Accidental Air Releases

Accidents or “upset” conditions can sometimes lead to unexpected and uncontrolled releases of gases into the atmosphere, which can result in concerns about human health impacts for workers or nearby residents and/or impacts to the environment and nearby ecosystems. Using air dispersion models to recreate airborne concentrations after an accidental release can help show where exposures happened, for how long, and at what levels. Air dispersion models can also be used for risk assessments of accidental releases near facilities handling flammable or toxic chemicals.

Exponent air modelers are experts in developing reliable and credible simulations of chemical transport and diffusion. We have modeled incidents including the release of pressurized gases from railcars or tanks, accidental flaring at oil and gas or chemical facilities, and releases from industrial stacks and fugitive sources. We have many years of experience using air dispersion models and developing modeling platforms. As a result, we have a deep understanding of how these models perform, how to use them in the most appropriate way (e.g., specific to source and/or airborne behavior), and how to develop credible and informative simulations. These studies have been used for both planning purposes and litigation.

CALPUFF is an advanced non-steady-state air dispersion model developed and maintained by former and current Exponent scientists. CALPUFF is widely used in for both research and regulatory purposes within the US and in countries around the world. As a non-steady state model, CALPUFF is capable of modeling changing meteorology and source terms on sub-hourly time scales. CALPUFF can simulate complex conditions including light winds, stagnation, and land types like complex terrain and coastlines. These refined features give CALPUFF significant benefits over steady-state models such as AERMOD when simulating a short-term incident.

Dense Gas Models

Some gases are “heavier than air,” causing them to settle near the ground. Dense gases can be heavier than air due to either their molecular weight or cold temperatures. For example, gases stored in liquid phase under pressure will dramatically cool when rapidly released to atmospheric pressure, a process known as auto-refrigeration. For dense gases, the force of gravity can become important when modeling dispersion. Exponent scientists are experienced with many of the most commonly used dense gas models, including DEGADIS, HGSYSTEMS, and Aloha, and are familiar with their limitations and proper application. For example, standard dense gas models will not evaluate the impact of terrain where dense gases can settle. For these cases, Exponent scientists will turn to more sophisticated tools, including Computational Fluid Dynamics (CFD) models, to fully model airborne releases.


Computational Fluid Dynamics (CFD) models use numerical techniques to solve the equations that govern the conservation of mass, momentum, and energy. These models predict the microscale flow of fluids around or through complex structures and can include chemical reactions and multi-phase physics such as the transport and evaporation of droplets resulting from the flash evaporation of depressurizing liquids. For accidental releases, CFD models are sometimes necessary to define the complex interaction of transported gases with terrain and the built environment. This could include the settling of dense gases into terrain features and the transport and mixing of pollutants in the near field around buildings or other structures. Exponent has experience with the STAR-CCM+, FLUENT, and Open-FOAM CFD platforms and has applied them to a variety of problems in the oil and gas, transportation, power generation, and chemicals industry.