science in ACTION
INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE
SEPA
www.epa.gov/research
EPA's Community
Multiscale Air Quality
Modeling System (CMAQ)
Tools for controlling air pollution
& studying climate change
What is CMAQ?
For more than a decade, EPA
and states have used EPA's
Community Multiscale Air
Quality (CMAQ) Modeling
System, a powerful
computational tool for air quality
management. The National
Weather Service also uses the
model to produce daily U.S.
forecasts for ozone air quality.
States use CMAQ to develop
and assess implementation
actions needed to attain National
Ambient Air Quality Standards
(NAAQS). The system
simultaneously models multiple
air pollutants, including ozone,
particulate matter, and a variety
of air toxics, to help regulators
determine the best air quality
management scenarios for their
communities, states and
countries.
CMAQ brings together three
kinds of models:
Meteorological models to
represent atmospheric and
weather activities.
Emission models to represent
man-made and naturally-
occurring contributions to the
atmosphere.


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A CMAQ map showing averaged ozone concentrations across tlie United States.
An air chemistry-transport
model to predict the atmospheric
fate of air pollutants under
varying conditions.
In October 2016, EPA released
the newest version of the
modeling system — CMAQ 5.2
for beta testing. A final version
is scheduled for release in June
2017. CMAQ 5.2 includes the
following new features:
New windblown dust emission
model: This physics based
model has been evaluated and
shown to be better with
observations than previous dust
emission components in CMAQ.
New pathways to organic
aerosol: New sources of
secondary organic aerosols have
been added to the model
(heterogeneous uptake of
glyoxal and methylglyoxal in
CB6) and properties of
traditional secondary organic
aerosol have been updated. The
volatility of both secondary and
primary organic compounds are
now treated consistently with
each other. In addition, a new
model species has been added to
account for the organic aerosol
compounds resulting from
combustion processes.
New gas-phase photochemistry
mechanism: A new way to
model gas-phase photochemistry
mechanisms has been added to
CMAQ 5.2. This allows for a
better treatment of rural and
remote chemistry, which is
particularly important for
modeling compliance strategies
for a lowered NAAQS standard.
CMAQ 5.2 also includes an
extension for modeling oceanic
halogen chemistry in
hemispheric simulations.
Instrumented diagnostic
capabilities: Several diagnostic
capabilities are included with
this model version that allow
users to probe source-receptor
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U.S. Environmental Protection Agency
Office of Research and Development

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relationships. The tools include:
1) Decoupled Direct Method in
Three Dimensions (DDM3D) for
calculating sensitivity
coefficients for user defined
parameters; 2) Integrated Source
Apportionment Method (ISAM)
for tracking contributions from
sources and regions; and 3)
Sulfur Tracking Model (STM)
for tracking sulfate production
pathway contributions. The
instrumented models are useful
tools for a variety of decision
support applications.
A Little History
Using data about land use,
meteorology, and emissions,
CMAQ provides detailed
information about the
concentrations of air pollutants
in a given area for any specified
emissions or climate scenario.
Since 1998, when the first
version was released, CMAQ
has been used to evaluate
potential air quality policy
management decisions. The
model provides reliable
information for decision makers
about the estimated impacts of
different air quality policies.
Since its inception, CMAQ has
been able to incorporate data
from related models that have
alternate mathematical
processes. This has allowed
inclusion of new science in the
model to address increasingly
complex air pollution issues.
Community Based Development
CMAQ stands out from other air
quality models because it
incorporates input from a large,
world-wide user community. To
support the CMAQ user
community, EPA and the
University of North Carolina at
Chapel Hill host the Community
Modeling and Analysis System
(CMAS) Center, which
distributes CMAQ software,
hosts user email exchanges, and
provides new user training on
the CMAQ modeling system.
This growing community, which
includes thousands of users in
more than 50 countries, has
helped assess and improve the
model's functionality. Users
include scientists, researchers
and air quality modelers, as well
as governmental air quality
managers who apply the
modeling system in their
environmental management
programs. Their input has helped
EPA scientists prioritize
modeling research to improve
CMAQ's capabilities.
CMAQ Model Application
CMAQ has been used to address
several major air pollution issues
in recent years. New fuel and car
standards finalized by EPA in
2014 were developed with input
from CMAQ. The new fuel
standards require the amount of
sulfur in gasoline to be reduced
by more than 60 percent starting
January 1, 2017. Under the new
rule, motor vehicle particulate-
matter emissions are to be
reduced by 70 percent and
nitrogen-oxides emissions by 80
percent. Once fully in place, the
standards will help avoid up to
2,000 premature deaths per year
and 50,000 cases of respiratory
ailments in children.
In developing the rule, CMAQ
was used to calculate 8-hour
ozone concentrations; daily and
annual PM2.5 concentrations;
annual nitrogen dioxide (NO2)
concentrations; annual and
seasonal (summer and winter)
air toxics concentrations;
visibility levels, and annual
nitrogen and sulfur deposition
total levels. These factors were
calculated for the years 2018 and
2030, with and without the rule.
CMAQ results were used in
combination with other tools to
determine how the new
standards would impact various
health outcomes.
For more information, visit:
EPA's CMAQ resource page:
http: //bit .lv/EP A-C MAQ
CMAS at UNC-Chapel Hill:
www.cmascenter.org
Download CMAQ:
www. github .com/U SEP A/CMAO
Technical Contact:
Jon Pleim
919-541-1336
pleim.ion@epa.gov
October 2016
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U.S. Environmental Protection Agency
Office of Research and Development

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