Conversion of units

Be aware that you cannot derive a surface concentration from a total column. See also this post. You can derive an integrated column [µg/m^2], but not a concentration [µg/m^3] (unless you want some average concentration for the full column, but I am not sure that will make a lot of sense for these molecules).

To derive a total column mass density, you will have to use the proper variable name for the mass density (which has a different name from the volume/number density), e.g. derive(SO2_column_density {time} [ug/m^2]).