AQS Air Dispersion Modeling 1 — Questions and Answers
Question 1: Which EPA-approved air dispersion model is the preferred regulatory model for most near-field (within ~50 km) stationary source applications?
- AERMOD (Correct answer)
- CALPUFF
- ISC3
- HYSPLIT
Correct answer: AERMOD
AERMOD (American Meteorological Society/EPA Regulatory Model) is the EPA's preferred regulatory model for near-field dispersion from stationary sources.
Question 2: In the Pasquill-Gifford atmospheric stability classification system, which stability class represents the most unstable atmospheric conditions?
- Class A (Correct answer)
- Class C
- Class E
- Class F
Correct answer: Class A
Class A is the most unstable Pasquill-Gifford stability class, occurring under strong solar radiation and light surface winds.
Question 3: The Gaussian plume model assumes that pollutant concentrations follow what type of statistical distribution in both the horizontal and vertical directions from the plume centerline?
- Logarithmic distribution
- Normal (Gaussian) distribution (Correct answer)
- Uniform distribution
- Exponential decay distribution
Correct answer: Normal (Gaussian) distribution
The Gaussian plume model assumes a normal (bell-curve) distribution of pollutant concentrations both crosswind and vertically from the plume centerline.
Question 4: The AERMET meteorological preprocessor is used to prepare which type of data for input into AERMOD?
- Terrain elevation data from digital elevation models
- Surface meteorological and upper-air sounding data (Correct answer)
- Stack emission rates and source parameters
- Receptor grid coordinates and elevations
Correct answer: Surface meteorological and upper-air sounding data
AERMET processes surface meteorological observations and upper-air soundings to derive planetary boundary layer parameters required by AERMOD.
Question 5: In air dispersion modeling, 'effective stack height' is defined as the sum of the physical stack height plus:
- The ambient mixing height at the time of release
- Plume rise due to momentum and buoyancy of exhaust gases (Correct answer)
- The height of nearby buildings causing downwash
- The height at which plume centerline reaches ambient temperature
Correct answer: Plume rise due to momentum and buoyancy of exhaust gases
Effective stack height equals physical stack height plus plume rise, which results from the upward momentum and buoyancy of hot exhaust gases.
Question 6: In the Gaussian dispersion equation, sigma-z (σz) represents the standard deviation of pollutant concentration in which direction?
- Horizontal crosswind direction
- Vertical direction (Correct answer)
- Along-wind (downwind) direction
- Temporal variation
Correct answer: Vertical direction
Sigma-z (σz) is the standard deviation of vertical concentration distribution, quantifying the degree of vertical plume spreading with distance from the source.
Question 7: Building downwash in air dispersion modeling occurs when stack emissions are captured by the aerodynamic wake created by:
- A temperature inversion layer aloft
- The source building or nearby structures (Correct answer)
- High ambient humidity causing plume impaction
- Wind shear between atmospheric layers
Correct answer: The source building or nearby structures
Building downwash occurs when the aerodynamic recirculation zone behind a building or structure captures and pulls the plume downward toward ground level.
AQS Air Dispersion Modeling 1
Which EPA-approved air dispersion model is the preferred regulatory model for most near-field (within ~50 km) stationary source applications?