United States Environmental Protection Agency

Office of Research and Development

National Exposure Research Laboratory
Research Abstract

Government Performance Results Act (GPRA) Goal #1
Annual Performance Measure #124

Significant Research Findings:

Updated CB4 Chemical Mechanism Included in Models-3/CMAQ

Scientific
Problem and
Policy Issues

Air quality (AQ) models describe the meteorological transport, atmospheric
chemistry, and fate of air pollutants like ozone, particulate matter, and air
toxics. These AQ models are used by EPA and state agencies to predict
current and future concentrations of these pollutants and to develop
strategies to bring pollutant concentrations below harmful levels. One
critical component of AQ models is the description of the complex
atmospheric photochemistry - the chemical mechanism. All chemical
mechanisms in AQ models are approximations of the real atmospheric
processes, condensing the myriad of atmospheric chemicals and chemical
reactions into a smaller number of representative chemicals and reactions -
in order to maintain computational speed while still representing the
atmospheric chemistry well. This research describes a major upgrade to the
most widely used AQ chemical mechanism, the implementation of the
upgraded mechanism in EPA's Community Multiscale Air Quality
(CMAQ) Model, and release of the upgraded mechanism for public use.

The Carbon Bond 4 mechanism (CB4) has been the most widely used AQ
chemical mechanism since its release in 1989. CB4 has been used in a
wide variety of AQ models and forms the basis for many of EPA's
regulatory policies for ozone and particulate matter (PM), and for State
Implementation Plans (SIPs). CB4 has also been a cornerstone of research
models that predict and analyze the production of secondary gas- and
aerosol-phase pollutants.

Despite its widespread use, CB4 has not seen a major update since 1989.
Although minor modifications have been made to it, the majority of
reaction rates are no longer consistent with current research. Since its
original development, researchers have also discovered important pathways
for ozone and PM production that are not adequately represented by the
existing condensed species in CB4. Because of the importance of this
mechanism to EPA and the public, we have performed a major update,
creating the CB05 mechanism. We have also implemented the updated


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mechanism in the newest version of EPA's air quality model, CMAQ.

With this updated description of chemistry in CMAQ and other air quality
models, EPA will be able to predict concentrations of criteria pollutants, in
urban and non-urban areas, using our most up-to-date knowledge of
atmospheric chemistry. While the original intent of this work was to
improve ozone predictions, the updated organic and inorganic chemistry
will also improve predictions of concentrations of particulate matter as well
as toxic air pollutants, such as formaldehyde, mercury, and benzene.

Research	The major objective of this work was to provide a comprehensive update of

Approach	the CB4 chemical mechanism, consistent with the most recent international

research, and to compare updated mechanism predictions against
experimental data. We strived to maintain a balance maintaining the low
computational cost of CB4, and enhancing the chemical detail that is
needed to accurately characterize the chemistry that occurs throughout a
wide range of atmospheric conditions. All reaction rates were reviewed
and updated to be consistent with values published by the International
Union of Pure and Applied Chemistry (IUPAC) and National Aeronautics
and Space Administration-Jet Propulsion Laboratory (NASA-JPL) data
evaluations.

At each step of the CB05 mechanism development, the work was reviewed
by both EPA/NERL and external scientists. All reaction rates were
calculated at three sets of temperature/pressure conditions and compared
with rates in the CB4 as well as in the SAPRC-99 mechanism, a second
widely used condensed chemical mechanism for AQ modeling. The
implementation of CB05 in CMAQ was tested by comparing results against
two separate box model implementations, one conducted by EPA staff and
another by external reviewers.

The final CB05 mechanism was implemented in the full CMAQ version 4.5
(September 2005 release). Simulations were performed using two 1-month
periods (January and July of 2001) in different seasons. The CB05 results
were compared with those obtained using the CB4.

Results and	Not only does CB05 use the most up to date reaction rates and mechanisms,

Impact	but it has significantly greater capacity to represent the large variety of

conditions found in ambient atmospheres - from urban to remote regions.
Some major features are:

•	All inorganic reaction rates have been modified - some minimally,
but many others substantially - so that they are now consistent with
the most recently published IUPAC and NASA-JPL values.

•	CB05 increases NOx recycling and provides more detail about


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peroxy radicals to improve simulations covering multiple days and
to describe the atmospheric chemistry that occurs under rural and
remote conditions much more accurately.

