CTMs.tex

\subsection{Chemical Transport Models}
Chemical Transport Models (CTMs) simulate production, loss, and transport of chemical species.
This is generally calculated using one or both of the Eulerian (box) or Lagrangian (puff) frames of reference.
CTMs normally solve the continuity equations simultaneously with chemical production and loss for chemicals under inspection. 
The continuity equations describe transport of a conserved quantity such as mass, which, solved together with production and loss of a chemical forms the basis for a CTM.
This basis enables a record of the chemical densities and transport over time as a model runs.
The general continuity equation links a quantity of a substance (q) to the field in which it flows and can be described by the formula:
\begin{eqnarray*}
    \frac{\partial \rho}{\partial t} + \nabla \cdot j &=& \sigma 
\end{eqnarray*}
where $\rho$ is density of q in the field, t is time, $\nabla$ is divergence, j is the flux (the amount of q per unit area per unit time entering or leaving the field), and $\sigma$ is the generation of q per unit volume per unit time.
Note that $\sigma$ can be positive or negative due to sources and sinks.

The type of model best suited to modelling the entire earth uses the Eulerian frame of reference, where the atmosphere is broken up into 3-D boxes with densities and transport calculated and stored for arbitrary sequential steps in time at each location.
The mass balance equation must be satisfied in any realistic long term box model and is as follows: 
\begin{eqnarray*}
    \frac{dm}{dt} &=& \sum{sources}-\sum{sinks} \\
    &=& F_{in} + E + P - F_{out} - L - D 
\end{eqnarray*}
where m is mass of a chemical, E and D are emission and deposition, P and L are production and loss, and F is chemical transport in and out, as shown in figure \ref{fig:boxModel}.
Many chemical species interact with each other through production and loss. 
Any large chemical model will solve this mass balance equation over highly coupled arrays of partial differential equations which can be complex and time consuming.

GEOS-Chem is a well supported global, Eulerian CTM with a state of the science chemical mechanism, with transport driven by meteorological input from the Goddard Earth Observing System (GEOS) of the NASA Global Modeling and Assimilation Office (GMAO).
GEOS-Chem simulates more than 100 chemical species from the earth's surface up to the edge of space (0.01 hPa) and can be used in combination with remote and in-situ sensing data to give a verifiable estimate of atmospheric gases and aerosols.
It was developed, and is maintained, by Harvard University staff as well as users and researchers worldwide. 
Several driving meteorological fields exist with different resolutions, the finest at 0.25 by 0.3125$^\circ$ horizontally at 5 minute time steps with 72 vertical levels.

Combining satellite data with model outcomes provides a platform for the understanding of natural processes to be tested now and into the future over Australia and anywhere with few in-situ measurements.
Due to the low availability of in-situ data covering most of the Australian continent, a combination of the models with satellite data may provide improved understanding of emissions from Australian landscapes.
Improved emissions estimates will in turn improve the accuracy of CTMs, providing better predictions of atmospheric composition and its response to ongoing environmental change.