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Community Multiscale Air Quality Modeling System (CMAQ)

Air-Surface Exchange Process Overview

Air-surface exchange overview

There is a constant exchange or flux of chemicals between the atmosphere and Earth’s surface. Pollutants such as sulfur dioxide (SO2HelpSO2A pungent, colorless, gaseous pollutant formed primarily by the combustion of fossil fuels. One of the six "criteria" pollutants for which EPA has set national ambient air quality standards.) can be emitted from sources such as smokestacks and vehicles, transported through the air, and then deposited to the ground. Other pollutants like ozone (O3HelpO3Ozone (O3) is a colorless gas with a pungent odor. It is found in two layers of the atmosphere, the stratosphere and the troposphere. In the stratosphere, ozone provides a protective layer shielding the Earth from ultraviolet radiation's potentially harmful health effects. At ground level (the troposphere), ozone is a pollutant that affects human health and the environment, and contributes to the formation of smog.) are formed in the atmosphere and are taken up by vegetation. Some pollutants are emitted from the surface as is the case with ammonia (NH3) that is used in agricultural fertilizers. The direction of the exchange or flux follows concentration gradients. If the concentration is higher in the air than the surface, the pollutant deposits. If the concentration in the air is lower than the surface, the pollutant will be emitted to the atmosphere.

Emissions processes

All emission sources can be classified as either natural or anthropogenic. Anthropogenic, or human-created sources are from human activities.  Natural source air emissions include volatile organic compounds (VOCsHelpVOCsOrganic chemicals that have a high vapor pressure (i.e. extremely low boiling point) at ordinary room temperature. VOCs include human-made and naturally occurring chemical compounds. Some VOCs are dangerous to human health or cause harm to the environment. Harmful VOCs typically are not acutely toxic, but continued exposure to them may have long-term health effects.), nitrogen oxides(NOxHelpNOxGases consisting of one molecule of nitrogen and varying numbers of oxygen molecules. Nitrogen oxides are produced in the emissions of vehicle exhausts and from power stations. In the atmosphere, nitrogen oxides can contribute to formation of photochemical ozone (smog), can impair visibility, and have health consequences; they are thus considered pollutants.), and greenhouse gases such as methane (CH4), nitrous oxide (N2O), ozone (O3) and carbon dioxide (CO2). Emission sources for all of these gases are natural processes occurring:

  • In vegetation and soils
  • In marine ecosystems, caused by geological activity like geysers or volcanoes
  • The of meteorological activity, such as lightning
  • From fauna, such as ruminants and termites

Although emissions resulting from activities associated with the agriculture industry are human-created, they are also included in the natural source category. These activities include fertilizer use, which triggers emissions from microbial activity, and agricultural biomass burning. 

There are five broad categories of emission sources that are needed or are a part of CMAQ:

  • Anthropogenic sources
  • Biogenic and natural sources
  • Fire sources
  • Wind-blown dust sources
  • Sea spray sources

Anthropogenic sources

Anthropogenic sources include all sources that are human-created and not included in the other three categories. These sources along with biogenic sources and fires sources are documented in the National Emissions Inventory (NEI).

Scientific approach

The NEI is a comprehensive listing by sources of six common air pollutants over the United States for a specific time interval. The NEI is created on a triennial basis: 1996, 1999, 2002, 2005, 2008, 2011, 2014, 2017, etc.

The six air pollutants are:

  • Carbon monoxide
  • Nitrogen oxides, a ground-level ozone precursor
  • Volatile Organic Compounds (VOCs), a ground-level ozone precursor
  • Particulate matter
  • Sulfur dioxide
  • Ammonia

Anthropogenic Sources can be further divided into seven broad sectors:

  • Agriculture
  • Fugitive dust
  • Fuel combustion
  • Industrial process
  • Mobile sources
  • Solvents
  • Miscellaneous

For additional technical information about the NEI, visit EPA's National Emission Inventory website.

