent at a rate governed by Fick's law, with no moving parts or power supply; active samplers use a calibrated pump to draw a known volumetric flow across a filter or through a sorbent/impinger. (2) Time resolution — passive methods integrate exposure over days to weeks and cannot resolve short-term peaks; active methods can be run over minutes to hours (or continuously, for real-time analyzers), resolving diurnal and event-driven peaks. (3) Cost, power and data quality — passive samplers are inexpensive, require no electricity, and are easy to deploy in large spatial networks, but have higher detection limits and uncertainty (uncontrolled uptake rate); active methods give better precision and lower detection limits but need power, calibration, and site infrastructure.

Passive is most appropriate when (a) mapping spatial concentration gradients over a large area with many sites (network density matters more than time resolution), and (b) monitoring in remote or unpowered locations where deploying pumps is impractical. Active is most appropriate when (a) a regulatory compliance measurement needs a defined short averaging period (e.g., 1-hour or 24-hour NAAQS-style limits), and (b) real-time source-attribution or episodic peak identification is required (e.g., during a smog alert).

(ii) Aerodynamic Diameter

The aerodynamic diameter ($d_a$) of an irregular, non-spherical particle of arbitrary density is defined as the diameter of a sphere of unit density (1000 kg/m³) that has the same terminal settling velocity as the actual particle. It collapses particle shape, true density and physical size into a single parameter that directly determines how a particle behaves in a moving air stream — its inertia, its settling behaviour, and (for the respiratory tract) how deeply it penetrates and deposits in the lung. Because every major particulate control technology (gravity settlers, cyclones, electrostatic precipitators, baghouses, wet scrubbers) separates particles by exploiting exactly this inertial/settling behaviour, engineered removal efficiency curves are always plotted against $d_a$ rather than physical (Feret/microscope) diameter. A direct engineered example is the cyclone separator (Problem 7(i) below): its cut diameter $[d_p]_{cut}$ is explicitly an aerodynamic-diameter concept — it is derived from a force balance between centrifugal force (proportional to particle mass, hence true density and volume) and drag, and the resulting efficiency curve is universal only when particle size is expressed as an aerodynamic (unit-density-equivalent) diameter.

(iii) PM_2.5 vs. PM₁₀ — Health and Aesthetic Differences

Health. (1) Penetration depth — PM₁₀ (coarse fraction, 2.5–10 µm) is largely filtered by the nose and upper airway and deposits mainly in the tracheobronchial region, aggravating asthma and upper-respiratory irritation; PM_2.5 (fine fraction, ≤2.5 µm, visible as the fine granular material in the micrograph above) penetrates past the upper airway defenses into the alveolar (gas-exchange) region and can cross into the bloodstream, driving the stronger epidemiological association with cardiovascular disease and premature mortality. (2) Chemical composition and toxicity — PM_2.5 is dominated by combustion-derived secondary sulfate/nitrate/organic aerosol and adsorbed toxic trace metals and PAHs (higher intrinsic toxicity per unit mass), while PM₁₀'s coarse fraction is dominated by mechanically generated crustal dust, road dust and pollen (lower intrinsic toxicity per unit mass).

Aesthetics. (1) Visibility/haze — PM_2.5 is close to the wavelength of visible light and scatters it efficiently (Mie scattering), so it is the dominant cause of regional haze and visibility degradation even at modest mass concentrations; the coarser PM₁₀ fraction scatters light far less efficiently per unit mass. (2) Soiling and dustfall — the coarse PM₁₀ fraction settles out quickly near the source, producing visible dustfall, soiling of surfaces and nuisance grit complaints; PM_2.5 stays airborne far longer and disperses more broadly rather than causing local soiling.