Chemical transport model

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

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, especially as the cost of computing declines over time, is for GCMs to incorporate CTMs for species of special interest to climate dynamics, especially shorter-lived species such as nitrogen oxides and volatile organic compounds; 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)^[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.

See also

• discussion of gridding in CLaMS
• Lagrangian and Eulerian coordinates
• discussion of the continuity equation in Jacob's Introduction to Atmospheric Chemistry online

Examples of Eulerian CTMs

• CAMx
• GEOS-Chem
• LOTOS-EUROS
• MOZART
• TOMCAT/SLIMCAT

Examples of Lagrangian CTMs

• CLaMS

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.

Examples of ozone CTMs

• CLaMS
• MOZART

Notes

1. ^ ^{a b} Jacob, Daniel (1999). Introduction to Atmospheric Chemistry (1st Edition ed.). Princeton University Press. pp. 75–85. ISBN 0-691-00185-5. http://www-as.harvard.edu/people/faculty/djj/book/.