Dispersion modeling uses mathematical algorithms to characterize or mimic the atmospheric processes that disperse a pollutant emitted by a source. Based on emissions and meteorological inputs, a dispersion model can be used to predict concentrations at selected downwind receptor locations. These air quality models are used to determine compliance with National Ambient Air Quality Standards (NAAQS), and other regulatory requirements such as New Source Review (NSR) and Prevention of Significant Deterioration (PSD) regulations. These models are addressed in Appendix A of EPA's Guideline on Air Quality Models (also published as Appendix W of 40 CFR Part 51).
Our Approach to Dispersion Modeling:
Air Compliance Testing will start by collecting all needed information about your facility, such as stack heights, point parameters, exact locations, and emission rates. Our next step is to perform preliminary modeling (screening) to determine whether a more refined modeling effort is required. If it is, then we will perform a sophisticated modeling analysis according to the guidelines of the state in which the facility is located. Our finished report to you will be suitable for submittal to your State Agency.
Dispersion Modeling Can Be Used During Your Permitting Process:
To verify that emissions from a new source will not exceed ambient air quality standards, or Maximum Ground Level or Fence Line Concentrations (Air Toxics requirements).
To determine appropriate stack configuration and/or additional control requirements.
To predict future pollutant concentrations from multiple sources.
To perform Air Quality Impact Studies.
To evaluate the effect of building downwash.
To analyze deposition patterns.
Modeling Can Be a Very Cost-Effective Approach:
When preliminary calculations indicate the need for excessively high stack heights that would be expensive to install;
When multiple discharges from stacks and vents are present;
When a better understanding of the impact of discharges is required.
When management controls are needed to minimize odor impacts. Dispersion Modeling can determine the conditions under which odor complaints are likely to occur, which can be used to plan a mitigation strategy;
When the impact of changes in emissions, emission controls or stack heights are being assessed. Potential changes can be input into the model to see the effect on the dispersion pattern.
Dispersion Models
Toxins Screening Model (TSCREEN) - A Gaussian model that implements the procedures to correctly analyze toxic emissions and their subsequent dispersion from one of many different types of possible releases for superfund sites. It contains 3 models: SCREEN3, PUFF, and RVD (Relief Valve Discharge).
AERMOD Modeling System - A steady-state plume model that incorporates air dispersion based on planetary boundary layer turbulence structure and scaling concepts, including treatment of both surface and elevated sources, and both simple and complex terrain.
CALPUFF Modeling System - A non-steady-state puff dispersion model that simulates the effects of time- and space-varying meteorological conditions on pollution transport, transformation, and removal. CALPUFF can be applied for long-range transport and for complex terrain.
The SLAB model treats denser-than-air releases by solving the one-dimensional equations of momentum, conservation of mass, species, and energy, and the equation of state. SLAB handles release scenarios including ground level and elevated jets, liquid pool evaporation, and instantaneous volume sources