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Biogenic and natural sources

Biogenic Sources are sources that originate from land based vegetation such as trees, shrubs, soil and crops. Biogenic sources, a subset of natural sources, include only those sources that result from some sort of biological activity. Biogenic emissions represent a significant portion of natural source emissions. VOCs, NOx, and the greenhouse gases can all be emitted from biogenic sources. Vegetation is the predominant biogenic source of VOCs and is typically the only source that is used to estimate biogenic VOC emissions. Microbial activity is responsible for the emission of NOx and the greenhouse gases CO2, CH4, and N2O. Soil microbial activity is responsible for NOx and N2O emissions from agricultural lands and grasslands. CH4 is emitted through microbial action in waterlogged soils or in other anaerobicHelpanaerobicAnaerobic respiration takes place without the use of oxygen and produces small amounts of energy. Alcohol or lactic acid or other compounds are produced as waste products depending on the kind of cells that are active., or oxygen-free, microenvironments. CO2 is released through the aerobicHelpaerobicAerobic respiration takes place in the presence of oxygen and produces a large amount of energy. Carbon dioxide and water are produced as the waste products. decay of biomass. 

There are four source categories for natural NOx emissions:

  • Soils
  • Lightning
  • Stratospheric injection
  • Oceans

Emissions from soils are the only biogenic source of NOx. Soils emit NOx through biological and abiological pathways, and emission rates can be categorized by land use. Most of the NOx emitted by soils is in the form of nitric oxide (NO). Agricultural lands and grasslands are the most significant emitters within this category. 

Scientific approach

Biomass burning, including forest fires, burning of agricultural wastes and other prescribed burning, is sometimes included as a natural source of VOC emissions. However, many of the biomass burning processes are anthropogenic activities, and are better grouped as a separate source. The quantity of NOx emissions from agricultural land is dependent on the rate of fertilizer application and the subsequent microbial nitrogen processing in the soil. Microbial nitrogen processing occurs naturally in soil, but the rates are greater when soil has been fertilized with chemical fertilizers.

Either the Biogenic Emission Inventory System (BEIS) or the Model of Emissions of Gases and Aerosols from Nature (MEGAN ) can be used to provide offline biogenic emissions to CMAQ. To calculate online biogenic emissions, CMAQ uses BEIS to calculate emissions resulting from biological activity from land-based vegetative species as well as nitric oxide emissions produced by microbial activity from certain soil types.  Further information on running CMAQ with online biogenics using BEIS can be found in the CMAQ User's Guide. 

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Fire sources

Fire sources are event-based sources and can be classified as wildfires, prescribed fires, crop residue burning, and rangeland burning.

  • Wildfire: An uncontrolled fire that burns any area and are often called forest fires, grass fires, peat fires depending on what is being burnt.
  • Prescribed fire: A fire that is planned for a predetermined area, under specific environmental conditions, to achieve a desired outcome.
  • Agricultural fire (crop residue burning): A fire that is planned by farmers as an inexpensive and effective method to remove excess residue to facilitate planting, control pests and weeds, and/or provide fast-acting ash fertilization prior to planting or re-seeding.
    • Pre-harvest burning for removal of leaves and other biomass (sugarcane).
    • Post-harvest burning for removal of ground-level senescent vegetation.

Scientific approach

Examples of agricultural fire types:

  • Established crop areas that produce food, fiber, and seeds
  • Fallow fields
  • Crop categories include Bluegrass, Corn, Cotton, Rice, Soy, Sugarcane, Wheat, Other, Fallow, Double Crops
  • Rangeland fires: a fire that is planned to burn over grassland and pasture areas (often these areas are used for cattle grazing)
    • Single Land use category in the underlying land use data set
    • Originally several categories that were merged into one (January 2014 update) due to inconsistency in reporting from state to state
  • Any fire that begins as a controlled fire but becomes uncontrolled  would be classified as a wildfire

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Windblown dust sources

Dust is created when high-speed wind blows near arid and semi-arid surfaces. Scientists are interested in studying dust emission, because windblown dust has important effects on the atmosphere, like change in visibility, and human health, like asthma attacks and skin irritation. Chemicals, airborne bacterial species, and trace metals can also be transported thousands of miles with dust particles.

The actual amount of dust emitted from an arid surface depends on wind speed, surface roughness, moisture content of the soil, vegetation coverage, soil type and texture, and air density.

