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Atmospheric Modeling and Analysis Research

CMAQ for Air Toxics and Multipollutant Modeling

In the past, chemical mechanism and air quality development have focused on Criteria Air Pollutants (CAPS) such as ozone and primary inorganic particulate matter. The Community Multiscale Air Quality (CMAQ) modeling system also has the capacity to predict concentration and deposition of many Hazardous Air Pollutants (HAPs) or air toxics. The capacity supplements a photochemical mechanism in CMAQ by adding explicit model species representing gas phase concentrations of several HAPs. It also expands the aerosol and cloud chemistry modules to predict hazardous species found in particulate matter. Previous and current versions of the CMAQ model provide the modified mechanisms and modules. In addition to HAPs explicitly listed in the Clean Air Act, research versions of CMAQ have been modified to model additional, potentially toxic compounds such as herbicides (atrazine) and hydrofluorocarbons (tetrafluoropropene).  EPA’s Air Toxics Web site provides more information on specific HAPs and EPA’s important role in managing them.

The Multipollutant version of CMAQ (CMAQ-MP) predicts a large number of HAPs, as listed in  Tables 1 and 2 below. The more notable HAPs include acrolein, formaldehyde and mercury compounds. EPA’s National Air Toxic Assessments (NATA) have listed the acrolein and formaldehyde as accounting for large fractions of health risk across the United States. Mercury is a well known neurotoxin. How CMAQ-MP simulates mercury is described on the webpage on mercury modeling.

The chemical and aerosol processes in CMAQ-MP allow allow scientists to determine how emission control strategies affect CAPs and HAPs within a single model application.  The results account for interactions between and feedbacks for CAPs and HAPs that can't be accurately assessed from separate applications with different models. CMAQ-MP can be used to answer critical questions such as:

  • What emission control strategies optimize human and ecological health regarding both short term (i.e. respiratory, cardiopulmonary) and long term risks (i.e. cancer)?
  • What are the likely concentrations of HAPs and CAPs in areas of the US where there are no measurements? 
  • Do the strategies developed use the best understanding of the processes that affect both CAPs and HAPs?
  • Do control strategies improve air quality for one pollutant but cause other pollutants to increase? 
  • What atmospheric processes dominate the interconnections between specific pollutants?
  • How can we respond rapidly to emerging issues regarding both CAPs and HAPs, such as new emission sources and meteorological conditions?

CMAQ-MP is not the only version or configuration that answers the above questions but its answers cover the largest range of CAPs and HAPs.

Gas Phase Hazardous Air Pollutants (HAPs)
2,4-Toluene Diisocyanate
Ethylene Oxide
Hexamethylene 1,6-Diisocyanate
Maleic Anhydride
1,1,2,2-Tetrachloride Ethane
Acetaldehyde Emissions Tracer
Propylene Dichloride
Acrolein Emissions Tracer
1,3-Dichloride Propene
Total Acetaldehyde
Formaldehyde Emissions Tracer
Total Acrolein
Total Formaldehyde
1, 3-Butadiene
Methylene Chloride
Vinyl Chloride
Elemental Mercury
Carbon Tetrachloride
Oxidized Mercury Compounds

Table 1.  Gas-Phase Hazardous Air Pollutants represented in the current CMAQ Multi-pollutant model.


Aerosol Phase Hazardous Air Pollutants (HAPs)

Beryllium Compounds

Mercury Compounds

Nickel Compounds

Cadmium Compounds

Chromium (III) Compounds

Diesel Emissions Tracer

Chromium (VI) Compounds


Lead Compounds


Manganese Compounds









Table 2.  Aerosol-Phase Hazardous Air Pollutants represented in the current CMAQ-MP model.

CMAQ-MP predictions for formaldehyde
CMAP-MP predicitions for acroleing

Figure 1. CMAP-MP predictions for formaldehyde (left) and acrolein (right) averaged over two weeks in July 2006.

Future Directions
One future direction is to use new knowledge to update the secondary production pathways of HAPs, such as formaldehyde and acrolein. This work would address shortcomings in predictions from the CMAQ-MP model and other versions of the CMAQ model that simulate HAPs. Comparisons against observations show underestimates. Error sources likely include inaccurate emission estimates and poorly resolved sub-grid processes, but the chemical mechanisms also play a role.  For example, underestimates of formaldehyde are linked to the uncertain chemistry of isoprene and other compounds emitted by biogenic sources.  For acrolein, errors in predictions can arise from production pathways besides 1,3-butadiene that chemical mechanisms do not represent.

