Chemical transport model

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A chemical transport model (CTM) is a type of computer numerical model which typically simulates atmospheric chemistry (though the term seems more general).

1 Chemical transport models and general circulation models
2 Types of chemical transport models
- 2.1 Methodologies
- 2.2 Species of interest
  - 2.2.1 Examples of ozone CTMs
3 Notes

[edit] Chemical transport models and general circulation models

While related general circulation models (GCMs) focus on simulating overall atmospheric dynamics (e.g. fluid and heat flows), a CTM instead focuses on the stocks and flows of one or more chemical species. Similarly, a CTM must solve only the continuity equation for its species of interest, a GCM must solve all the primitive equations for the atmosphere; but a CTM will be expected to accurately represent the entire cycle for the species of interest, including fluxes (e.g. advection), chemical production/loss, and deposition. That being said, the tendency is (especially as the cost of computing declines over time) for GCMs to incorporate CTMs for species of special interest to climate dynamics (e.g. carbon dioxide, methane).

[edit] 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).

[edit] Methodologies

Jacob (1999)^[1] classifies CTMs as Eulerian/"box" or Lagrangian/"puff" models, depending on whether the CTM in question focuses on ^[1]

(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 continuity equations using a global/fixed frame of reference, while a Lagrangian CTM uses a local/moving frame of reference.

[edit] Species of interest

CTMs typically focus on one species, but in order to realistically model its dynamics, the CTM may be forced to account for many related species, such as precursors or tracers. E.g. the MOZART model focuses on ozone, but additionally models over 100 related species (including aerosols) and several hundred reactions.