# AIR POLLUTION MODELLING • Dispersion diffusion modelling for single and multiple point sources • Photochemical modelling for regional air quality • Receptor models BOX

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```					      AIR POLLUTION MODELLING

• Dispersion (diffusion) modelling for single and
multiple point sources
• Photochemical modelling for regional air quality
• Receptor models
BOX MODELS

• Conservation of mass principle applied to
relatively large scale systems such as an
urban airshed
• INPUT - OUTPUT + GENERATION -
CONSUMPTION = ACCUMULATION
Figure 6.1 de Nevers
• Simple box model of a rectangular city
SIMPLE BOX MODEL OF A CITY
qL
cb
uH
b = pollutant concentration in entering air
q = pollutant mass emission rate
per unit area in the city (area source)
L = length of box in direction of wind
H = mixing height
u = wind speed
c = pollutant concentration in air leaving box
• Pollutants of interest: “smog” components O3 and
secondary PM, i.e. reactive species

• Smaller boxes are required to characterize the “well
mixed” conditions

• Steady state rarely of interest, we are usually
interested in modelling, explaining, predicting,
preventing severe air pollution episodes of a transient
nature

• Wind, emission, and ambient monitoring data
required for meaningful modelling work
Figure 4-A Wark & Warner
• Development of mass balance equation with
 ci ( uci ) ( vci ) ( wci )     ci     ci         
+        +        +        = KX       +  KY     +  K V ci 
t     x       y        z    x  x  y     y  z 
         z 

+ R i + S i + Di + W i

ci          =       concentration of pollutant i,
a function of space (x,y,z) and time (t)
• u,v,w               =        horizontal and vertical wind speed
components
• KX, KY     =        horizontal turbulent diffusion coefficients
• KV         =        vertical turbulent exchange coefficients
• Ri         =        net rate of production of pollutant i by
chemical reactions
• Si          =       emission rate of pollutant i
• Di          =       net rate of change of pollutant i due to
surface                       uptake processes
Wi          =       net rate of change of pollutant i due to wet
deposition
2 DIMENSIONAL DISPERSION MODEL
Eulerian approach to the point source problem

 c       2c        2c
u      Ky       + Kz
 x       y 2
 z2
- u constant, v = w = 0
- advection term much greater than dispersion term in x
direction
-General solution:

K'       y2   z 2  u 
c ( x, y , z )     exp          
x         K
  y Kz     4 x 

                    
Q             y2   (z  H )2  
c            exp   2            
2 u y z       2 y   2 z  
2

Solution for windspeed of u m/s and continuous release
of Q g/s of pollutant at : x = y = 0 (stack location)
and z = H (the effective stack height)

H = h +h                                       2Kx
h : physical stack height,           
h : plume rise due to buoyancy
u
Figure 6.10 de Nevers
• UAM scheme
MULTIPLE BOX MODEL OF A CITY
THE URBAN AIRSHED MODEL - UAM
• Mass balances (including generation and
consumption terms) written for many boxes of
typically 2-5 km square and ~ 102 meters high.
• Each box is considered to be well mixed.
• Boxes can have mass fluxes to/from all adjacent
boxes.
• Inputs are time variant emission and wind patterns as
well as solar flux (for ozone photochemistry)
• Outputs are time variant concentrations of pollutant in
each box.
The region to be simulated is divided into several three-
dimensional grids covering the region of interest.

A base coarse grid covering the entire domain must first
be defined; then finer nested grids within the coarse grid
may be defined for regions in which more refined
analyses are desired.
Photochemical, multiple box modelling

• Given temporal and spatial variation of emissions and
atmospheric conditions (usually obtained from
specialized emission and meteorological models,
including solar flux etc), estimate the spatial and
temporal variation of ozone and fine PM
• Consider a complex array of anthropogenic and
natural emissions
• Consider complex chemistry among atmospheric
chemicals
Gas-Phase Chemistry
• Hundreds of organic compounds and thousands of
reactions participate in the formation of ozone in the
atmosphere.
• Most photochemical kinetic mechanisms treat organic
compounds in groups, often on the basis of the
reactive functional groups they contain.
• Carbon-bond approach: propylene, butene, and 1-
pentene would be split into one olefinic bond (OLE)
and one, two, and three paraffinic bonds (PAR),
respectively.
• ~80 reactions involving ~30 compounds or pseudo-
compounds
The major factors that affect ozone air
quality include:

• The spatial and temporal distribution of
emissions of NOx and volatile organic
compounds (VOC) (both anthropogenic and
biogenic)
• The composition of the emitted VOC and
NOx
• The chemical reactions involving VOC, NOx,
and other important species
• The spatial and temporal variations in the
wind fields
The major factors that affect ozone air
quality include:

• The dynamics of the boundary layer,
including stability and the level of mixing
• The diurnal variations of solar insolation
and temperature
• The loss of ozone and ozone precursors by
dry and wet deposition
• The ambient background of VOC, NOx, and
other species in, immediately upwind, and
above the region of study.

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 views: 134 posted: 1/21/2010 language: English pages: 16