A second direction of future work is predicting additional toxic components in particulate matter, such as Polycyclic Organic Matter, a high priority urban air toxic. The group is believed to be mostly Polycyclic Aromatic Hydrocarbons (PAHs) emitted into the atmosphere as well as their daughter products. Simulating their fate and transport offers several challenges for model development. Many PAHs divide between the gas and aerosol phases based on their sub-cooled vapor pressures.  The gas phases undergo an ill-defined chemistry that produces other multiphase PAHs and Secondary Organic Aerosols (SOA). For example, naphthalene is believed to produce a significant fraction of SOA from mobile sources and its daughter products are more toxic. Another challenge comes from estimating the surface to air fluxes. Currently, the fluxes in CMAQ do not consider pathways unique to liphilic compounds such as PAHs. Estimating the pathways to improve these fluxes is also a potential area for future work.

In addition to the above areas, future efforts involve extending the CMAQ model to address emerging issues such as the use of biofuels and emissions of new compounds.  This might include both upgrading pathways of emitted compounds that are HAP precursors and including additional HAPs that impact human and ecological health.

Related Links:
  • Bash, J.O., 2010. Description and initial simulation of a dynamic bidirectional air-surface exchange model for mercury in Community Multiscale Air Quality (CMAQ) model. Journal of Geophysical Research, 115, D06305, 15 pp.
  • Cooter, E. J., W.T. Hutzell, W.T. Foreman, W. T. and M.S. Majewski, 2002. A Regional Atmospheric Fate and Transport Model for Atrazine, Part II: Evaluation. Environmental Science and Technology, 36, 4593-4599.
  • Cooter, E. J., and W.T. Hutzell. 2002. A Regional Atmospheric Fate and Transport Model for Atrazine, Part I: Development and Implementation. Environmental Science and Technology, 36, 4091-4098.
  • Hutzell, W.T., D.J. Luecken, K.W. Appel, W.P.L. Carter, 2012. Interpreting predictions from the SAPRC07 mechanism based on regional and continental simulations, Atmospheric Environment, 46, 417-429
  • Hutzell, W.T., D.J. Luecken, 2008. Fate and transport of emissions for several toxic metals over the United States, Science of the Total Environment 396, 164-179.
  • Luecken, D.J., W.T. Hutzell, M.L. Strum, G.A. Pouliot, 2012. Regional sources of atmospheric formaldehyde and acetaldehyde, and implications for atmospheric modeling, Atmospheric Environment, 47, 477-490.
  • Luecken, D. J., R.L. Waterland, S. Papasavva, W.T. Hutzell, K.N. Taddonio, J. Rugh, and S.O. Andersen, 2010. Ozone and TFA Impacts in North America from degradation of 2,3,3,3-tetrafluoropropene (HFO-1234yf), a potential greenhouse gas replacement. Environmental Science and Technology, 44(1), 343-348.
  • Luecken, D.J. and A.J. Cimorelli, 2008. Co-dependencies of reactive Air Toxic and criteria pollutants on emission reductions. Journal of the Air and Waste Management Association, 58, 693-701.
  • Luecken, D., A. Cimorelli, C. Stahl, and D. Tong, 2008. Evaluating the effects of emission reductions on multiple pollutants simultaneously, Air Pollution Modeling and its Applications XIX, ed.C. Borrego and A.I. Miranda, NATO Science for Peace and Security Series, Springer, The Netherlands.
  • Luecken, D.J., W.T. Hutzell, G.L. Gipson, 2006. Development and analysis of air quality modeling simulations for hazardous air pollutants, Atmospheric Environment 40, 5087-5096.

  • Papasavva, S. T., D.J. Luecken, K.N. Taddonio, R.L. Waterland, and S.O. Andersen, 2009. Estimated 2017 refrigerant emissions of 2,3,3,3-tetrafluoropropene (HFC-1234yf) in the United States resulting from automobile air conditioning. Environmental Science and Technology, 43(24), 9252-9259.

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