Lecture 12 - Air Pollution Models

Question 1

Applications of Air Quality Monitors

Answer 1

1. assess impacts of sources
2. siting of new sources
3. emission control evaluation
4. contingency planning
5. emergency response
6. source apportionment
7. impact and risk assessment

Question 2

Types and components of air quality models

Answer 2

wind tunnels, simulation models, receptor models,

simulation models need emission inventory meteorology, physical and chemical processes

Question 3

Useless notes about simulation models

Answer 3

advection dispersion equation (conservation of mass), navier stokes equation (energy conservation in flow), heat expansion (assuming adiabatic expansions),

d[C] = advection, dispersion, settling, emissions, decay,

Question 4

Gaussian puff model assumptions and initial conditions

Answer 4

• instantaneous point source located at x = y = z = 0
• no particle settling
• infinite fluid (complete absorption at surfaces) and no reflections
• constant fluid flow along x axis and non isotropic, nonhomogeneous
turbulence
• first order decay

initial conditions: need an intial concentration (either 0 or C), boundaries are infinitely far away, conservation of mass is required, gaussian distribution in each direction

Question 5

Gaussian Plume model assumptions

Answer 5

mostly the same as gaussian puff model

• steady state, averaged concentrations (1 hour)
• constant U at all locations
• vertical and crosswind distributions are known & Gaussian
• negligible mass diffusion in x direction
• conservative pollutants (no transformation)
• no deposition and gravitational settling

of course you can modify this model to include:
1. Perfect absorption
2. Total reflection
3. Mass transfer approach for air ground fluxes (Use Vd
, deposition velocity)

Question 6

what do the dispersion coefficients in the gaussian plume equation mean? what are they based on

Answer 6

how much spreading there is each direction. They may be estimated empirically as a function
of stability category; downwind distance; and axis.

• wind speed;
• surface roughness;
• height above the surface;
• heat flux (solar angle, cloud cover, surface thermal properties,
anthropogenic heat production and surface slope relative to solar angle);
• sampling time & modeling scale;
• orographic barriers (terrain features)

Question 7

Stability Class

Answer 7

• A or 1 - very unstable (rapid dispersion)_
• B or 2 - moderately unstable
• C or 3 - slightly unstable
• D or 4 - neutral (most common)
• E or 5 - somewhat stable
• F or 6 - stable (least dispersion)
Pasquill-Gifford-Turner (PGT) Typing System

Question 8

What is plume rise

Answer 8

the difference between the stack height and the height of the plume centerline. a measure of the bouyancy of the plume and the wind speed

Question 9

What sort of information do you need from sources

Answer 9

a mass emission rate. for line sources its mass per km per time, for area sources its mass per area per time.

you also need the length or area of your source

Question 10

How is deposition incorporated into models? settling?

Answer 10

wet deposition scavenging coeff, dry deposition settling velocity in different regions (surface, laminar, transfer)

incorporation of a Stoke’s settling velocity based on flow regime and particle diameter

Question 11

What does he want us to know about Screen3

Answer 11

Screen3 is a short-term Gaussian plume model for non-reactive pollutants designed to model a single source, and determine downwind concentrations. It is a screening
model designed to estimate one-hour average worst-case concentrations with a low intensity effort requiring few parameters. It can also calculate concentrations at
selected distances, but only directly downwind.

Question 12

definitions of Screening model, critical wind speed, critical concentrations, receptors,

Answer 12

• Screening Model: Model used for initial calculations, often used to compare with NAAQS or other levels to determine if detailed modeling is required.
• Critical Wind Speed: Wind speed that produces highest concentration at specified distance and stability class.
• Critical Concentrations: Highest concentration that can be produced in any wind speed U, stability class SC, and downwind distance X.

Question 13

what options can you set in screen 3

Answer 13

gradual plume rise, stack tip downwash, bouyancy induced dispersion, terrain type, wind profile exponents, disperion coefficients

Question 14

Option 2. Stack tip downwash

Answer 14

This accounts for downwash due to top of
stack when flue gas has a low exit velocity in comparison to winds.The effect of stack tip downwash is to increase concentrations for sources with low flue gas velocities. Cavity problems

Question 15

Option 3. Buoyancy Induced Dispersion

Answer 15

Represents growth in plume due to
buoyancy, turbulence and entrainment of air immediately after stack release . Usually the effect of buoyancy induced dispersion is small, unless Hs is small

Question 16

Option 4. Terrain Type.

Answer 16

simple flat terrain, simple elevated terrain, complex terrain

Question 17

Option 1. Gradual plume rise. -

Answer 17

The effect of gradual plume rise is to increase concentrations near source. However, there is
a potential problem that the dispersion curves were
developed for horizontal plumes. So, the general
procedure is to compare concentrations with and without
plume rise.

Question 18

option 6 - dispersion coefficients

Answer 18

urban or rural. The choice of urban or rural is made by using the predominant land use.
Urban dispersion coefficients grow much faster with distance, due to increased
thermal and mechanical turbulence.