The software CLASS (Chemistry Land-surface Atmosphere Soil Slab model) is designed to be user friendly and self-explanatory. Since CLASS has a lot of options and possibilities, the user is free to design new experiments in addition to the ones presented in the previous chapters. However, it is important to bear in mind that although some of the solutions are mathematically possible, not all of them have a sound physical or chemical explanation. Therefore, in this chapter, we begin by explaining the basic steps to design a research plan to be efficient in the use of CLASS. Thereafter, we describe the main options related to the atmospheric physics, land surface, chemistry, and plant physiology.

16.1 Putting in Practice the Research Plan: The Numerical Experiment

– New experiment: Before using CLASS it is important that you define a research question, and set up a numerical experiment that enables you to answer it. On the basis of the design of the experiment, you have to adjust the switches and the variables (see next sections in options). Remember also to give each specific experiment a representative name. The following options are available:

1. • BASIC: Time integration, boundary layer characteristics, and state variables (potential temperature, specific humidity).

2. • Wind, carbon dioxide, and inert scalar.

3. • Radiation/Geography and Clouds: Geographical location, time, and boundary layer cloud type (stratocumulus and shallow cumulus including the dynamic and radiation effects).

4. • Surface: Includes the basic properties of vegetation and soil.

5. • Chemistry: Photolysis and reaction rate specifications, and the conditions of reactants.

6. • Reactions: Chemical scheme: none, simple, or complex.

7. • Advanced surface: Additional specifications for the vegetation and surface properties.

We describe now some basic and useful tips to navigate in the graphical user interface CLASS. We indicate whether an option is switched ON or OFF at the bottom of the main CLASS window. For each variable the complete variable name can be shown as a tooltip by hovering the mouse over the input field for a few seconds.