Air Dispersion Modeling: CALPUFF

What Is the CALPUFF Modeling System?

CALPUFF is an advanced, non-steady-state meteorological and air quality modeling system maintained and distributed by Lakes Environmental. It is listed by the U.S. Environmental Protection Agency (EPA) as an alternative dispersion model for assessing long-range transport of pollutants and their potential air quality and visibility impacts, including impacts on Federal Class I areas. Unlike steady-state Gaussian models, CALPUFF simulates pollutant transport by following the pollutant plume (or “puffs”) as meteorological conditions vary over time and space.

The CALPUFF modeling system is composed of three integrated components:

• • CALMET – A diagnostic three-dimensional meteorological model that generates spatially and temporally varying wind, temperature, and boundary-layer parameters
  • CALPUFF – A non-steady state air quality dispersion model that simulates pollutant transport, chemical transformation, and removal
  • CALPOST – A post-processing package used to generate concentration fields, visibility metrics, and regulatory reporting outputs

CALPUFF Modeling Optimal Applicability

CALPUFF is well-suited for modeling scenarios involving complex or non-steady atmospheric conditions, including:

• • Near-field impacts in complex flow or dispersion situations
  • Complex terrain environments, including:
    • Stagnation, inversion, recirculation, and fumigation conditions
    • Overwater transport and coastal meteorological effects
    • Light wind speed and calm wind conditions
  • Long-range transport of pollutants
  • Visibility assessments and Federal Class I area impact studies
  • Criteria pollutant modeling, including support for State Implementation Plan (SIP) development
  • Secondary pollutant formation and particulate matter modeling
  • Buoyant area and line sources, such as wildfires and industrial processes

CALPUFF Technical Algorithms & Key Features

CALPUFF provides a comprehensive suite of algorithms and modeling features to simulate pollutant dispersion under complex meteorological and terrain conditions. These capabilities enable accurate assessment of near-field and long-range impacts (typically >50 km), and chemical transformation, and deposition processes.

Core technical algorithms include:

1. 1. Dry Deposition – Full resistance model for calculating dry deposition rates of gases and particulate matter, accounting for geophysical parameters, meteorological conditions, and pollutant species
   2. Wet Deposition – Simulates pollutant removal by precipitation, including rain and snow scavenging
   3. Chemical Transformation – Models chemical reactions and secondary pollutant formation during transport
   4. Subgrid-Scale Complex Terrain – Captures terrain effects smaller than the model grid, including hills, valleys, and local topography
   5. Puff Sampling Functions – Tracks pollutant “puffs” over time and space for non-steady state dispersion simulations
   6. Wind Shear Effects – Accounts for vertical changes in wind speed and direction, influencing plume rise and dispersion
   7. Building Downwash – Simulates the influence of nearby structures on pollutant plumes
   8. Overwater and Coastal Interaction Effects – Models the effects of land-sea interactions on plume dispersion
   9. Dispersion Coefficient Options – Provides flexibility to represent various atmospheric stability and turbulence conditions

Key Modeling Features:

1. 1. Source Types
      1. Point Source
      2. Line Source
      3. Volume Source
      4. Area Source
   2. Non-Steady State Emissions and Meteorology – Gridded 3D fields of meteorological variables (winds, temperature) that incorporates time and space-varying emissions
   3. Plume Rise – Simulates vertical movement of emissions due to buoyancy or momentum
   4. Interface to Emissions Production Model (EPM) developed by the U.S. Forest Service – Time-dependent emissions and heat release data for use in modeling controlled burns and wildfires
   5. Graphical User Interface (GUI) – Available from third-party software companies at cost –Supports setup, visualization, and post-processing of model results efficiently

Core Advantages of CALPUFF

CALPUFF is frequently selected when modeling objectives require a more realistic representation of atmospheric behavior than steady-state models can provide. Key advantages include:

• • Applicability to a wide range of far field and complex dispersion scenarios
  • Three-dimensionally varying meteorological fields for complex terrain and coastal regions
  • Ability to model both near-source and long-range impacts within a single framework
  • Suitability for Federal Class I area assessments and specialized regulatory applications
  • Comprehensive pre- and post-processing tools that support efficient and transparent modeling workflows

Note: From 2003 to 2017, CALPUFF was EPA’s preferred model for long-range transport assessments related to NAAQS and PSD increments. While it is no longer designated as the preferred model, it remains an important alternative for projects involving complex meteorological conditions.