Navigating the New Air Quality Landscape: MCAQD Updates Modeling Triggers in Latest Permitting Handbook

Environmental Consulting

February 17, 2026

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The Maricopa County Air Quality Department (MCAQD) recently released an updated Permitting Handbook (November 2025), introducing significant changes to the “Regulatory Triggers” that determine when an applicant must conduct an ambient air quality impact assessment (air dispersion modeling). These updates are particularly relevant for facilities, such as data centers, asphalt terminals, and potentially other facilities with smaller electric generating units (EGUs) that have historically accepted voluntary emission limits to remain below modeling thresholds.

Understanding the Shift: Old vs. New Thresholds

Under the Minor New Source Review (NSR) Regulatory Triggers section, the permitting handbook revision marks a transition from requiring air modeling on new and modified sources with a PTE greater than the “Minor NSR Modification Thresholds” to instead new sources which exceed the “Significance Level Thresholds.” Under the previous guidance, the thresholds for triggering a required air quality assessment were notably more stringent. As part of the recent update, MCAQD has revised the air modeling requirement to the significance levels which are twice the Minor NSR thresholds. The table below illustrates the difference in the tons per year (TPY) from the previous and revised handbook with respect to the air modeling regulatory triggers for several key pollutants:

Table 1.

Pollutant	Minor NSR Threshold (tpy)	Significance Level (tpy)
PM2.5	5.0	10
PM10	7.5	15
SO2	20	40
NOX	20	40
VOC	20	40*
CO	50	100
Pb	0.3	0.6

_{*Note: While VOCs have a significance level of 40 TPY, they do not currently trigger modeling requirements because no National Ambient Air Quality Standard (NAAQS) exists for VOCs as a standalone pollutant.}

The Importance of Regulatory Triggers

Under Rule 241 (Minor New Source Review), new sources or existing sources proposing modifications must demonstrate that their emissions will not interfere with the attainment or maintenance of the National Ambient Air Quality Standards (NAAQS). The primary mechanism for this demonstration is air dispersion modeling.

Because air dispersion modeling—ranging from simple screening assessments (AERSCREEN) to complex refined modeling (AERMOD)—requires significant time, technical expertise, and detailed input data (such as stack parameters and meteorological data), these thresholds serve as a critical “practical first approximation” for assessing permitting requirements and efforts. Staying below these triggers can drastically simplify the permitting process for many clients and reduce the time to acquire new permits or modifications to existing permits.

Strategic Impact for Clients

The doubling of the modeling trigger from the Minor NSR thresholds to the Significance Level thresholds is a major development for the regulated community. Many facilities frequently accept enforceable limits to stay “synthetic minor” and avoid the additional regulatory modeling burden. These higher threshold values may provide more operational flexibility or allow for facility expansions that would have previously required air modeling to show compliance with the NAAQS. However, it is vital to remember that MCAQD retains discretionary authority to require an ambient air impact assessment if they believe a source could interfere with the NAAQS, particularly in cases involving the clustering of multiple small-to-moderate sources at a single location.

What This Means for Your Permitting Strategy

While the increased thresholds are a welcome change for those looking to streamline their permitting needs, compliance remains a multi-step process:

PTE Analysis: Applicants must first determine their Potential to Emit (PTE) to see if they meet the significance level thresholds.
Screening vs. Refined Modeling: If thresholds are exceeded, a tiered approach starting with screen modeling is used to compare impacts against Significant Impact Levels (SILs).
BACT/RACT Requirements: Regardless of modeling, sources exceeding specific thresholds must still implement Best Available Control Technology (BACT) or Reasonably Available Control Technology (RACT).

As the air quality regulatory landscape in Arizona continues to evolve, understanding these handbook changes is essential for effective strategic planning. Our team is ready to help you evaluate your emission profiles and determine the most efficient path forward under these new MCAQD guidelines.

For specific office support or questions, reach out to Major Kindsfater or Maheshwar Mane in Trinity’s Phoenix office.