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Community Multiscale Air Quality (CMAQ) Modeling System

A tool to support air quality management and inform policy to protect public health

What is CMAQ?

The Community Multiscale Air Quality
(CMAQ) Modeling System is EPA's
premier modeling system for studying air
pollution from local to hemispheric
scales. For more than two decades, EPA
and states have used CMAQ—a powerful
computational tool translating
fundamental atmospheric science
principles to policy scenarios—in support
of air quality management. CMAQ is
continually updated to incorporate
knowledge on the state-of-the science
and harness high performance computing
power to more effectively and efficiently
characterize air quality and protect
human health and the environment.

States use CMAQ to develop and assess
implementation actions needed to attain
National Ambient Air Quality Standards
(NAAQS) defined under the Clean Air
Act. CMAQ simulates air pollutants of
concern—including ozone, particulate
matter (PM), and a variety of air toxics—
to optimize air quality management.
Deposition values from CMAQ are used
to assess ecosystem impacts such as
eutrophication and acidification from air
pollutants. In addition, the National
Weather Service uses CMAQ to produce
twice-daily forecast guidance for ozone
air quality across the U.S.

CMAQ unites the modeling of
meteorology, emissions, and chemistry to
simulate the fate of air pollutants under
varying atmospheric conditions. Other
kinds of models—including crop
management and hydrology models—
can be linked with the CMAQ simulations,
as needed, to more holistically simulate
pollution across environmental media.

What's new in CMAQ?

In August 2019, EPA released a major
update to CMAQ. CMAQ version 5.3
includes the following new features:

•	A more detailed representation of
the characteristics of PM: CMAQ
5.3 improves modeling of PM
composition, size distributions, and
optical properties. It also enhances
the simulation of human-influenced
secondary organic aerosols by
considering newer laboratory and
observational data.

•	Expanded chemistry for ozone and
PM formation from global-to-local
scales: CMAQ 5.3 updates the
science behind the interactions of
chemicals in the air and clouds.

These advances reflect the state of
the science and are more inclusive of
chemical processes not just within the
U.S., but under different climatic
conditions across the globe.

•	More complex land and
atmosphere interactions to support
both air quality and ecosystems
applications: CMAQ 5.3 includes two
new options for simulating the
exchange of pollutants between the
land and the atmosphere, improving
linkages of CMAQ for ecological
applications.

•	Increased emphasis on pollutants
originating outside the U.S.: While
air quality has improved through EPA
regulations under the Clean Air Act,
understanding the fate of air
pollutants originating in other
countries remains critical for
addressing air quality in the U.S.
CMAQ 5.3 better captures the
influences of reactive chemical
species originating from the oceans,
and it increases the emphasis on
more accurate characterization of
pollutants transported through the air
from distant sources.

1

U.S. Environmental Protection Agency

Office of Research and Development

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•	Increased scientific consistency
between meteorology and
chemistry models: As the state of
the science in the meteorology model
evolves, changes are introduced into
CMAQ to represent the atmospheric
processes as consistently as possible
between these models. The
meteorology model used by CMAQ
was updated by adding scientific
complexity, incorporating new data
sources, and changing the
representation of the atmosphere
above the troposphere—that is, above
where most of the weather occurs.
This change is important because
intermittent infusions of high
concentrations of ozone into the lower
atmosphere occur through physical
processes at the top of this layer —
the tropopause.

•	Greater flexibility to support
increasingly diverse uses of
CMAQ: Some of the software in the
CMAQ modeling system has been
restructured so that both users and
developers can more readily extend
CMAQ to meet their needs. For
example, a new emissions interface
allows for substantial flexibility in the
way emissions are mapped, scaled,
and checked for quality and can
greatly simplify the task of assessing
air quality improvements resulting
from emission changes.

•	Improved efficiency for CMAQ-
ISAM for isolating source
contributions to air quality: The

science algorithms used by the
Integrated Source Apportionment
Method (CMAQ-ISAM) to track
contributions from different emission
sources to ozone and PM have been
updated. Code improvements have
led to substantially faster run times to
support its practical applications.

•	Fully revised documentation to
better reflect CMAQ's current
structure and capabilities: A new

comprehensive user's guide provides
instructions on setting up and running
the model, including guidance on
what model options are
recommended for different types of
modeling applications.

CMAQ Support Tools

New versions of software tools needed
for preparing input datasets and
evaluating output were released
concurrent with CMAQ 5.3:

•	Meteorology-Chemistry Interface
Processor (MCIPv5.0): Prepares
atmospheric fields from the
meteorology model for use in CMAQ.

•	Fertilizer Emission Scenario Tool
for CMAQ (FEST-Cv1.4): Generates
fertilizer application input for CMAQ
bi-directional ammonia modeling.

•	Atmospheric Model Evaluation
Tool (AMETv1.4): Used to analyze
air quality and meteorology models.

•	Visual Environment for Rich Data
Interpretation (VERDIv2.0):

Visualizes gridded output from MCIP
and CMAQ.

Community Engagement

The CMAQ community has thousands of
users in more than 50 countries. These
users include scientists and air quality
managers across government, academia,
and the private sector. Their input has
contributed to developing a more robust
model.

To support the CMAQ user community,
EPA contracts with the University of
North Carolina at Chapel Hill to host the
Community Modeling and Analysis
System (CMAS) Center, which provides

user training and support for the CMAQ
modeling system. In 2018, EPA worked
with CMAS to launch a new online user
forum to connect users with model
developers and the international user
community to collaborate on using CMAQ
and to share feedback for science and
feature improvements.

For more information, visit:

EPA's CMAQ website:
www.epa.gov/cmaq

CMAQ source code on GitHub:
https://aithub.com/USEPA/CMAQ

CMAQ Support Tools:

https://www.epa.gov/cmag/cmag-

resourcesutilities-model-users

CMAS Center at UNC-Chapel Hill:
www.cmascenter.org

CMAS User Forum
https://forum.cmascenter.org

NOAA/NWS Air Quality Forecasts using
CMAQ

https://airgualitv.weather.gov

August 2019

2

U.S. Environmental Protection Agency

Office of Research and Development

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