Air Quality Models
Air Quality Modeling
April 4, 2013
- 1 Air Quality Models
- 1.1 Overview
- 1.1.1 Configuration
- 1.1.2 Sources
- 1.1.3 Transport and Removal
- 1.1.4 Transformation
- 1.2 Available Eulerian Grid Models
- 1.3 Running CAMx
- 1.4 Running CMAQ
Chapter 1: Air Quality Models
OOOOThis section gives a high level overview of Air Quality Models (AQMs) and focuses on AQMs used for NAAQS regulatory modeling. The most commonly used regulatory models are the Comprehensive Air Quality Model with extensions (CAMx) and the Community Multiscale Air Quality model (CMAQ). Both CAMx and CMAQ numerically simulate sources, transport, removal, and transformation of atmospheric pollutants (see Figure 1.1). Both models share basic configuration, processes, and inputs.
Figure 1.1: Overview of model inputs and processes.
OOOOEulerian photochemical grid models use the finite difference element to represent a defined space and time. The finite difference element assumes that a continuum can be represented by a series of discrete elements. Thus, time be discretized into hours and space will be discretized into vertical layers, east-west rows, and north-south columns.
OOOOOf course, we will not discretize infinite time or infinite space. Instead, we define the simulation spatio-temporal boundaries. Outside of these boundaries pollutant concentrations will not be modeled; they will be provided as boundary condition inputs. In space, a simulation will typically focus horizontally on an urban region and extend from the ground into the free troposphere. Horizontal boundary conditions are typically referred to as boundary conditions. In time, a simulation will typically be on the order of weeks.
Boundary Condition Files:
Finite Difference Elements
OOOOThe space and time within the boundaries are descretized uniformly in the horizontal plane and with decreasing resolution vertically.
OOOOThe first basic modeling process represents the sources of pollutants. Sources are represented by emission files. Emission files come in two forms: point and low-level.
OOOOPoint sources include stacks and flares. Stacks and flares emit intensely enough to require separate representation. Each stack or flare must have temperature (K), velocity (m/s), flow (m^2/s), and speciated mass of emissions (moles/h). From height, temperature, and velocity of emission you can calculate the plume height.
OOOOLow-level emissions include all other emissions. Low-level emissions, like point sources are in mol/hr. The emissions must first be converted to ppm, then transported, then converted to molecules/cm^3 before chemistry.
1.1.3 Transport and Removal
OOOOTransport and removal are physical processes that move pollutants in, around, and out of our simulation boundaries. Transport and removal processes are advection, diffusion, wet scavenging, and dry deposition.
OOOOAdvection is directly a function of wind speeds in 3-D. Typically the east-west component is referred to as U, the north-south component is V and the up-down component is W. The U and V (and W for CMAQ) components are provided as inputs to the model. Figure 1.1.3 shows that wind components are stored on the edges of the cell. This is an important difference from what weve talked about so far; concentrations, emissions, temperature, pressure, etc are all at the center.
Figure 1.2: Arakawa C grid configuration.
OOOOWet scavenging and wet deposition is a partitioning process between the gas phase and the liquid vapor and droplets. The liquid vapor and droplet content of the air is provided as inputs. These inputs include the following types of water in g/m^3: cloud, rain, snow, and graupel (do you know what graupel is?).
OOOODry deposition is the rate at which gas and particles in the air hit the ground and become ground phase. Dry deposition is calculated using Equation 1.1 and is heavily dependent on the land use for surface properties.
Vd = 1/(ra+rb+rs)OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO(1.1)
- ra is bulk resistence, a function of radiation,surf roughness, and near surface temperature lapse
- rb is a function of chemical species
OOOOOOO(a) Over land: a function of stomatal/mesophylic portions of plants, pathway into upper canopy, pathway into lower canopy, and pathway to ground
OOOOOOO(b) Over water: a function of Henrys Law Constant, Surface Temp, and u
OOOOThe properties of land are parameterized for broad categories. The landuse input file assigns each grid cell a fractional coverage of each category.
OOOOAir pollutants undergo transformation in the atmosphere via chemistry. The chemical component of the model is one of the most complex and will only briefly be covered here. First, the chemistry is simplified by generalization, distortion and deletion.
• generalization: all alkanes are the same
• deletion: ignore pernitric acid
• distortion: radicals are in pseudo-steady state (i.e. ∆ ppm/s=0)
OOOOThe remaining chemistry can be separated into photolytic and kinetic reactions. Photolysis is a function of chemical species concentrations, cross-section, quantum yield, and the available sunlight. Kinetic reactions are a function of temperature, pressure, and concentration of reactants.
Kinetic Rate Constant Forms:
• Arrhenius equation
OOOO-K = A x (−T/300)B × e−T/C
• Reverse equilibrium
OOOO- K = Kn / (A × (−T/300)B × e-T/C)
OOOO- K = A(1.0 + 0.6P)
OOOO- K = K0 + K0[M]/(1 + K2[M]/K1)
OOOO- K = K1 × K0[M]
• Fall off
OOOO- K = K0[M]/(1 + K0[M]/K1) × F1/(1/N + log10(K0[M]/K1)^2)
1.2 Available Eulerian Grid Models
1.3 Running CAMx
Table 1.1: Eulerian models
OOOOBroadly speaking, there are 7 steps to running CAMx (see below). In our lab, steps 1 through 3 are generally prepared for us for the base simulation. Step 4 only has to be run occasionally. Steps 5 and 6 are generally created for us, but must be modified for use at UF. Steps 5 and 6 are also frequently combined (e.g. in the test case). Step 7 will always be run at UF.
- Create meteorology inputs
- Create emission inputs
- Create other inputs
- Compile CAMx executable
- Configure options for CAMx
- Create job script
- Run job script