Question 3 of 7: Measurement of Air Pollutants and Particulate Health/Aesthetic Effects

Question 3: Measurement of Air Pollutants and Particulate Health/Aesthetic Effects (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) Infrared Absorption / Gas Chromatography for Ambient Pollutant Measurement

Nondispersive infrared (NDIR) absorption is the standard continuous-monitoring principle for gases such as CO and CO₂: each gas absorbs infrared radiation at characteristic vibrational-rotational wavelengths (CO at ~4.6 µm, for example), so an IR source is passed through a sample cell and a detector measures the attenuation at that wavelength; by the Beer–Lambert law the absorbance is proportional to concentration, $A = \varepsilon b c$, so the instrument's response calibrates directly to ppm concentration once the path length $b$ and molar absorptivity $\varepsilon$ (or an empirical calibration curve) are fixed. A "nondispersive" design uses a broadband source with a narrow-band optical filter selective to the target gas's absorption line, avoiding the cost of a full spectrometer while remaining specific to that pollutant; NDIR CO analyzers are the backbone of roadside/urban CO monitoring networks under the National Air Pollution Surveillance (NAPS) program.

Calibration of an NDIR analyzer is performed with certified reference gas standards: a zero gas (pollutant-free nitrogen or "zero air") establishes the baseline (zero absorbance) reading, and one or more span gases of accurately known concentration (traceable to a NIST- or equivalent-certified cylinder) are introduced to set the slope of the instrument's concentration-vs-signal response; a multi-point calibration curve is then verified across the expected ambient range, and the zero/span check is repeated on a routine schedule (daily to weekly) to correct for detector drift, matching the quality-assurance protocol required for regulatory ambient monitoring data.

(ii) Significance of Particle Size Distribution

Particle size distribution governs both where a particle deposits in the atmosphere and body, and which control technology can capture it, because the physics of transport (gravitational settling, inertial impaction, diffusion) all scale with particle diameter in different, competing ways. Environmentally, larger particles (>10 µm) settle out rapidly near the source under gravity (Question 5(ii)) and dominate local nuisance/soiling deposition, while fine and ultrafine particles (<2.5 µm, and especially the sub-0.1 µm nucleation mode) have settling velocities low enough that they remain airborne for days, undergo long-range transport, and participate in cloud-condensation-nuclei and secondary-aerosol chemistry (sulfate/nitrate formation, Question 1(i)). For engineering control selection, the size distribution directly dictates which device is effective: gravity settling chambers and cyclones (Question 7(i)) are efficient only above roughly 10–20 µm because collection efficiency depends on inertial forces that scale with $d_p^2$; fine particles below a device's cut diameter pass through largely uncaptured and require a fundamentally different mechanism — fabric filtration (diffusional/interception capture) or electrostatic precipitation (charge-driven migration, effective across a very wide size range including the sub-micron fraction that mechanical collectors cannot reach).

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

Two key differences distinguish the two size fractions. Health: PM10 (the coarse-to-fine fraction up to 10 µm) is large enough to be substantially filtered by the nasal passages and upper airway, so its principal health effect is irritation and aggravation of the upper respiratory tract and existing conditions like allergic rhinitis; PM2.5 (fine particulate) penetrates past the upper airway defenses deep into the alveolar region of the lung and can cross into the bloodstream, driving the cardiovascular and cardiopulmonary mortality/morbidity endpoints (heart attack, stroke, reduced lung function) that dominate the epidemiological burden attributed to particulate matter, and it is regulated with a tighter concentration standard under the CAAQS for that reason. Aesthetics: PM10's coarser, often mineral/wind-blown or mechanically generated composition (road dust, construction, agriculture) is visually obvious as settled grit, soiling of surfaces, and localized haze near the source; PM2.5, being predominantly combustion- and secondary-aerosol-derived and remaining suspended far longer, is the dominant contributor to regional visibility impairment (light scattering by sub-micron particles is most efficient near the wavelength of visible light) and to the persistent regional haze observed over urban airsheds, rather than to localized surface soiling.