Question 2 of 7: Measurement Techniques and PM2.5/PM10 Health and Aesthetic Considerations

Question 2: Measurement Techniques and PM2.5/PM10 Health and Aesthetic Considerations (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 Ambient Air Samplers

A passive sampler (e.g., a diffusion tube or passive dosimeter badge) has no pump: the target gas reaches an internal sorbent purely by molecular diffusion through a static air gap or membrane, at a rate governed by Fick's law and the sorbent's uptake rate constant. It needs no power or maintenance visit beyond deployment and retrieval, is cheap enough to deploy in large numbers, and integrates concentration over days to weeks — but it cannot resolve short-term peaks and its uptake rate is sensitive to wind speed and temperature. Passive samplers are the right tool for a spatial screening/siting survey — e.g., mapping NO₂ gradients across a city with fifty simultaneous tube deployments to site a new fixed monitoring station — where many locations and a long integration time matter more than fine time resolution.

An active sampler draws a metered volume of air through a filter, sorbent tube, or continuous analyzer using a calibrated pump, giving quantitative, time-resolved concentration data (often continuous, sub-hourly) traceable to a known sample volume. It needs power, routine calibration and a technician to service the pump/flow controller, and is correspondingly far more expensive per site. Active samplers are the right tool for a regulatory compliance/reference monitoring station — e.g., a continuous PM2.5 beta-attenuation or SO₂ UV-fluorescence analyzer at a National Air Pollution Surveillance (NAPS) station — where legally defensible, time-resolved data against a NAAQS/CAAQS averaging period is required.

(ii) Significance of Terminal Settling Velocity and Particle Size Distribution

Terminal settling velocity and particle size distribution (PSD) together determine how long a particle survives airborne, how far it travels from its source, and where in the human respiratory tract it deposits — and therefore both what environmental/health outcome to expect and what control technology can remove it. Since $v_t\propto d_p^2$ (Stokes regime) or a weaker power for larger, non-Stokesian particles, coarse particles (large $d_p$) settle out near the source within minutes to hours, dominating local dustfall/soiling nuisance, while fine particles (small $d_p$) have settling velocities of only mm/s or less and can remain suspended for days, undergoing long-range atmospheric transport and, once inhaled, penetrating past the nose/throat into the bronchioles and alveoli.

Engineering control follows directly from this: a device is sized around the PSD of the specific emission (grain dust, fly ash, diesel soot), because each removal mechanism has its own characteristic efficiency curve versus particle size — gravity settlers and simple cyclones excel on the coarse tail of the distribution (as in Question 1(iii) and Question 5(i)) but fall off sharply below a few microns, while electrostatic precipitators and high-efficiency fabric filters are chosen specifically because their efficiency curves remain high into the sub-micron range where settling-based devices fail. Knowing the full PSD, not just a single mean diameter, is essential because a device that looks "efficient" on a mass basis can still pass through the numerous, health-relevant fine-particle tail almost untouched.

(iii) PM2.5 vs. PM10: Health and Aesthetic Differences