Air and Noise Pollution and Control: Dispersion and Abatement
Air pollution questions test dispersion modelling, stability classification, stack design and control-device performance; noise questions test decibel arithmetic, equivalent levels and attenuation. Calculations are usually short but unit-sensitive.
Core rules and concepts
- Stability and dispersion: Pasquill-Gifford classes A (very unstable) to F (very stable); stable conditions give small dispersion coefficients and high ground-level concentrations. Mixing height and inversions trap pollutants.
- Gaussian plume: for ground-level centreline concentration with effective stack height H, C = Q/(pi u sy sz) exp[-H^2/(2 sz^2)]; the factor 2 for ground reflection is already embedded in this form.
- Effective stack height: H = physical height + plume rise (Briggs equations depend on buoyancy flux and wind speed).
- Control devices: cyclones (cut diameter), ESPs (Deutsch equation, efficiency = 1 - exp(-wA/Q)), baghouses (air-to-cloth ratio), scrubbers and adsorbers.
- Noise: L = 10 log(I/I0) with I0 = 10^-12 W/m2; levels add logarithmically; Leq is the energy-average over a period; point sources attenuate 20 log(r2/r1).
Worked example
A stack emits Q = 100 g/s at effective height H = 0 with u = 2 m/s, sy = 50 m, sz = 30 m. Ground-level centreline concentration is C = 100/(pi x 2 x 50 x 30) = 100/9425 = 0.0106 g/m3, i.e. about 10,600 ug/m3. If H is raised to 60 m, the exponential term exp[-3600/(2 x 900)] = exp(-2) = 0.135, cutting the concentration to roughly 1,430 ug/m3. The key step is using effective, not physical, stack height.
Traps that cost marks
Adding decibels arithmetically (two 70 dB sources give 73 dB, not 140 dB); using stack height instead of effective height; mixing units of sy and sz; and ignoring that ESP efficiency depends on both drift velocity and specific collection area.