A chemical transport model (CTM) is a type of computer numerical model which typically simulates

atmospheric chemistry Atmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary approach of research and draws on environmental chemistry, physics, meteoro ...

and may give

air pollution forecasting Air pollution forecasting is the application of science and technology to predict the composition of the air pollution in the atmosphere for a given location and time. An algorithm prediction of the pollutant concentrations can be translated into ...

Chemical transport models and general circulation models

While related general circulation models (GCMs) focus on simulating overall atmospheric dynamics (e.g.

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

and

heat flow Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

s), a CTM instead focuses on the stocks and flows of one or more

chemical species A chemical species is a chemical substance or ensemble composed of chemically identical molecular entity, molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels ...

. Similarly, a CTM must solve only the

continuity equation A continuity equation or transport equation is an equation that describes the transport of some quantity. It is particularly simple and powerful when applied to a conserved quantity, but it can be generalized to apply to any extensive quantity. S ...

for its species of interest, a GCM must solve all the

primitive equations The primitive equations are a set of nonlinear partial differential equations that are used to approximate global atmospheric flow and are used in most atmospheric models. They consist of three main sets of balance equations: # A ''continuity equ ...

for the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

; but a CTM will be expected to accurately represent the entire cycle for the species of interest, including fluxes (e.g.

advection In the field of physics, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion of a fluid. The properties of that substance are carried with it. Generally the majority of the advected substance is al ...

), chemical production/loss, and deposition. That being said, the tendency, especially as the cost of computing declines over time, is for GCMs to incorporate CTMs for species of special interest to

climate dynamics ''Climate Dynamics'' is a peer-reviewed scientific journal published by Springer Science+Business Media. It covers all aspects of the dynamics of global climate systems, including analytical and numerical modeling research on the structure and be ...

, especially shorter-lived species such as

nitrogen oxides Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide *Nitrogen trioxide (), or ni ...

and

volatile organic compounds Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a ...

; this allows feedbacks from the CTM to the GCM's radiation calculations, and also allows the meteorological fields forcing the CTM to be updated at higher time resolution than may be practical in studies with offline CTMs.

Types of chemical transport models

CTMs may be classified according to their methodology and their species of interest, as well as more generic characteristics (e.g. dimensionality, degree of resolution).

Methodologies

Jacob (1999) classifies CTMs a
Eulerian/"box"
o

models, depending on whether the CTM in question focuses on * (Eulerian) "boxes" through which fluxes, and in which chemical production/loss and deposition occur over time * (Lagrangian) the production and motion of parcels of air ("puffs") over time An Eulerian CTM solves its

s using a global/fixed

frame of reference In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system whose origin, orientation, and scale are specified by a set of reference points― geometric points whose position is identified both mathema ...

, while a Lagrangian CTM uses a local/moving frame of reference.

Examples of Eulerian CTMs

CCATT-BRAMS

WRF-Chem
*

CMAQ CMAQ is an acronym for the Community Multiscale Air Quality Model, a sophisticated three-dimensional Eulerian grid chemical transport model developed by the US EPA for studying air pollution from local to hemispheric scales. EPA and state envir ...

CMAQ Website

CAMx

GEOS-Chem

MATCH
* MOZART: (Model for OZone And Related chemical Tracers) is developed jointly by the ( US)

National Center for Atmospheric Research The US National Center for Atmospheric Research (NCAR ) is a US federally funded research and development center (FFRDC) managed by the nonprofit University Corporation for Atmospheric Research (UCAR) and funded by the National Science Foundatio ...

(NCAR), the

Geophysical Fluid Dynamics Laboratory The Geophysical Fluid Dynamics Laboratory (GFDL) is a laboratory in the National Oceanic and Atmospheric Administration (NOAA) Office of Oceanic and Atmospheric Research (OAR). The current director is Dr. Venkatachalam Ramaswamy. It is one of se ...

(GFDL), and the

Max Planck Institute for Meteorology The Max Planck Institute for Meteorology (''Max-Planck-Institut für Meteorologie''; MPI-M) is an internationally renowned institute for climate research. Its mission is to understand Earth's changing climate. Founded in 1975, it is affiliated w ...

(MPI-Met) to simulate changes in

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

concentrations In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...

in the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

. MOZART was designed to simulate

tropospheric The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

chemical and transport processes, but has been extended (MOZART3) into the

stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...

and

mesosphere The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...

. It can be driven by standard

meteorological Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...

fields from, for example,the

National Centers for Environmental Prediction The United States National Centers for Environmental Prediction (NCEP) delivers national and global weather, water, climate and space weather guidance, forecasts, warnings and analyses to its Partners and External User Communities. These prod ...

(NCEP), the

European Centre for Medium-Range Weather Forecasts The European Centre for Medium-Range Weather Forecasts (ECMWF) is an independent intergovernmental organisation supported by most of the nations of Europe. It is based at three sites: Shinfield Park, Reading, United Kingdom; Bologna, Italy; an ...

(ECMWF) and the Global Modeling and Assimilation Office (GMAO), or by fields generated from

general circulation model A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms f ...

s. MOZART4 improves MOZART2's chemical mechanisms,

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

scheme,

dry deposition In the physics of aerosols, deposition is the process by which aerosol Particulates, particles collect or deposit themselves on solid surfaces, decreasing the concentration of the particles in the air. It can be divided into two sub-processes: ' ...

mechanism,

biogenic A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...

emissions and handling of

aerosols An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthrop ...

. * TOMCAT/SLIMCAT
CHIMERE

POLYPHEMUS
* TCAM (Transport Chemical Aerosol Model; TCAM): a mathematical modelling method (computer simulation) designed to model certain aspects of the Earth's atmosphere. TCAM is one of several chemical transport models, all of which are concerned with the movement of chemicals in the atmosphere, and are thus used in the study of air pollution. ::TCAM is a multiphase three-dimensional eulerian grid model (as opposed to lagrangian or other modeling methods). It is designed for modelling dispersion of pollutants (in particular

photochemical Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400–7 ...

and

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...

) at mesoscales (medium scale, generally concerned with systems a few hundred kilometers in size). ::TCAM was developed at the

University of Brescia The University of Brescia ( it, Università degli Studi di Brescia) is an Italian public research university located in Brescia, Italy. It was founded in 1982 and is branched in 4 Faculties. The University of Brescia was officially established ...

in Italy.ESMA (Environmental Systems Modeling and Assessment) group of University of Brescia
/ref>

Examples of Lagrangian CTMs

CLaMS Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shel ...

FLEXPART

Examples of Semi-Lagrangian CTMs

MOCAGE

Examples of ozone CTMs

* MOZART

Notes

External links

* *MOZART: ** UCAR'
MOZART page
** MPI-Met'
MOZART page

Larry Horowitz's MOZART page