  • The main mechanism behind strong dust storms is called “saltation bombardment” or “sandblasting.” The physics of saltation include the movement of sand particles due to wind, the impact of these particles to the surface that removes part of the soil volume, and the release of smaller dust particles.
  • CMAQ first calculates friction velocity at the surface of the Earth. Once this friction velocity exceeds a threshold value, saltation, or horizontal movement, flux is obtained. Finally, the vertical flux of the dust is calculated based on a sandblasting efficiency formulation – a vertical-to-horizontal dust flux ratio.
  • CMAQ uses satellite information from the Moderate Resolution Imaging Spectroradiometer or MODIS to obtain realistic time-varying vegetation coverage. The model obtains MODIS vegetation, soil moisture and wind speed from the meteorological model, WRF.
  • Using the satellite-based vegetation data together with a newly developed relation for the surface roughness length, the effects of solid elements, such as pebbles, and vegetation non-erodible elements in local wind acceleration, drag partitioning, and protective coverage, is formulated in a consistent manner.

Additional technical details can be found in Foroutan et al. (2017).

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Ocean and sea spray sources

Sea spray is an important component of particles in the atmosphere of coastal location. The sea salt in these particles can react with anthropogenic pollution in urban areas near the coast, changing the way it’s transported and deposited. To accurately simulate the interaction between sea salt and anthropogenic pollution, realistic sea spray particle emissions are needed in air quality models.

Scientific approach

  • Because sea spray particles are emitted during wave breaking and bubble bursting at the ocean surface, the main factor affecting the emission rate is the wind speed.
  • The temperature of the ocean also affects bubble bursting and subsequent emission rate of sea spray particles.
  • Wave breaking is enhanced near the surf zone just offshore, and CMAQ accounts for this by increasing sea spray particle emission rates in the surf zone.

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Deposition processes

Atmospheric deposition are the processes that remove atmospheric gases and particles by direct deposition to the surface water, vegetation, or soil (dry deposition) or the absorption or interception of gases and particles by precipitation (wet deposition). Some important gas pollutants, which are eventually deposited back to the surface, are generated naturally on land or from bodies of water. The exchange of these gases between the air and surfaces is dependent on the difference between the atmospheric concentration and the concentration in the surface media. Because the pollutants can be either deposited or emitted, we call the processes bidirectional exchange. Deposition and bidirectional exchange processes can be a significant source of nutrients or acidifying chemicals to various ecosystems. The introduction of these chemicals can contribute to adverse ecosystem impacts, e.g. eutrophication, soil acidification, biodiversity loss, etc. 

Dry deposition

Dry deposition is the process of pollutants impacting the ground, a plant, a building, a body of water or another surface and subsequently being removed from the atmosphere. Because it is dry, this process is entirely driven by winds and gravity, not rainfall or fogs. DepositionHelpDepositionWhen chemicals like acids or bases fall to the Earth's surface. Deposition can be wet (wet deposition, such as rain or cloud fog), as well as particle and gas deposition (dry deposition). can be an important pathway for pollutants to be transferred from the atmosphere to an ecosystemHelpecosystemThe interacting system of a biological community and its non-living environment.. An excess of deposition can alter the chemical composition of an ecosystem and cause effects such as acidificationHelpacidificationRefers to reducing something's pH, making it more acidic; also means the loss of ANC. and eutrophicationHelpeutrophicationThe process by which lakes and streams are enriched by nutrients (usually phosphorus and nitrogen) which leads to excessive plant growth.. When an ecosystem's chemical composition is significantly altered, species loss or a shift in the prevalence of individual species can result. Specific examples of damaging effects include increases in the frequency of harmful algal blooms and decreased forest growth.

Scientific approach

Dry deposition is determined as the product of the atmospheric concentration and the deposition velocity. The deposition velocity is modeled in CMAQ using the electrical resistance paradigm where resistances are defined along pathways from the atmosphere to the vegetation or surface and act in series and parallel. Some literature refers to "conductances" which are simply the inverse of the resistance. The deposition pathways modeled in CMAQ are shown in the figure below from Pleim and Ran, 2011

Deposition pathways modeled in CMAQ

Wet deposition

Wet deposition is the removal of atmospheric gases or aerosols by precipitation. The wet deposition processes include in-cloud scavenging and the below-cloud interception of gases and aerosols by precipitation. Wet deposition processes can more efficiently remove small aerosols (with a particle diameter less than 2.5 µm) than dry deposition processes can. In non-arid areas, wet deposition can contribute to more than half of the total deposition of nutrients or acidifying pollutants to the land and surface waters. 

