Most mobile source dispersion models predict air pollution concentrations near roadways resulting from motor vehicles traveling under free-flow conditions. BREEZE ROADS is an enhanced version of the CALINE4, CAL3QHC, and CAL3QHCR series of models that incorporates methods for estimating queue lengths and the contribution from idling vehicles.


CALINE4 is a line source air quality model developed by the California Department of Transportation (Caltrans). It is based on the Gaussian diffusion equation and employs a mixing zone concept to characterize pollutant dispersion over the roadway.

The purpose of the model is to assess air quality impacts near transportation facilities. Given source strength, meteorology and site geometry, CALINE4 can predict pollutant concentrations for receptors located within 500 meters of the roadway. In addition to predicting concentrations of relatively inert pollutants such as carbon monoxide (CO), the model can predict nitrogen dioxide (NO2) and suspended particle concentrations. It also has special options for modeling air quality near intersections, street canyons, and parking facilities.

  • Include similar model theory as is in CAL3QHC, but has an advanced method for calculating NO2 concentrations using the Discrete Parcel Method
  • Predict pollutant concentrations for receptors located
  • Compute NO2 concentrations without the need for an external background concentration file
  • Ability to model the standard five roadway types (at-grade, depressed, fill, bridges, and intersections), as well as street canyons, bluffs, and parking lots


ROADS CALINE4 Screenshot

Users have the option to select between using the CALINE4, CAL3QHCR, and CAL3QHC models in the Control Options dialog box. Each model requires that certain parameters are entered in order to complete a model run.


The CAL3QHC model is a roadway model that predicts air pollutant concentrations near highways and arterial streets due to emissions from motor vehicles operating under free-flow conditions and idling vehicles. In addition, CAL3QHC incorporates methods for estimating traffic queue lengths at roadway intersections.

BREEZE CAL3QHC permits the user to specify roadway links and receptor locations within an XYZ plane. The Y-axis is aligned due north, with wind angle inputs to the model following accepted meteorological convention (e.g. 270E represents a wind from the west). The positive X-axis is aligned due east. A link can be specified as either a free flow or a queue link. The program automatically sums the contributions from each link to each receptor. Surface roughness and meteorological variables (such as atmospheric stability, wind speed, and wind direction) are assumed to be spatially constant over the entire study area.

ROADS CAL3QHC option screenshot

When selecting the CAL3QHC model, users are prompted to enter the averaging time and pollutant.


The CAL3QHCR model is an enhanced version of CAL3QHC. This version can process up to a year of hourly meteorological, vehicular emissions, and traffic volume and signalization data in one model run. In addition, 1-hour and running 8-hour averages of CO or 24-hour and annual block averages of PM can be calculated. CAL3QHCR can also be used as an analysis tool for high concentration episodes or for examining the effects that changing meteorology and traffic conditions have on concentrations. Other features include:

  • Conduct daily to seasonal runs with the CAL3QHCR model
  • Use a two-tiered approach to process data according to the day of the week
  • Produce output that provides a detailed synopsis of MET and traffic conditions on air quality


ROADS CAL3QHCR option screenshot

When selecting CAL3QHCR, users are prompted to select the pollutant and identify traffic patterns.

Additional Functions and Capabilities

More information on the various functions and capabilities of the CALINE4, CAL3QHC, and CAL3QHCR models can be seen in the tables below:

Averaging Periods CALINE4 CAL3QHC CAL3QHCR
CO 1 hr 30-60 min 1 hr and Running 8 hr
PM 1 hr 30-60 min 1 hr, 24 hr, Period, and Annual Block
NO2 1 hr n/a 1 hr and Annual Block    
Other inert pollutants 1 hr 1 hr  


Available Features CALINE4 CAL3QHC CAL3QHCR
One and two tier approaches     x
Can be used as an analysis tool for high concentration episodes or for examining the effects changing meteorology and traffic conditions have on concentrations     x
ISC CALMPRO algorithm included     x
Urban and rural modes     x
Options to use ISCST3 meteorological data files     x
Can specify range of dates to model     x
Requires external background concentration file for computing NO2 concentrations     x
Can process up to 120 roadway links x x x
Can process up to 200 receptors x x  
At-grade roadways x x x
Depressed roadways x x x
Fill (raised roof) roadways x x x
Bridges x x x
Intersections / Queuing links x x x
Street canyons and bluffs x    
Parking lots x    

Request a Trial

Related Products & Services

  • Model-ready meteorological data for the ISC and CAL3QHCR models

  • Model air quality impacts due to industry emissions for permitting and planning purposes

  • A multi-layer, multi-species non-steady-state puff dispersion model

  • Custom EHS, modeling, and programming solutions

Upcoming Training