INTRODUCTION
Ambient Air Quality and Planetary Boundary Layer
Major Air Pollutants
Building of Mathematical Model
Air Quality Modeling
A. Ambient Air Quality and Planetary Boundary Layer
The ambient air quality is the concentration of air pollutant within the atmospheric
boundary layer or the mixing layer. The lowest layer of the atmosphere is called the
troposphere. The troposphere can be divided into two parts: a planetary boundary layer,
PBL, extending upward from the surface to a height that ranges anywhere from 100 to
3000 m, and above it, the free atmosphere. The boundary layer is directly influenced by
the presence of the Earth's surface, responding to such forcings as frictional drag, solar
heating, and evaporation. Each of these forcings generates turbulence of various-sized
eddies, which can be as deep as the boundary layer itself, lying on top of each
other. Consider the case of solar heating: solar heating of the ground on a sunny day
creates thermals of warmer air that rise over colder air causing vertical mixing and
turbulence. Therefore, in any weather prediction model, the PBL must be parameterized
as a mechanism for turbulence. Stull (1988) defines the atmospheric boundary layer as
``the part of the troposphere that is directly influenced by the presence of the earths
surface, and responds to surface forcings with a time scale of about an hour or less.''
The following figure illustrates a typical daytime evolution of the atmospheric boundary
layer in high pressure conditions over land. The solar heating causes thermal plumes to
rise, transporting moisture, heat and aerosols. The plumes rise and expand adiabatically
until a thermodynamic equilibrium is reached at the top of the atmospheric boundary
layer. The moisture transferred by the thermal plumes forms convective clouds.
Drier air from the free atmosphere penetrates down, replacing rising air parcels. The part
of the troposphere between the highest thermal plume tops and deepest parts of the
sinking free air is called the entrainment zone. The convective air motions generate
intense turbulent mixing. This tends to generate a mixed layer, which has potential
temperature and humidity nearly constant with height. When buoyant turbulence
generation dominates the mixed layer, it is called a convective boundary layer (CBL).
The lowest part of the ABL is called the surface layer. In windy conditions, the surface
layer is characterized by a strong wind shear caused by friction.
The boundary layer from sunset to sunrise is called the nocturnal boundary layer. It is
often characterized by a stable layer, which forms when the solar heating ends and the
radiative cooling and surface friction stabilize the lowest part of the ABL. Above that, the
remnants of the daytime CBL form a residual layer. The nocturnal boundary layer may
also be convective when cold air advects over a warm surface.
Factors governing concentrations within system:
Sources
Transformation
processes
Transport
processes
Removal
mechanisms
Concentrations
Adverse effects
Four groups of factors control the concentrations of contaminants in environmental
systems. In turn, the concentrations of contaminants have a major influence on adverse
effects, although, as indicated by the dashed line, there may be other important factors.
Notes:
1. The effects of sources and removal mechanisms are included in the governing
equation or the boundary conditions depending on their characteristics.
2. The transformation processes include chemical and biological reactions.
3. The mechanisms of transport processes include convection, diffusion, radiation (for
heat transport), and so on.
Types of units for concentration of air pollutant in the atmosphere:
(a). mass per unit volume or mass density: ug/m3;
(b). volume mixing ratio: ppb; (The advantage of volume mixing ratio is that the
magnitude is independent of temperature and pressure.)
(c). number of molecules per unit volume: molecule/cm3;
(d). relative humidity, RH;
For idea gas: PV=nRT or ρ = PM/RT. Thus, 1 ppbv = (M/V) μg/m3, where M is
molecular weight and V is the molar volume at the corresponding state (temperature and
pressure).
B. Major Air Pollutants
Types of air pollutants:
(a). by emission sources: stationary source and mobile source;
(b). by emission inventory: point source, line source, and area source;
(c). by pollutants: primary pollutant and secondary pollutant;
Note that primary pollutant is emitted directly from source to the atmosphere; therefore,
the physical and chemical properties of primary pollutants in the atmosphere are the same
as those in the emission sources, like stack flue gas. The secondary pollutants are
formed in the atmosphere by transformation processes. Thus, the physical and chemical
properties of secondary pollutants are different from those of their precursors. Note that
the concentrations of primary pollutants are determined by the three factors only: sources,
transport processes, and removal mechanisms.
1. Gaseous pollutants
Gaseous air pollutants can be classified into four groups based on the chemical species:
sulfur-containing species, nitrogen-containing species, hydrocarbon, and ozone. The air
pollutants for each types are:
Sulfur-containing species: H2S, DMS, SO2
Nitrogen-containing species: NH3, NO, NO2, N2O, HNO3, PAN, …….
Hydrocarbon: C6H6, HCHO,…….
Ozone
Note for unit conversion: ug/m3 = ppb*MW/24.5 at 1 atm and 25oC.
Note that except for ozone and some VOCs species, like HCHO, most of the gaseous air
pollutants are primary pollutants. The ambient ozone are only formed by photochemical
reactions and the HCHO are mixtures of primary emission and photochemical reaction
product.
2. Particulate matter:
Term
Descriptions
Size
Aerosol
Condensed matter suspended in air
nm to mm
Dust
Particles generated by mechanical forces, eg. wind, car,
supermicron
Smoke
Particles from uncomplete combustion, mainly elemental carbon
submicron
Fog
Liquid droplet from the condensation of water vapor
supermicron
Mist
Liquid droplet
supermicron
Fume
Particles formed by condensation of metallic vapor
submicron
Haze
Particles formed by photochemical reactions in the atmosphere
submicron
Smog
Particles mixtures of primary and secondary compositions
submicron
In additional to concentrations, there are two types of special characteristics of particles:
particle size distribution and chemical compositions.
Major chemical compositions and their precursors for the ambient particles
NOx
NH3
Airborne Particles
hv
O3
VOCs
SO2
N(V), S(VI), SOC
Ox
ida
nts
H
2O
2 ,O
3 ,e
tc.
氣
固
相
反
應
液
相
反
應
SO4=
Secondary
光
化
學
反
應
10~30%
-
3~20%
NH4+
4~15%
SOA
5~20%
POA
5~10%
NO3
Primary
Primary particles,
NaCl , dust etc.
Major chemical compositions and their formation pathways for the ambient particles
25~60%
C. Building of Mathematical Model
Types of air quality modelings include physical models and mathematical models.
Physical models are small scale, laboratory representations of the phenomena, like wind
tunnel and water tank. Mathematical models used a set of analytical/numerical
algorithms that describe the physical and chemical aspects of the problem. The
mathematical models can be classified into:
 deterministic models: based on fundamental mathematical descriptions of
atmospheric processes, in which effects (i.e., air pollutant concentrations) are
generated by causes (i.e., emissions);
 statistical models: based on semiempirical statistical relations among available data
and measurements.
Building of deterministic mathematical models include the followings major components:
Physical Processes
Chemical Processes
Biological Processes
Differential Equation
Solution
The procedures are:
1. To define the system, initial conditions, and boundary conditions;
2. To derive the differential equations from the physical/chemical/biological rules
with various assumptions; (Note that biological processes are generally not
important in air quality modelings.)
3. To solve the mathematical equations. For some complex simulations, the post
analyses are needed to handle the big data base and to show the visual results.
Example: Gaussian plume dispersion from a continuous and steady stack emission into an
uniform and stationary domain.
Categories of modeling approaches: Eulerian and Lagrangian methods
 Eulerian method: Coordinate system is fixed with respect to the same point in all
dimensions (spatial and temporal).
 Lagrangian method: Coordinate system is defined with respect to particle, that is,
traveled with particle.
D. Air Quality Modeling
Purposes of air quality modelings include, but not limited to the followings:
 To establish emission control legislation; i.e., to determine the maximum allowable
emission rates that will meet air quality standards;
 To evaluate proposed emission control techniques and strategies; i.e., evaluating the
impacts of future control, like SIP;
 To evaluate the source-receptor relationships for the existing air pollution problems.
Example: The basic structures for air quality management
Based on the source-receptor relationships, the air quality models can be divided into
dispersion or source-oriented models and receptor or receptor-oriented models.
Dispersion models estimate the ambient pollutant concentrations from various emission
sources. On the other hand, receptor models evaluate the source apportionment from
the measured ambient results. Types of dispersion models are Gaussian dispersion
models, like ISC, AERMOD, CALPUFF, box models, like EKMA, and
three-dimensional grid models or airshed models, like Models3/CMAQ, CAMx.
Examples of receptor models are principal component analysis (PCA), chemical mass
balance (CMB), and positive matrix factorization (PMF).
Characteristics of air quality modelings include spatial three-dimensional variations and
multi-scales.
Various scales of air quality modelings: Note that modeling scale is determined by the
pollutant and the study domain.
The major components of air quality modelings:
The ambient air pollutant
concentrations are determined by four factors: sources, transformation processes,
transport processes, and removal mechanisms. The inputs to air quality models can be
classified into two categories: meteorological data and source emissions. Note that
topography (terrain and surface land use) is generally specified included within the
meteorological data.
Example: Modeling system structure of AERMOD.
Example: Structure of Models-3/CMAQ
Procedures for air quality modeling work: