**Instructor's notes.**

The model used in the activity " *Using
mass balance model to understand CFCs*** " **is
based on the simple mass balance relation.

** (eqn. 1) **

Where C is the global atmospheric concentration of CFC-12 (in pptv),** S** is the
emission source strength (ppt/yr), and **t**
is the atmospheric lifetime (yrs).

The general solution to equation 1 for any arbitrary time dependent emission source S(t) is:

** (eqn.
2) **

We have solved equation 2 assuming an emission source strength of the form,

** (eqn. 3) [see solution]**

An interactive online program has been written in which students can modify the input values ( Co, t, So, and R) and then generate a graph of C vs t. A table of values for C and S as functions of time is also generated. Students first "calibrate" the model to fit recent observations and then use the model to explore future emission scenarios. Although I have used this assignment after a brief in-class discussion of the model basics and online modeling environment, the two activities below provide a solid background of the mass balance concept and its application to global trace gas concentrations.

For classes with limited mathematics ability, I describe the mathematics of
the model using only the finite difference form of equation 1. Although it
is tempting to also discuss Euler's number **e, **I purposely avoid this for
classes with weak math skills as I believe that it adds little (if anything) to
their understanding of the mass balance physical processes.** **

To give students a better feel for the model you may want to use some or all
of the introductory water bucket model activity at: http://www.atmosedu.com/physlets/GlobalPollution/WaterBucket.htm.
I often use a 2-liter pop bottle, with flow from a sink into the top and a
hole in the bottom, as a physical model during an in class discussion of mass
balance. The lifetime for this water bucket model is then related to the
hole size in the bottle and viscosity of the water. |

- A similar modeling activity has also been written to explore carbon dioxide build-up in the atmosphere using the same model structure given by equations 1-3. http://serc.carleton.edu/quantskills/workshopactivities/co2_global.html