When a regional wildfire breaks out, the immediate threat is obvious: smoke columns visible for miles, compromised air quality, and communities on high alert. But for the technical teams at State, Local, and Tribal air agencies, a secondary, prolonged challenge begins long after the fire is contained.
Under the Clean Air Act, air agencies are legally responsible for meeting the National Ambient Air Quality Standards (NAAQS) for criteria pollutants like fine particulate matter (PM2.5) and ozone (O3). When a monitor records a spike that exceeds these standards, the agency has a limited window to “flag” the event as exceptional—an unusual atmospheric phenomenon, such as a wildfire, that temporarily degrades air quality. If the U.S. Environmental Protection Agency (EPA) accepts the agency’s demonstration under the Exceptional Events Rule (EER), the affected monitoring data can be excluded from federal compliance determinations, so communities aren’t penalized for pollution spikes entirely outside their control.
The catch: proving an event actually qualifies has historically meant a heavy lift of custom modeling and data-wrangling for agency staff.
Why This Matters
Successfully proving an exceptional event requires a rigorous, technical demonstration to determine if the anomalous data can be excluded from regulatory determinations. To alleviate the substantial computational and administrative burden placed on agencies, the U.S. EPA, in coordination with the U.S. Forest Service (USFS), has rolled out three enhanced, user-friendly data tools designed to expedite determinations. The newly-released tools offer centralized, pre-computed modeling data, historical mapping, and standardized meteorological trajectory tools.
The EMBER Tool
The Expedited Modeling of Burn Events Results (EMBER) tool is a U.S. EPA screening dataset that provides results of photochemical air quality modeling with and without wildfire contributions to isolate how regional wildfires impact ground-level O3 concentrations.
The EMBER tool serves as a critical asset for complex O3 demonstrations because its newly expanded version now fully supports comprehensive O3 exceptional events demonstrations. The original EMBER tool provided modeled O3 impacts at United States (U.S.) monitoring locations originating from wildland fires across the U.S., Canada, and Mexico for 2023. The expanded version provides the same data for 2021 through 2025. This expanded dataset gives agencies a repository of historical impact data, reducing the need to perform resource-intensive, localized regional modeling runs from scratch.
The HYSPLIT Trajectory Tool for Exceptional Events
To prove a causal relationship, an agency must demonstrate that the air mass passing over the active fire is the same air mass that arrives at a monitor with elevated concentrations attributable to the wildfire. The National Oceanic and Atmospheric Administration’s (NOAA) Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model is the “go-to” for this type of analysis.
The HYSPLIT Trajectory Tool for Exceptional Events is a U.S. EPA interface that provides streamlined access to atmospheric wind trajectories, fire locations, and smoke plume geometries. By integrating NOAA’s Hazardous Mapping System (HMS) smoke polygons, it allows air agencies to quickly verify and document exactly when transported wildfire smoke reached a monitoring site, standardizing the causal analysis required for regulatory data exclusion.
The Historic AirNow Fire and Smoke Map
Through the new Historic AirNow Fire and Smoke Map, users can now look back at past conditions by date to analyze historical events. The map includes data from monitors in the U.S., Canada, and Mexico for particulate matter with a diameter less than 10 and 2.5 micrometers (PM10 and PM2.5) and O3 measurements from permanent regulatory monitors and temporary low-cost sensors directly alongside satellite-observed smoke plumes. Finally, the user can download this data for use in an EER analysis.
Conclusions
The objective of these tools isn’t just to simplify paperwork—it’s to optimize agency resources. When local and state environmental teams spend less time on back-end data processing for events outside their borders, they free up valuable technical capacity. As EPA Assistant Administrator for Air and Radiation Aaron Szabo emphasized in U.S. EPA’s June 26, 2026 press release, “These three tools will greatly reduce burdens faced daily by our air partners as EPA continues to work on our revision to the Exceptional Events Rule.” That shift matters well beyond the paperwork. For technical teams preparing upcoming NAAQS compliance demonstrations, these tools represent a significant shift toward a more predictable, data-driven, and streamlined exclusion process.
Another objective of these tools is to make it easier to exclude atypical activities from monitor design values that represent background conditions. The exclusion of atypical activities related to wildfires can create more “headroom” between the background and the NAAQS and facilitate air quality analyses associated with permitting projects.
If your company is working with an air agency to prepare an exceptional events demonstration or weighing how these tools could affect an upcoming NAAQS compliance or permitting analysis, ALL4’s Air Quality Modeling and Monitoring (AQMM) team can help you put them to use. We have a strong team with extensive experience modeling many types of projects for construction permitting, operating permit renewals, and source-specific state implementation plans (SIPs) and we tailor our modeling approach to the specific situation. Please reach out to Matt Stresing to strategize around your next project.
