AIR POLLUTION MODELLING

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AIR POLLUTION MODELLING

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

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BOX MODELS

• Conservation of mass principle applied to
  relatively large scale systems such as an urban
  airshed
• INPUT - OUTPUT GENERATION - CONSUMPTION
  ACCUMULATION

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Figure 6.1 de Nevers

• Simple box model of a rectangular city

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SIMPLE BOX MODEL OF A CITYsteady state, no
chemical reactions
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• 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

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Figure 4-A Wark Warner

• Development of mass balance equation with
  diffusion and advection components

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• 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

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Figure 6.10 de Nevers

• UAM scheme

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MULTIPLE BOX MODEL OF A CITYTHE 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.

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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.  
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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

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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

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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

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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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