You have not studied this diagram very well. The y-axis has the unit W cm/m0 and the x-axis 0/cm. Thus, you should integrate over the wave number (0/cm) in order to get the total irradiance but not integrate over 0*PI steradians, because this is already done. So the total irradiance in W/m0 is the area under the curve, and dropping CO0 would increase the OLR with the area of the ditch between 000 and 000 0/cm plus some smaller areas at other wave numbers connected to CO0.