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^eDsv^	U.S. Environmental Protection Agency	18-P-0241
*	Office of Inspector General	Septembers, 2018
^ At a Glance
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Why We Did This Project
We conducted this audit to
assess the effectiveness of the
U.S. Environmental Protection
Agency's (EPA's) process for
reviewing and approving air
quality dispersion models it
recommends for use by state,
local and tribal air pollution
control agencies.
Air quality dispersion models
predict the air quality impact of
pollutants released into the
atmosphere. The EPA's review
and approval process for
designating preferred models is
outlined in Appendix W of
40 CFR Part 51. The goal of
this process is to identify the
best-performing model as the
preferred model, and the
appendix lists preferred
models. Appendix W was
originally promulgated in 1978
and most recently revised in
2017. The American
Meteorological Society/EPA
Regulatory Model (AERMOD)
is the EPA preferred model for
most regulatory uses listed in
Appendix W. AERMOD
predicts the air quality impact
of pollutants from sources up to
50 kilometers downwind, and
was first designated as a
preferred model in the 2005
revision of Append ixW.
This report addresses the
following:
• Improving air quality.
Send all inquiries to our public
affairs office at (202) 566-2391
or visit www.epa.gov/oiq.
EPA Can Strengthen Its Process for
Revising Air Quality Dispersion Models
that Predict Impact of Pollutant Emissions
Air quality estimates
derived from air quality
dispersion models are
used to make important
decisions to protect
public health, such as
setting emissions limits.
What We Found
Although the agency has prepared guidance on the
recommended procedures for reviewing the
development and evaluation of new air quality
dispersion models, similar guidance is not available
for model revisions. The development of standard
operating procedures (SOPs) and quality
assurance project plans (QAPPs) or equivalent
documents for model revisions could assure
consistent application of quality assurance and
quality control activities.
From 2006 through 2016, the EPA issued 12 Model Change Bulletins revising
AERMOD with enhancements, bug fixes and/or miscellaneous changes to
improve the model. The 12th Model Change Bulletin was associated with the
2017 Appendix W revisions that included adding new regulatory uses to
AERMOD. However, the quality assurance and control activities undertaken for
these revisions were not as extensive as what EPA guidance recommends for
new model development and evaluation. For example, the agency used peer-
reviewed journal articles to satisfy peer-review requirements, while AERMOD
received a panel peer review when it was developed. In one instance, the agency
proposed a new regulatory option for AERMOD, which lacked peer-review
literature and later needed additional evaluation. Development of SOPs, as well
as QAPPs or equivalent documents, could assure that consistent and
appropriate quality control and assurance activities are conducted when revising
preferred models by helping assure that the predicted results are of sufficient
quality. This is especially important because AERMOD is used by all 50 states,
as well as tribes and territories, to predict air quality impacts for regulatory
purposes under the Clean Air Act.
Recommendations and Planned Agency Corrective Actions
We made four recommendations to the Assistant Administrator for Air and
Radiation. These recommendations involved developing SOPs to guide and
document its process for reviewing and approving revisions to preferred air
quality dispersion models, developing QAPPs or equivalent documents to
describe results of systematic planning for air quality dispersion model revisions,
updating the Office of Air Quality Planning and Standards' Quality Management
Plan, and training staff. The agency agreed with our recommendations and
provided acceptable corrective actions.
Listing of OIG reports.

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