# Air Quality Module

The Air Quality Module mainly deals with the prediction of pollution concentrations at a disaggregate level. SMARTPLANS performs these predictions at the grid cell level. The user can define the spatial resolution of the input grid cells to be used in the estimation of pollution concentration. The estimated concentrations will also be used as input into the Health Benefits Module. The pollutant included in the model are: Hydro Carbon (HC), Carbon Monoxide (CO), Nitrogen Oxide (NO_{x}), fine particulate matter (PM_{2.5}), particulate matter (PM_{10}), Sulfur Dioxide (SO_{2}), and Carbon Dioxide (CO_{2}). The Air Quality Module allows the user to choose one of two different Pollution Concentration Models to predict the concentrations of the various pollutants at a disaggregate level: (1) Land Use Regression (LUR) model, or (2) Gaussian Dispersion model.

### Land Use Regression (LUR) Modeling Approach

The LUR approach utilizes a multivariate linear regression model to estimate pollution concentration. Here, the concentration is calculated as follows:

where betas are the estimated parameters for pollutant p, while X’s are significant variables that influence pollutant p.

### Gaussian Dispersion Modeling Approach

The Gaussian Dispersion modeling approach estimates the concentration of pollutant p at a receptor point (x, y, z) (i.e., centroid of a grid cell) from a given source (i.e., center of road links) that lie at an angle (theta > 0) along the wind direction w. Ground Level concentration C for pollutant p at crosswind distance y and downwind distance x is calculated as follows:

where:

- C
_{p}is the concentration of a specific pollutant p at location x, y, and z - H is the height (m) of the release of the emissions from emission sources (i.e., vehicles). This is set to 0.035 m
- Y is the distance (m) along the normal from the emission source to the plume center line
- Q
_{p}is the simulated road link emission rate (g/m/s) of pollutant p - μ is the average wind speed (m/s)
- σ
_{y}is the standard deviation of the horizontal concentration in the plume - σ
_{z}is the standard deviation of the vertical concentration in the plume - D
_{p}is an optional term capturing the molecular weight of pollutant p

D_{p} can be set to either a power or exponential function.

where E_{p} is a positive parameter > 0; β is less than or equal to 0; and W_{p} is the molecular weight of p.

The dispersion parameter σ_{y} and σ_{z} vary by “Stability Class”, which are dependent on weather conditions and average wind speed. For example, if the stability class is set to Day with moderate incoming solar radiation and wind speed greater than 5 m/s, then the stability class will be D. Consequently, the standard deviations of the horizonal and vertical concentrations will be calculated as follows: