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

Document Sample
AIR POLLUTION MODELLING • Dispersion diffusion modelling for single and multiple point sources • Photochemical modelling for regional air quality • Receptor models BOX Powered By Docstoc
					      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
    steady state, no chemical reactions
      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
  diffusion and advection components
 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
- Steady state
- 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 
                                               
                                               
  2-D STEADY DISPERSION MODEL
            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.