Define which gases contribute to SOA formation
In ARCA the gases, or vapours as they are called in this context, condense and evaporate on particles according to the so called Köhler equation. There the key property governing some compounds tendency to go to particle phase is the saturation vapour pressure Psat. We calculate Psat using the Multiphase system online property prediction. These temperature dependent values for each condensing vapour are saved as fitting parameters into the "vapour file", and from there the numerical model loads the list of vapours, their molar mass and the two parameters that are used in the Antoine equation: log10(Psat) = A- B/T where T is the gas temperature.
The Create vapour and elemental composition tool does all this for you.
The "Vapours" and "Elements" file can contain compounds that are not used in the current chemistry, in that case they will just be ignored. Therefore it is completely ok to create a vapour file which contains all possible (known) vapours, even if you use a chemistry scheme that has only part of those compounds. Adding more vapours increases the computational burden of the model, as well as the size of output. For all practical purposes it is rarely useful to include vapours that have high Psat, if they will also not reach very high concentrations. A reasonably safe limit has been Psat<10-6 atm, this could vary in some special cases which you would know.