Scientific approach

In CMAQ, clouds can be grid-scale and sub-grid (convective). The precipitation rate is used directly from the meteorological model (typically WRF) as is the location of grid-scale clouds. Sub-grid clouds are rediagnosed in CMAQ based on the precipitation rate. Scavenging, wet deposition, and below-cloud rainout/washout are modeled for sub-grid clouds. The treatment for grid-based clouds does not currently include below-cloud rainout/washout, but this is a priority research area for model improvment. 

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Bidirectional exchange processes

Semivolatile pollutants that are produced or remain in vegetation, soil and/or water can be re-emitted from the surface to the atmosphere if the concentration at the surface is greater than the atmospheric concertation. Alternatively, these pollutants will deposit to the surface if the atmospheric concentration is greater than the concentration at the surface. This bidirectional nature of air-surface exchange can modify the transport and environmental impact of these pollutants. For example, ammonia (NH3) is soluble, readily partitions to the aerosol phase, and has the chemical characteristics that indicate it should deposit to the surface quickly. However, NH3 is also semivolatile and can be produced or retained in soil, vegetation, and water through the decomposition of organic matter or through fertilizer application. This means it can deposit or be re-emitted or produced when conditions are right, leading to impacts of NH3 pollution in areas distant from large NH3 emission sources. 

Scientific approach:

  • Currently bidirectional exchange in CMAQ is parameterized for ammonia (Bash et al. 2013, Pleim et al. 2013) and mercury (Hg) (Bash 2010). 
  • Sub models, for Hg, and USDA Environmental Policy Integrated Climate (EPIC) model fertilization rates and soil pH, for NH3 (Cooter et al. 2012), are used to estimate the concentrations of Hg and NH3 in the soil, vegetation, and water surfaces.  The Fertilizer Emission Scenario Tool for CMAQ (FEST-C) system is used to simulate daily fertilizer application information using the EPIC model for a defined CMAQ domain.
  • Fluxes, or the sum of dry deposition and emissions, are calculated based on the air-surface concentration gradient and an estimated transfer velocity similar to the dry deposition velocity. 

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Related links


Appel, K.W., Pouliot, G., Simon, H., Sarwar, G., Pye, H.O.T., Napelenok, S., Akhtar, F., & Roselle, S. (2013). Evaluation of dust and trace metal estimates from the Community Multiscale Air Quality (CMAQ) model version 5.0. Geoscientific Model Development6(4), 883-899.

Baker, K., Woody, M., Tonnesen, G., Hutzell, W., Pye, H., Beaver, M., Pouliot, G., & Pierce, T. (2016). Contribution of regional-scale fire events to ozone and PM 2.5 air quality estimated by photochemical modeling approaches. Atmospheric Environment140, 539-554.

Bash, J.O. (2010). Description and initial simulation of a dynamic bi-directional air-surface exchange model for mercury in CMAQ, J. Geophys. Res., 115, D06305. doi: 10.1029/2009JD012834EXIT

Bash, J.O., Cooter, E.J., Dennis. R.W., Walker, J.T., & Pleim. J.E. (2013). Evaluation of a regional air-quality model with bi-directional NH3 exchange coupled to an agro-ecosystem model, Biogeosciences, 10, 1635-1645 doi: 10.5194/bg-10-1635-2013EXIT

Cooter, E.J., Bash, J.O., Benson V., & Ran, L.-M. (2012). Linking agricultural management and air-quality models for regional to national-scale nitrogen deposition assessments, Biogeosciences, 9, 4023-4035. doi: 10.5194/bg-9-4023-2012 EXIT

Foroutan, H., Young, J., Napelenok, S., Ran, L., Appel, K.W., Gilliam, R.C. & Pleim, J.E. (2017). Development and evaluation of a physics-based windblown dust emission scheme implemented in the CMAQ modeling system, J. Adv. Model. Earth Syst., 9, 585–608, doi: 10.1002/2016MS000823Exit

Janssens-Maenhout, G., Crippa, M., Guizzardi, D., Dentener, F., Muntean, M., Pouliot, G., Keating, T., Zhang, Q., Kurokawa, J., & Wankmüller, R. (2015). HTAP_v2. 2: a mosaic of regional and global emission grid maps for 2008 and 2010 to study hemispheric transport of air pollution. Atmospheric Chemistry and Physics15(19), 11411-11432.

