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Technical Analysis:
Serious Area PM2.5 SIP


The Serious PM2.5 SIP Development is very much an iterative process. The technical foundation of any SIP involves numerous emissions inventories, air quality modeling assumptions, potential emission controls, and ever-fluctuating design values recorded throughout the air monitoring network. The PM2.5 Implementation Rule is very prescriptive about how these numbers must fit together to comprise an approvable plan. Various components are compiled early in the process only to be pieced together with other components later in the process. Corrections and adjustment to the various pieces is almost always necessary – even right up to the very end. Nothing appearing in these Serious PM2.5 SIP development pages during the process of SIP development is to be regarded as a final product. The UDAQ is making this DRAFT work available for public review in an effort to ensue transparency throughout the SIP development process. We encourage the public to review and provide feedback on these intermediary products with the understanding that they are not yet final.


In addition to the provisions submitted to meet the Moderate Area planning requirements, States addressing Serious Area planning requirements must also submit, as part of a Serious Area SIP revision, a demonstration, including air quality modeling, that shows:

  • The plan provides for attainment of the relevant NAAQS by the applicable attainment date.


  • A demonstration, in the case of any area seeking an extension of the attainment date of up to five additional years, that attainment by such date would be impracticable AND that the plan provides for attainment by the most expeditious alternative date practicable.

This quantitative demonstration using air-quality modeling is the foundation of the Serious Area SIP revision. The demonstration will depend in large part on the Emissions Inventories developed to assess control strategies and projected growth. The air-quality model is able to translate these changes in emissions into predictions about changing PM2.5 concentrations within the airshed.

Emissions Inventories

Emissions inventories are a fundamental component of any SIP demonstration. Not only do they give an indication of trends and status, they feed the air quality model the quantitative information necessary to make predictions and draw conclusions concerning the efficacy of potential control strategies.

Many different inventories are used throughout the development of a SIP. There are episodic inventories, a baseline inventory and projection inventories for various years. Each is constructed so as to include all of the relevant pollutants. For PM2.5 this also includes four PM2.5 precursors: SO2, NOx, VOC, and Ammonia. Each inventory is also adjusted to reflect the winter conditions during which Utah’s northern valleys experience the temperature inversions that serve to trap air contaminants and drive the chemistry that creates secondary PM2.5.

Episodic inventories are used to evaluate the performance of the air quality model. These emissions are paired in time with a representation of the actual meteorological conditions incurred during a multi-day period (episode) in which ambient air monitors recorded elevated concentrations of PM2.5. The air quality model is then expected to reproduce the PM2.5 concentrations that were actually monitored.

The base year inventory depicts the current state of emissions at a time when the PM2.5 standard was violated. More importantly, it serves as a reference point within the air quality modeling, to be compared with the emissions in some future year, as represented by one of the projection year inventories.

It is important to keep in mind that UDAQ regularly produces inventories throughout the state, and makes these available on its website. These inventories serve different purposes within the air program, and so direct comparisons with PM2.5 SIP-specific inventories may not be appropriate. Some of the reasons these comparisons may disagree include: 1) adjustments for seasonality in SIP inventories, 2) differences in geographic boundaries (some nonattainment boundaries include collections of counties or partial counties), 3) differing definitions of what constitutes a “point source,” and 4) the aggregation of certain pollutants appearing in simplified pie-charts.

Posted Inventories

  1. SIP Inventories Presented Here (52 KB)
  2. 2014 Base Year Inventory Table (10 KB)
  3. DRAFT 2014 Salt Lake NAA Inventory (327 KB)
  4. DRAFT 2014 Provo NAA Inventory (231 KB)
  5. PM2.5 SIP 2014 Point Source Baseline Emissions (26 KB)
  6. DRAFT Inventory Tables for SLC SIP (2016, ’17, ’19, and 2020) (440 KB)


Predicting air pollution in Utah is difficult. During wintertime inversions, most of the small particulate (PM2.5) we breathe is created by complex chemical reactions in the air. Meteorological conditions and topography also play a significant role in determining the formation of PM2.5 in Utah’s valleys.

Considering the incredibly large number of variables that affect wintertime air pollution, we must rely on sophisticated computer models to predict future air quality. The Utah Division of Air Quality (DAQ) uses the most state-of-the-art air quality models available.

Utah DAQ has made a number of significant investments in improving our model performance. Some of these improvements include: improved meteorological modeling, updated emissions profiles, and realistic snow chemistry. This effort provides confidence in assessing our response to federal standards.

In summary, air quality modeling is how we determine if Utah is on track to meet federal regulations. By staying on top of the latest technology and research, Utah DAQ can better serve public health and the regulated community.