•	Increasing the number of species used to track aldehydes,
peroxyacyl nitrates, and internal olefins helps the mechanism better
represent the chemistry of these species - all of which are critically
important to oxidant formation in both urban and remote areas.

•	Initial tests show approximately 13 percent increase in ozone, while
the timing of peak concentration is unchanged. Average aerosol
production in the test (nitrate, sulfate, and organic) generally
decreases on average between 3 and 9 percent. These changes
should improve the AQ simulations, and further testing is being
performed against observations.

•	Two options have been included as part of the CB05; either one or
both can be added to the base mechanism. These include an explicit
chlorine mechanism and an explicit air toxics mechanism.

•	The CB05 has been evaluated against two large databases of smog
chamber experiments at the University of North Carolina, Chapel
Hill and the University of California, Riverside. CB05 simulations
matched the observed data better than other mechanisms for many
of the experiments.

The CB05 mechanism has been successfully implemented in the latest
release of CMAQ and made available to the public.

This work contributes toward the long-term goal of producing advanced
and accurate modeling tools that can be used by the Agency to support the
implementation of current ozone standards and to improve the science used
in reviewing these standards. Accurate and defensible chemical
mechanisms are a key component of air quality models used to develop
SIPs demonstrating compliance with National Ambient Air Quality
Standards (NAAQS).

Improving the predictions of criteria pollutants, ozone and PM, as well as
air toxics, will help EPA and states develop more scientifically sound
approaches for assessing and characterizing environmental exposure and
risk, and developing strategies for reducing concentrations of harmful
pollutants. The Office of Air Quality Planning and Standards will benefit
from improvements in the scientific basis by which they evaluate air
pollutant control strategies. Because CB4 is in widespread use both
nationally and internationally, we expect that CB05 will be embraced by
many in the research and regulatory community. External groups impacted
by this research include academia, the NARSTO program, private
consultants, and anyone performing air quality modeling.


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The scoping effort for this work, technical direction, error checking,
implementation of CB05 into CMAQ, and initial analysis was done by
EPA/NERL scientists. The inclusion of updated rate constants, new
species (split from lumped model species), and emissions mapping was
performed under contract #RT-04-00675 with Yocke and Co. Emissions
processing for CB05 was provided under contract #68-W-01-032 with
Computer Sciences Corporation.

The updated CB05 mechanism was presented to the public at the
Community Modeling and Analysis System Center's 4th Annual CMAS
Models-3 User's Conference, Sept. 26-28, 2005. The presentation at the
conference described the results of the CB05 implementation into CMAQ,
together with an initial analysis of differences between CB4 and CB05.
This conference traditionally draws hundreds of participants in air quality
modeling for three days of intensive discussion and interaction on
application, improvements, and results from atmospheric models. The
products of this work include the CB05 mechanism, a detailed contractor
report describing the mechanism and evaluations, and files to implement
CB05 in CMAQ, which have been archived in the CMAQ model archive.
The entire package is scheduled for a formal, public, beta-release in
October 2005.

Recent publications and presentations associated with this research task
include the following:

Sarwar, G., D. Luecken, G. Yarwood, G.Whitten and W. Carter, 2005. "Implementing an Updated
Carbon Bond Mechanism into the Community Multiscale Air Quality Model," presented at the 4th
Annual CMAS Models-3 User's Conference, Chapel Hill, NC, September 26, 2005.

Yocke and Co., 2005. Updates to the Carbon Bond Mechanism: CB05. RT-04-0065. July 29, 2005.

Future Research Future research efforts must focus on identifying and understanding

differences in concentrations and relative reduction factors between CB4
and CB05. Since previous SIP implementation and regulatory development
within EPA relied on CB4, it is critical to understand the implications of
switching to an alternative mechanism. We must focus on evaluating air
quality models using the CB05 mechanism against all available monitoring
data, over a variety of conditions, temperatures, seasons, and regions.
Additional research will evaluate both the impacts of the new mechanism
on computational speed and the improved accuracy in the chemical profiles
and predictions.

Research
Collaboration and
Research
Products

Contacts for	Questions and inquiries can be directed to:

Additional

Information


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Deborah J. Luecken

U.S. EPA, Office of Research and Development
National Exposure Research Laboratory
Atmospheric Modeling Division, E243-03
Research Triangle Park, North Carolina 27711
Phone: 919/541-0244
E-mail: luecken.deborah@epa.gov

Funding for this project was through the U.S. EPA's Office of Research
and Development, National Exposure Research Laboratory, and the work
was conducted by the Atmospheric Modeling Division.


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