Differences between SO2 and tcSO2

I am quite new to atmospheric data andI am working with CAMS reanalysis data to do my diploma thesis. I noticed there are two different parameters for SO2 (but the same for CO, NO2, O3..)
I would like to know what is the difference between Sulphur dioxide (kg kg^-1) and Total column Sulphur dioxide kg m^-2. 
The first mentioned can be found in CAMS Model levels, second one is from CAMS surface level. 

Is there any good documentation of the parameters from CAMS reanalysis (except of https://apps.ecmwf.int/codes/grib/param-db

Thank you very much for your help!


Total column (in kg m-2) is available at the surface (model level 60 for MARS users). Total column refers to the total amount of the selected variable in a column of air extending from the surface of the Earth to the top of the atmosphere (model level 1 for MARS users). Total column can also be referred to as total <selected variable>, or vertically integrated <selected variable>.