Air Quality

Air Dispersion Modeling

Unexpected Events/Episode Modeling


In case of unexpected releases of criteria and toxic pollutants, regulatory agencies often require to conduct air dispersion modeling to determine the magnitude and area of impact of the event on the off-site properties. Examples of modeling include emission events reporting under State Implementation Plans or the Clean Air Act (CAA) Section 112(r) (Emergency Management) provisions codified in Title 40 Code of Federal Regulations Part 68 (40 CFR 68, Chemical Accident Prevention Provisions), a.k.a. Risk Management Program or RMP.

The types of goals for which modeling may be required include:

  • Violations of the National Ambient Air Quality Standards (NAAQS);
  • Violations of State air quality standards for criteria pollutants, Hydrogen Sulfide, etc.;
  • Effects reviews for releases of toxic chemicals;
  • Off-site Consequence Analyses for development of Risk Management Programs;
  • Litigation support;
  • Back-calculations of the amounts of materials released; and
  • Other.

Sage Environmental personnel have extensive experience in modeling of emission events and risk management planning using simple and complex air dispersion modeling software.

Our Expertise:

Air dispersion modeling tools used in Unexpected Events/Episode Modeling and mastered by Sage Environmental personnel include:

  • SCREEN3;
  • RMP*Comp;
  • ISCST3;
  • SLAB;
  • ALOHA;
  • PHAST;
  • and others.

Our experience includes in-depth knowledge of the publications developed by the U.S. EPA and other organizations in support of the models listed above. Specific modeling tasks completed by Sage personnel in the last ten years included:

  • Documentation of worst-case and alternative release scenarios in support of Risk Management Plans for stationary sources with one to several dozen RMP-regulated processes;
  • Strategic planning modeling in support of regulatory applicability;
  • Post-event support for excessive emission events;
  • Development of certified time-specific and site-specific meteorological data for the duration of the events;
  • NAAQS and State regulation modeling in different states;
  • Complex release scenarios for toxic and flammable substances;
  • Litigation support; and
  • Other.

The Work:

Our efforts for a project requiring Unexpected Events/Episode Modeling typically starts with an evaluation of the release impacts using simple models requiring minimal efforts to set up and run (e.g., SCREEN3 or RMP*Comp). If the modeling results meet the goals of the review, a modeling report is prepared to document all applicable assumptions and model settings and to present the modeling results in concise and defendable manner.

In case the results produced by simple screening models do not meet the objectives of the study, Sage provides to our clients a variety of options to meet the goal using modeling refinements. As a rule, multiple model refinement options are available for selection. Upon receiving a client’s approval, modeling progresses to more and more complex stages, as required to meet the goal. The modeling assumptions, settings, and results are summarized in a modeling report. Specific data elements (e.g., weather conditions) may be certified as necessary. Top-level experienced Sage personnel may then serve as an expert witness or provide other litigation support.

Our Reach:

Sage Environmental personnel have completed hundreds of Unexpected Events/Episode Modeling exercises. References are available upon request.

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