Sources and References

Primary texts — Cooper, C.D. and Alley, F.C., Air Pollution Control: A Design Approach, 4th ed., Waveland, 2011; de Nevers, N., Air Pollution Control Engineering, 3rd ed., Waveland, 2017.

Supplementary texts — Wark, K., Warner, C.F., and Davis, W.T., Air Pollution: Its Origin and Control, 3rd ed., Addison-Wesley, 1998; Seinfeld, J.H. and Pandis, S.N., Atmospheric Chemistry and Physics, 3rd ed., Wiley, 2016.

Online resources — MIT OCW 1.84J “Atmospheric Chemistry”; U.S. EPA AP-42 emissions factors; NIOSH Pocket Guide; ISO 14644 cleanroom standards (public summaries); Environment and Climate Change Canada public air quality documents.

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Chapter 1: Air Pollutants and Their Sources

1.1 Criteria Pollutants

The US EPA defines six criteria pollutants with National Ambient Air Quality Standards (NAAQS): PM (PM\(_{10}\), PM\(_{2.5}\)), SO\(_2\), NO\(_2\), CO, O\(_3\) (secondary), and Pb. Canadian and European equivalents use similar or stricter standards. Hazardous air pollutants (HAPs) form a separate list of toxic chemicals with specific emission limits.

1.2 Sources

• Stationary combustion: power plants, industrial boilers, refineries, cement kilns.
• Mobile: on-road vehicles, off-road equipment, marine vessels, aircraft.
• Industrial processes: chemical plants, mineral processing, metallurgy, agriculture.
• Fugitive: leaks, dust from handling, evaporation losses.

Emission inventories (NEI in US, NPRI in Canada) aggregate sources for regulatory and planning purposes.

1.3 Formation Mechanisms

• Thermal NO\(_x\): Zeldovich mechanism, strongly temperature-dependent (> 1500 °C).
• Fuel NO\(_x\): oxidation of fuel-bound nitrogen.
• Prompt NO\(_x\): Fenimore mechanism in flame fronts.
• SO\(_2\): oxidation of fuel sulfur.
• PM: incomplete combustion (soot), ash, mineral condensation, secondary photochemistry (ammonium nitrate, sulfate).
• VOCs: evaporation, incomplete combustion, fugitive leaks; precursors to tropospheric ozone via NO\(_x\) / VOC photochemistry.

1.4 Regulatory Structure

Canada: CEPA, Canada-wide standards, provincial regulations. US: Clean Air Act with Title V permitting, NAAQS, NESHAP (HAPs), NSPS (new source performance), Acid Rain (SO\(_2\)/NO\(_x\) cap-and-trade). EU: Industrial Emissions Directive, Best Available Techniques (BAT) reference documents. International: CLRTAP, Montreal Protocol, Paris Agreement.

Chapter 2: Atmospheric Dispersion

2.1 The Plume Model

A steady, continuous point source disperses downwind under prevailing winds. The Gaussian plume equation for concentration at \( (x, y, z) \):

\[ C(x,y,z) = \frac{Q}{2\pi u \sigma_y \sigma_z}\exp\!\left[-\frac{y^2}{2\sigma_y^2}\right]\left\{\exp\!\left[-\frac{(z-H)^2}{2\sigma_z^2}\right] + \exp\!\left[-\frac{(z+H)^2}{2\sigma_z^2}\right]\right\}. \]

\( H \) is effective stack height (physical stack + plume rise); \( \sigma_y(x), \sigma_z(x) \) are Pasquill-Gifford dispersion coefficients functions of atmospheric stability class (A-F).

2.2 Plume Rise

Buoyancy-driven (Briggs): \( \Delta h = 1.6 F^{1/3} x^{2/3}/u \) with buoyancy flux \( F = g v_s d_s^2 (T_s - T_a)/(4 T_s) \). Momentum-dominated plumes use different relations. Downwash, stack-tip effects, and building wakes require CFD or wind-tunnel study in complex environments.

2.3 Stability and Mixing

Atmospheric stability classes from surface wind speed and solar radiation (daytime) or cloud cover (nighttime). Very unstable (A) disperses fastest; very stable (F) trap emissions. Inversions (ground-based radiation inversions, subsidence inversions) cap vertical mixing and concentrate pollutants. Thermal mixing height limits plume rise and dilution.

2.4 Regulatory Modeling

AERMOD (US EPA) is the regulatory workhorse for near-field dispersion. CALPUFF handles long-range, non-steady conditions. Photochemical models (CMAQ, CAMx) simulate secondary pollutant formation on regional scales. Model-observation comparison informs emission reduction targets.

Chapter 3: Particulate Control

3.1 Particle Characteristics

Aerodynamic diameter integrates shape and density into an equivalent-sphere basis. Particle size distributions are typically log-normal; coarse (2.5-10 µm) and fine (PM\(_{2.5}\)) modes dominate human exposure concerns. Coarse particles deposit by impaction and settling; fine by diffusion and interception. Penetrate to alveoli when < ~3 µm.

3.2 Cyclones

Centrifugal separation: dusty gas enters tangentially, rotates in outer vortex, particles spiral to wall and fall out; clean gas exits via central vortex. Critical cut diameter from Lapple:

\[ d_{50} = \sqrt{\frac{9 \mu b}{2 \pi N_e v_{in}(\rho_p - \rho_g)}}. \]

Cut diameters 5-15 µm; pressure drops 500-2000 Pa. High-efficiency cyclones collect to ~90% at 5 µm; used as precleaners for finer devices.

