Trinity Consultants

Air Dispersion Modeling



For over 30 years, Trinity Consultants has performed dispersion modeling studies to assist industrial facilities in assessing the impact of their emissions on ambient air quality. Regulatory requirements for dispersion modeling remain the primary motivation for the majority of modeling studies in the U.S. New federal modeling guidelines, more complex models that simulate the transformation of pollutants and more accurately represent dispersion, and stringent air quality standards have made regulatory modeling more demanding.  Companies are also realizing the value of dispersion modeling as a planning tool to assess the feasibility of major capital projects. Modeling enables companies to analyze the air quality impacts of operational changes and incorporate the costs of required controls into initial project planning.

Trinity Consultants has performed dispersion modeling analysis in support of projects throughout the U.S. With extensive knowledge of state and federal modeling requirements, Trinity saves clients time and money by performing analyses the right way the first time. Trinity’s unique insight into dispersion modeling allows us to develop innovative modeling strategies to increase operational flexibility while demonstrating compliance with air quality standards. This often results in simple operational or design changes that save companies significant costs associated with additional air pollution control equipment.

Proper model selection is the first step in developing a comprehensive modeling strategy. Trinity draws upon its thorough understanding of available models to help clients determine the most appropriate tools for their specific needs. For facilities with difficult sources such as material handling equipment, open pits, tanks, and fugitive emission points, Trinity’s technical staff evaluates complex scenarios to pinpoint suitable modeling applications for each situation. Appropriate model selection is key to demonstrating compliance with air quality standards.

Regulatory Modeling

Trinity’s consultants have performed thousands of air dispersion modeling studies in support of air permit applications. Trinity closely follows developments in U.S. Environmental Protection Agency (EPA) and state modeling guidelines. In 2003, EPA promulgated a final rule adopting CALPUFF as its preferred long-range air dispersion model. In 2005, EPA promulgated a final rule adopting AERMOD for short-range dispersion modeling (with a one year transition period for ISCST3). Trinity’s technical staff is well versed in the use of both models—they have submitted AERMOD dispersion modeling analyses to support both state permits and federal Prevention of Significant Deterioration (PSD) permit applications and have conducted CALPUFF modeling for both PSD and Class I area analyses. Both AERMOD and CALPUFF utilize detailed meteorological data inputs that require subjective analysis to achieve their improved estimation of air quality impacts.  Trinity’s staff maintains ready access to raw meteorological and geophysical data and is proficient in analyzing and processing these data for input to AERMOD as well as CALPUFF, which requires considerable expertise and computing resources to execute.

Accidental Release Modeling

Accidental releases can include scenarios such as toxic exposure to vapor gas, dense gas, and liquid spill releases; overpressure and thermal exposure from explosions; and thermal exposure from flammables. The nature of the release, including the substance and the emission rate, and the type of release dictate the most appropriate model for estimating downwind concentrations.

Trinity’s technical staff has performed many types of accidental release analyses, including off-site consequence analyses required by the Section 112(r) Risk Management Program. Trinity has also performed hazards analyses to assist companies with internal planning and to support legal proceedings. These types of analyses can be technically challenging and are performed by Trinity’s hazards modeling specialists, including scientists with doctorate degrees.

Engineering/Business Assessments

Dispersion modeling is ideal for performing “what-if ” analyses to determine the effects of potential operational changes, particularly with the productivity enhancements available in commercial dispersion modeling software. By performing a series of model runs, Trinity assists clients with plant and equipment siting, stack height design, and evaluation of control equipment options. These analyses help ensure that operational changes are cost-effective and do not adversely
impact permit requirements, project scheduling, or community relations. Furthermore, with Class I area analyses required for new and modified facilities subject to PSD review, air quality modeling is becoming a key component of “fatal flaw” siting considerations.
Specialized Modeling Studies

Trinity has also performed modeling studies to meet the requirements of many specialized and often technically challenging scenarios, including:

  • Predicting the impact of roadway air emissions with the MOBILE emission factor model and dispersion models such as CALINE3 and CAL3QHC
  • Predicting chronic risk/health effects associated with air toxics
  • Performing offshore modeling to meet Mining and Minerals Management Service requirements
  • Conducting off-site impact and deposition studies to support litigation activities

Model Development and Training

Since 1983, Trinity has provided the market leading PC-based BREEZE® air dispersion modeling software to environmental professionals. These models are productivity-enhanced versions of regulatory models from EPA and other sources. Trinity also provides professional training courses in dispersion modeling, including classes for the newest models, AERMOD and CALPUFF, taught by senior-level consultants who are dispersion modeling experts.  Trinity develops customized modeling applications and teaches custom courses designed to meet the needs of individuals and organizations.

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