Air free CO is a special case of COn.  In this case the reading is normalised to 0% oxygen.

Example:

measured CO = 40ppm (COm)

measured O2 = 5.5% (O2m)

local regulation requires normalisation to 0% O2. (O2r)

COn = COm x (20.0 minus O2r) / (20.9 minus O2m)

COn = 40 x (20.9 minus 0) / (20.9 minus 5.5) = 40 x 20.9 /15.4 = 54.3ppmn

See also:  What should O2 ref be set to? and What does COn mean?