The inversion problem of particle size distribution determination and aerosol spectrometers calibration problem are considered in the light of cross-channel interference existing in these instruments. It is shown that the latter phenomenon calls for changing the calibration concept for spectrometers. In the absence of any a priori knowledge about continuous size distributions the ill-posed data inversion problem is not amenable to solution by any known regularization algorithm. It is only the coarse grained (rather than fine structure) information about the size distribution that can be extracted from instrument measurements. The determination of such information in the form of "physical" size interval population distribution is suggested.