Koplitz, S. N., Nolte, C. G., Pouliot, G. A., Vukovich, J. M., & Beidler, J. (2018). Influence of uncertainties in burned area estimates on modeled wildland fire PM2. 5 and ozone pollution in the contiguous US. Atmospheric Environment, 191, 328-339.

Pleim, J.E., Bash, J.O., Walker, J.T., & Cooter, E.J. (2013). Development and testing of an ammonia bi-directional flux model for air-quality models, J. Geophys. Res.  118, doi: 10.1002/jgrd.50262EXIT

Pleim, J. & Ran, L. (2011). Surface Flux Modeling for Air Quality Applications, Atmosphere, 2, 271-302. doi:10.3390/atmos2030271 Exit

Pouliot, G., Pace, T.G., Roy, B., Pierce, T., & Mobley, D. (2008). Development of a biomass burning emissions inventory by combining satellite and ground-based information. Journal of Applied Remote Sensing, 2(1), 021501-021517.

Pouliot, G., & Pierce, T. (2009). Integration of the Model of Emissions of Gases and Aerosols from Nature (MEGAN) into the CMAQ Modeling System, paper presented at 18th International Emission Inventory Conference. Baltimore, Maryland.

Pouliot, G., Pierce, T., van der Gon, H.D., Schaap, M., Moran, M., & Nopmongcol, U. (2012). Comparing emission inventories and model-ready emission datasets between Europe and North America for the AQMEII project. Atmospheric Environment53, 4-14.

Pouliot, G., Rao, V., McCarty, J.L., & Soja, A. (2017). Development of the crop residue and rangeland burning in the 2014 National Emissions Inventory using information from multiple sources.  Journal of the Air & Waste Management Association, 67, 613-622.

Pouliot, G., van der Gon, H.A.D., Kuenen, J., Zhang, J., Moran, M.D., & Makar, P.A. (2015). Analysis of the emission inventories and model-ready emission datasets of Europe and North America for phase 2 of the AQMEII project. Atmospheric Environment115, 345-360.

Pouliot, G., Wisner, E., Mobley, D. & Hunt Jr, W. (2012). Quantification of emission factor uncertainty. Journal of the Air & Waste Management Association62(3), 287-298.

Ran, L., Yuan, Y., Cooter, E., Benson, V., Yang, D., Pleim, J., Wang, R. and Williams, J. (2019). An integrated agriculture, atmosphere, and hydrology modeling system for ecosystem assessments. Journal of Advances in Modeling Earth Systems, 11(12), 4645-4668. DOI:

Schwede, D., Pouliot, G., & Pierce, T. (2005). Changes to the biogenic emissions inventory system version 3 (BEIS3). 4th Annual CMAS Models-3 Users' Conference, September, 26-28.

Wilkins, J. L., Pouliot, G., Foley, K., Appel, W., & Pierce, T. (2018). The impact of US wildland fires on ozone and particulate matter: a comparison of measurements and CMAQ model predictions from 2008 to 2012. International Journal of Wildland Fire, 27, 684-698.

Xing, J., Mathur, R., Pleim, J., Hogrefe, C., Gan, C.-M., Wong, D., Wei, C., Gilliam, R., & Pouliot, G. (2015). Observations and modeling of air quality trends over 1990–2010 across the Northern Hemisphere: China, the United States and Europe. Atmospheric Chemistry and Physics15(5), 2723-2747.

Xing, J., Pleim, J., Mathur, R., Pouliot, G., Hogrefe., C., Gan, C.-M., & Wei, C. (2013). Historical gaseous and primary aerosol emissions in the United States from 1990 to 2010. Atmospheric Chemistry and Physics13(15), 7531-7549.

Yienger, J.J., & Levy II, H. (1995). Empirical model of global soil-biogenic NO x emissions. J. Geophys. Res., 100(D6), 11447-11464.

Zhou, L., Baker, K. R., Napelenok, S. L., Pouliot, G., Elleman, R., O'Neill, S. M., Urbanksi, S. P. & Wong, D. C. (2018). Modeling crop residue burning experiments to evaluate smoke emissions and plume transport. Science of the Total Environment, 627, 523-533.