Question 3 of 5: Measurement Techniques and Particulate Characteristics

Question 3: Measurement Techniques and Particulate Characteristics (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.

(i) Three Measurement Techniques

1. Nondispersive infrared absorption (NDIR) — for CO. A sample gas stream passes through a cell exposed to an infrared source beam; a narrow bandpass filter isolates CO's characteristic ~4.6 µm absorption band, and the fractional attenuation of the beam (Beer–Lambert law) is proportional to CO concentration along the optical path. Advantage: continuous, real-time, non-consuming measurement well suited to continuous emission monitoring. Limitation: other IR-absorbing species (CO₂, water vapour) can cross-interfere if optical filtering/reference compensation is inadequate, and the cell optics need periodic zero/span calibration and can foul in a dusty stream.

2. Chemiluminescence — for NO_x. Sample air is reacted with ozone generated on-instrument; NO + O₃ → NO₂* + O₂, and the electronically excited NO₂* relaxes by emitting light in proportion to the NO concentration present, detected by a photomultiplier tube. Total NO_x is obtained by first passing a second sample stream through a thermal/catalytic converter that reduces any NO₂ to NO, then differencing the two channel readings to recover NO₂ by subtraction. Advantage: highly sensitive and selective, the reference method for regulatory ambient NO_x monitoring. Limitation: the converter can also reduce other reactive nitrogen species (PAN, HNO₃), which is read as a positive interference/bias on the "NO₂" channel; requires an on-board ozone generator and periodic converter-efficiency checks.

3. High-volume gravimetric sampling (Hi-Vol) — for PM₁₀/TSP. A calibrated blower draws a large, known volume of ambient air (with a size-selective inlet cutting at 10 µm for PM₁₀) through a pre-weighed filter over a fixed sampling period (typically 24 h); the filter is re-weighed under controlled temperature/humidity, and the mass gain divided by the sampled air volume gives the time-averaged mass concentration directly. Advantage: a simple, robust, directly traceable mass measurement that is the regulatory reference method against which continuous PM monitors are calibrated. Limitation: it is not continuous (one integrated 24-h value per run, with lab turnaround before the result is known) and gives no real-time or sub-daily resolution, so it cannot support an alert/warning program the way a continuous beta-attenuation or optical monitor can.

(ii) Origin, Health and Aesthetic Considerations of Particulate Matter

Origin. Particulate matter is emitted directly (primary PM — combustion soot and ash, fugitive road/construction dust, sea salt) or forms in the atmosphere from gas-phase precursors (secondary PM — sulfate and nitrate aerosol from SO₂/NO_x oxidation, secondary organic aerosol from VOC photochemistry).

Health considerations. Health effects scale strongly with how deep a particle penetrates the respiratory tract, which is governed by aerodynamic diameter: coarse particles are largely captured in the upper airways, while fine particles (PM_2.5) penetrate to the alveoli, and some ultrafine fraction can cross into the bloodstream — driving respiratory irritation, asthma exacerbation, and (for chronic fine-PM exposure) elevated cardiovascular disease and premature mortality risk.

Aesthetic considerations. Fine particulate, being close in size to the wavelength of visible light, scatters light very efficiently (Mie scattering) and is the dominant driver of regional haze, degrading visibility across a wide area; particulate deposition also soils building facades, vegetation and outdoor surfaces near a source.

(iii) PM_2.5 and PM₁₀: Definitions and Three Key Differences

PM₁₀ is particulate matter with an aerodynamic diameter at or below 10 µm (50% collection efficiency cut point at 10 µm). PM_2.5 is the finer subset, at or below 2.5 µm (the same 50%-efficiency cut definition applied at 2.5 µm).

Difference 1 — origin. PM₁₀ (its coarse fraction, 2.5–10 µm) is predominantly mechanically generated — road dust, construction activity, pollen, sea salt. PM_2.5 is predominantly combustion-derived and secondary — vehicle exhaust, industrial combustion, and photochemically formed sulfate/nitrate/organic aerosol.

Difference 2 — health effect severity and depth of deposition. PM₁₀ deposits mainly in the upper respiratory tract, where natural clearance mechanisms remove much of it, producing mostly irritation/asthma aggravation. PM_2.5 penetrates to the alveoli (and its ultrafine tail into the bloodstream), and at comparable mass concentration is associated with substantially more severe long-term outcomes (cardiovascular disease, lung cancer, premature mortality).

Difference 3 — aesthetics/visibility. PM_2.5 is the dominant driver of regional haze because its size is close to the wavelength of visible light (efficient Mie scattering) and it settles far more slowly than coarse PM₁₀, so it persists and travels farther; PM₁₀'s aesthetic impact is more localized soiling/visible dust nearer its source, with a comparatively minor regional-haze contribution.

(iv) Total Suspended Particulate, Condensable and Filterable Particulate

Total suspended particulate (TSP) is all particulate matter suspended in ambient air, historically measured by a Hi-Vol sampler with no size-selective inlet, so it captures everything up to roughly 25–45 µm aerodynamic diameter — a broader, less health-relevant metric than PM₁₀/PM_2.5, largely superseded by size-selective sampling in current regulatory practice.

Filterable particulate matter is solid or liquid material that is already a particle at stack/duct temperature and is captured directly on a filter during source testing — a primary emission. Examples: fly ash, soot/carbon char, entrained metal oxide dust.

Condensable particulate matter is material that exists as a vapour at stack temperature but condenses into solid or liquid particles only after the flue gas cools and dilutes in the ambient atmosphere just past the stack exit — it is not captured by a standard hot-filter test and must be measured separately (e.g. by an impinger/condensation train). Examples: inorganic condensables such as sulfuric acid mist/ammonium sulfate forming from SO₃ and residual ammonia as the gas cools; organic condensables forming from semi-volatile unburned hydrocarbons.