CAMS Surface concentration of a given species

Users who want get the surface concentration of a CAMS given species, should be taking the mixing ratio (kg per kg dry air) on the lowest model level (L60 or L137, depending on the dataset) and multiplying by the dry air density to convert this to concentration (kg/m3).

PM is an exception because this is a diagnostic output that it is already converted to concentration in kg/m3 within the model.

Can I ask you to provide more specifics about this? What is meant by dry air density ? 

Thank you very much.