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National Ambient Air Quality Standards (NAAQS) Update

Posted: May 22nd, 2020

Authors: Steven M. 
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The Clean Air Act (CAA) the U.S. Environmental Protection Agency (U.S. EPA) to establish National Ambient Air Quality Standards (NAAQS) for six common air pollutants (also known as “criteria air pollutants”). The six criteria pollutants are particulate matter, sulfur dioxide, nitrogen dioxide, carbon monoxide, ground-level ozone and lead. The NAAQS for each pollutant includes primary and secondary standards. The primary standards are designed to protect public health while the secondary standards protect the public against adverse environmental effects (i.e., public welfare).

The CAA requires U.S. EPA to conduct a review of the NAAQS once every five years. During each five-year review U.S. EPA considers scientific studies published since the last review in an Integrated Science Assessment (ISA), which provides additional information regarding what air quality levels are protective of human health and welfare. The U.S. EPA then prepares a Policy Assessment (PA) which evaluates the potential policy implications of the information within the ISA. The PA also includes the findings of the independent review of the ISA by the Clean Air Scientific Advisory Committee (CASAC).

U.S. EPA is currently conducting five-year reviews of the NAAQS for particulate matter (PM) and ground-level ozone (ozone). The NAAQS for PM and ozone are perhaps the two most critical NAAQS when it comes to obtaining air permits for new greenfield sites and major expansions of existing facilities. This is due to the relatively large number of nonattainment areas and the shrinking “gap” between the ambient background values and the standards in attainment areas.

Significance of Nonattainment Designations

A nonattainment designation is undesirable due to the increased complexity, difficulty, and length of time required to obtain nonattainment new source review (NNSR) air quality permits. New projects located within a nonattainment area must evaluate the most stringent air pollution controls (known as lowest achievable emission rate or LAER), demonstrate the project will not increase the concentration of the nonattainment pollutant more than a de minimis amount using air dispersion models, obtain off-sets for the increase in emissions of the nonattainment pollutant (or the pollutant pre-cursors in the case of reactive pollutants like ozone), and demonstrate the proposed project cannot be constructed outside the nonattainment area. Existing sources within a nonattainment area must conduct a cost-benefit analysis of additional air pollution controls (known as reasonably available control technology or RACT) to help bring the area back into attainment.

Significance of Lowering the NAAQS

In areas designated as attainment, proposed projects that are subject to prevention of significant deterioration (PSD) permitting requirements are required to demonstrate the project does not cause or contribute to a violation of the NAAQS using air dispersion models. This can be very challenging in many attainment areas because the ambient background values for PM with an aerodynamic diameter less than 2.5 microns (PM2.5) and ozone are more than 75% of the current NAAQS, leaving little room for the facility to successfully model attainment. If the facility is located near a nonattainment area the facility may also be required to demonstrate there is a de minimis air quality impact from the proposed project at the nonattainment area.

Background on PM NAAQS

The PM NAAQS was first established in 1971 for total suspended particulate (TSP). The NAAQS was changed from TSP to coarse particulate matter, more commonly known as PM10, in 1987.  PM10 is a subset of TSP smaller than 10 microns in aerodynamic diameter and represents the inhalable portion of TSP. In 1997, the PM10 standard was augmented with standards for fine particulate matter, known as PM2.5, which are particles smaller than 2.5 microns in aerodynamic diameter. PM2.5 poses a greater risk to human health and contributes to regional haze and reduced visibility in national parks.

Following the 1987 change to PM10, U.S. EPA has designated 89 areas of the country as nonattainment. Fifty-nine of those areas have since been redesignated as maintenance areas, which means those areas are now meeting the 1987 PM10 standards of 150 micrograms per cubic meter (µg/m3) (24-hour) and 50 µg/m3 (annual – abolished following the 2006 review).

