Question 3 of 7: Measurement of Air Pollutants and Particulate Characteristics

Question 3: Measurement of Air Pollutants 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) Nondispersive Infrared Absorption (NDIR) for CO Measurement

Main principle. Carbon monoxide has a characteristic infrared absorption band near 4.6 µm. In an NDIR analyzer, a sample gas stream is drawn continuously into a measurement cell through which an infrared source beam passes; a narrow optical bandpass filter isolates CO's own absorption wavelength, and a detector compares the beam intensity through the sample cell against a sealed reference cell (or a reference wavelength where CO does not absorb). By the Beer–Lambert law, the fractional attenuation of the beam at CO's absorption wavelength is proportional to the CO concentration along the optical path, giving a continuous, calibrated concentration reading.

Advantage. NDIR gives continuous, real-time, non-consuming (non-destructive) measurement with good sensitivity and selectivity once properly filtered, making it well suited to continuous emission monitoring (CEM) of a stack or ambient CO station.

Limitation. Other infrared-absorbing species with overlapping bands (notably CO₂ and water vapour) can cross-interfere and bias the reading if the optical filtering or reference compensation is inadequate, and the sample-cell optics require periodic zero/span calibration and are susceptible to fouling by particulate in a dusty flue-gas stream.

(ii) Biological, Chemical and Physical Characteristics of Particulates

Biological. Particulate matter can carry adsorbed or embedded bioaerosols — bacteria, fungal spores and endotoxins — on its surface. Because fine particles have a large surface-area-to-volume ratio, they act as a vector that carries these biological agents deep into the respiratory tract, triggering infection or an inflammatory/allergic response well beyond what the particle mass alone would predict.

Chemical. Particulates commonly adsorb toxic chemical species onto their surface — heavy metals (Pb, Cd, As) or polycyclic aromatic hydrocarbons (PAHs) from incomplete combustion. The particle again acts as a vector, transporting these low-volatility toxins deep into the alveolar region, where they desorb and are absorbed systemically — a combined effect greater than either the particle or the adsorbed chemical would cause alone.

Physical. Particle shape and biopersistence — a fibrous morphology (the clearest example being asbestos) — makes long, thin, durable fibres resistant to the lung's normal clearance mechanisms (the mucociliary escalator, macrophage engulfment), so they lodge permanently in alveolar tissue and cause chronic inflammation, fibrosis (asbestosis) or malignancy (mesothelioma) over a period of decades, even though the fibre material itself may be chemically and biologically inert.

(iii) PM2.5 vs. PM10: Health Effects and Aesthetics

Similarity. Both fractions are defined by an aerodynamic-diameter cut point (PM10: 50% collection efficiency at 10 µm; PM2.5: the same at 2.5 µm), and both cause respiratory/cardiovascular health effects and reduce visibility.

Main difference. PM10 (the coarse fraction, roughly 2.5–10 µm) is mostly mechanically generated — road dust, construction activity, pollen, sea salt — and deposits mainly in the upper respiratory tract (nose, throat, upper airways), where the body's natural clearance mechanisms remove much of it; health effects are largely irritation and asthma aggravation. PM2.5 (the fine fraction) is predominantly combustion-derived and secondary in origin — vehicle exhaust, industrial combustion, and secondary sulfate/nitrate/organic aerosol formed photochemically — and is small enough to penetrate past the upper airways deep into the alveoli, with some ultrafine fraction crossing into the bloodstream; at comparable mass concentration it is associated with substantially more severe long-term outcomes (cardiovascular disease, lung cancer, premature mortality). Aesthetically, PM2.5 is the dominant driver of regional haze because its size is close to the wavelength of visible light, making it a very efficient light scatterer (Mie scattering), and because it settles out far more slowly than coarse PM10, it persists longer in suspension and travels farther, worsening regional visibility well beyond the immediate source area.