3.3 Electrostatic Precipitators

Corona discharge from wires charges particles; collection plates hold them. Deutsch-Anderson efficiency:

\[ \eta = 1 - \exp(-\omega A/Q), \]

with migration velocity \( \omega \) (2-20 cm/s). ESPs achieve > 99.5% efficiency on fly ash; require low particle resistivity (gas conditioning with SO\(_3\) or ammonia can be necessary). Capital-intensive but low operating cost.

3.4 Fabric Filters (Baghouses)

Filtration through woven or felt fabrics with periodic cleaning (reverse-gas, shake-deflate, pulse-jet). Cake filtration captures > 99.9% of fine particles. Air-to-cloth ratio (typical 1-3 ft/min) sizes the filter. Fabric choice depends on temperature, moisture, chemical compatibility. Limited by temperature; advanced ceramic filters extend to > 800 °C.

3.5 Wet Scrubbers

Venturi scrubbers, spray towers, packed columns, impingement/entrainment scrubbers. Particles impact water droplets; collection efficiency improves with liquid-to-gas ratio and velocity differential. Used when particulates are also soluble gases, flammable, or sticky. Drawback: wastewater handling.

Chapter 4: Gaseous Pollutant Control

4.1 Absorption

Soluble acid gases (SO\(_2\), HCl, HF, NH\(_3\)) absorbed in water or reagent solution. Column design from Kremser/HTU-NTU framework:

\[ N_{TU} = \int_{y_{out}}^{y_{in}} \frac{dy}{y - y^*}, \]

with driving force \( y - y^* \) from equilibrium. Packing selection by BAT guidance; plastic random packings common for corrosive service.

4.2 Adsorption

VOCs removed by activated carbon, zeolites, polymeric resins. Breakthrough curves characterize bed capacity; regeneration by steam (continuous adsorption with rotating wheels or twin-bed systems) recovers solvent and restores capacity. Typical VOC removal > 95% at face velocities 0.1-0.5 m/s.

4.3 Thermal Oxidation

Direct-fired thermal oxidizers (DFTO) burn VOC-laden streams at 700-1000 °C; regenerative thermal oxidizers (RTO) use ceramic heat storage to preheat incoming gas, dramatically reducing fuel use. Catalytic oxidizers operate at 250-500 °C with Pt/Pd catalysts. Trade-offs: DFTO (simple, expensive fuel), RTO (capital-intensive, fuel-efficient), CATOX (catalyst deactivation risk).

4.4 Biofiltration and Absorption into Biological Media

Biofilters (peat, compost, wood chips): VOCs and odor compounds biodegraded on biofilm. Low operating cost, good for large-volume dilute streams (odor control, WWTP off-gas). Residence times 30-60 s; removal > 90% for water-soluble compounds.

Chapter 5: NO\(_x\) and SO\(_x\) Control

5.1 Combustion-Based NO\(_x\) Reduction

Low-NO\(_x\) burners (staged combustion), flue gas recirculation, over-fire air, reburning. Reduce formation at the source. Typical reductions 30-70%.

5.2 Selective Catalytic Reduction (SCR)

NH\(_3\) or urea injected, reduces NO\(_x\) to N\(_2\) + H\(_2\)O over V\(_2\)O\(_5\)-WO\(_3\)/TiO\(_2\) catalyst at 300-400 °C:

\[ 4 NO + 4 NH_3 + O_2 \to 4 N_2 + 6 H_2O. \]

Achieves > 90% NO\(_x\) reduction. Ammonia slip must be controlled; catalyst poisoning (As, alkali) and fouling considerations.

5.3 Selective Non-Catalytic Reduction (SNCR)

Urea or ammonia injected into high-temperature zone (900-1100 °C); requires no catalyst. Typical reduction 30-60%; operating window narrow but capital low. Often combined with low-NO\(_x\) burners.

5.4 Flue Gas Desulfurization (FGD)

Wet limestone FGD: CaCO\(_3\) slurry scrubs SO\(_2\) to produce gypsum (CaSO\(_4\)·2H\(_2\)O); > 95% efficient, dominant global technology. Seawater FGD: alkalinity of seawater neutralizes; coastal use. Dry FGD: lime injection, sodium-based sorbents; moderate efficiency, no wastewater. Spray dry absorber: intermediate option. Regenerable processes (Wellman-Lord) produce concentrated SO\(_2\) for acid or sulfur manufacture.

Chapter 6: Indoor Air Quality

6.1 IAQ Pollutants

Volatile organic compounds from building materials, furnishings, and cleaning products; formaldehyde from pressed-wood; radon from soil; biologicals (mold, dust mites, dander); combustion products; particulate matter ingress from outdoors. Sick building syndrome and building-related illnesses are known hazards; occupants spend > 90% of time indoors.

6.2 Ventilation Standards

ASHRAE 62.1 and 62.2 specify minimum ventilation rates. Air changes per hour (ACH) and outdoor air fraction both matter. Demand-controlled ventilation (CO\(_2\) sensors) reduces energy penalty of over-ventilation while maintaining IAQ.

6.3 Indoor Air Measurement

Direct-reading instruments (photoionization, photoacoustic, electrochemical), passive samplers (diffusion tubes), active sampling onto sorbents followed by GC/MS. Particle counters (optical, light scattering) for PM and airborne microbial assessment. Radon with alpha-track detectors or continuous monitors.

6.4 Control Strategies

Source control (low-emission materials, avoid spray solvents), ventilation, air cleaning (filtration, adsorption, UV-C upper-room for airborne pathogens), humidity management (40-60% RH suppresses dust mites and mold while preserving comfort). Post-COVID reassessment emphasized higher MERV filters, HEPA portable units, and increased outdoor air rates.