In 1997, U.S. EPA added the PM2.5 standard and designated 39 areas as nonattainment. Thirty-five of those areas have since been redesignated as maintenance areas, which means all but four of those areas are now meeting the 1997 PM2.5 standards of 65 µg/m3 (24-hour) and 15.0 µg/m3 (annual). In 2006, U.S. EPA lowered the 24-hour PM2.5 standard to 35 µg/m3, and 32 areas were designated as nonattainment. Eighteen of those areas have now been redesignated as maintenance areas. Following the last five-year review, U.S. EPA lowered the annual PM2.5 standard to 12.0 µg/m3 in 2012.  Nine areas were designated nonattainment and six remain nonattainment areas. In 2016, U.S. EPA revoked the 1997 standard except in the four remaining areas designated as nonattainment.

U.S. EPA Proposed Action on 2020 PM NAAQS

The final PM ISA and CASAC review were completed in December of 2019. U.S. EPA published the final PA for the PM NAAQS in January 2020. The final PA evaluated retaining the current PM2.5 annual standard of 12.0 µg/m3 and alternative annual standards between 10.0 and 8.0 µg/m3 based on the updated science. The final PA also evaluated the current 24-hour PM2.5 standard of 35 µg/m3 and one alternative standard of 30 µg/m3. The final PA concluded the available science supports retaining the current 24-hour PM10 standard of 150 µg/m3. No alternative standards were considered for PM10 in the PA.

The U.S. EPA Administrator proposed retaining the current NAAQS for PM on April 14, 2020. U.S. EPA is accepting public comments on the proposed PM NAAQS through June 29, 2020.  Comments in support of retaining the current PM NAAQS may be critical to final action on the PM NAAQS. The timing of the final rulemaking is uncertain due to the Presidential Election on November 3, 2020. If the final action is not completed prior to the election and there is a change in administration, the new U.S. EPA Administrator may review the PA and arrive at a different recommendation for the PM NAAQS. Additional tightening of the existing PM NAAQS would make future air permitting efforts more complex, difficult, and time consuming.

Background on Ozone NAAQS

The ozone NAAQS was first established in 1971 as a 1-hour standard of 0.08 ppm for total photochemical oxidants. In 1979, the standard was converted into ozone and the 1-hour concentration limit was increased to 0.12 ppm.

The ozone NAAQS was changed to an 8-hour average of 0.08 ppm in 1997 and the 1-hour standard was revoked. In 2008, the 8-hour ozone standard was lowered to 0.075 ppm resulting in 47 nonattainment areas with 10 areas being redesignated as maintenance areas. The 1997 standard was revoked in 2015 following implementation of the more stringent 2008 standard. The 8-hour ozone standard was lowered again in 2015 to 0.070 ppm. Fifty-two areas were designated nonattainment and 51 areas remain nonattainment.

U.S. EPA Status on Ozone NAAQS

The ozone draft ISA was completed in September of 2019. U.S. EPA published the draft PA for the ozone NAAQS in October 2019. The draft PA concluded the available science supports retaining the current 8-hour ozone standard of 0.070 ppm (70 ppb). No alternative standards were considered in the draft PA. The CASAC provided comments on the draft ISA and draft PA in February 2020. The final ozone ISA was released in April 2020. Be on the lookout for the final PA and the proposed action on the ozone NAAQS five-year review over the next few months.

U.S. EPA Releases Updated Permit Modeling Guidance

U.S. EPA released DRAFT Guidance for Ozone and Fine Particulate Matter Permit Modeling in February 2020.  The draft guidance updates the 2014 PM2.5 modeling guidance to reflect 2017 revisions to the 40 CFR Part 51 Appendix W (Guideline on Air Quality Models). The draft guidance also consolidates the 2016 guidance on use of chemical transport models, the 2017 guidance on photochemical grid models, the 2018 guidance regarding significant impact levels (SILs), and the 2019 modeled emission rates for precursors (MERPs) guidance. The guidance is expected to be finalized in late 2020.

One notable change in the draft guidance is the requirement to only include those precursors emissions in a MERPs analysis that are greater than PSD significant emissions rate (SER) thresholds.  This is an update from previous guidance which required all precursor pollutants to be included in a MERPs analysis.  ALL4 has recently assisted clients with this complex set of modeling requirements and working through various modeling options with the permitting authority and U.S. EPA to successfully model attainment of the PM2.5 and ozone NAAQS.


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