Optical devices for detection and sizing of aerosol microparticles (aerosol particle counters) employ the fact that the scattering efficiency, and accordingly, the scattering signal amplitude grows with the particle’s size. This principle implies that all the analyzed particles “feel” the same optical field intensity. However, frequently the light beam intensity in the measured volume depends on the coordinates, and identical particles produce different scattering signals depending on their trajectory with respect to the light beam axis. In such situations, typical for laser light sources, the unambiguous relation between the particle size and the scattering signal amplitude is destroyed. However, the original particle-size distribution can be retrieved via a special mathematical processing, provided that statistical parameters of the particles’ trajectories are known, as well as the spatial inhomgeneity of illumination. We develop the mathematical model describing formation of the scattering signals’ set. It leads to an integral equation involving the probability densities of: (i) the scattering signal amplitude, A, distribution P(A); (ii) the particle radius, a, distribution W(a) in the original aerosol; the particle-trajectories’ spatial distribution n(x) with respect to the light beam axis x = 0; and the spatial distribution of the light intensity I(x) within the measuring volume. For the practical case of Gaussian I(x) and rectangular n(x), the equation can be reduced to the known Abel equation, which enables to retrieve W(a) from the experimentally measured P(A) via the exact analytical expression. However, in many practical situations a numerical approximate approach may be more efficient. To this end, we describe in detail a simplified numerical procedure that enables to find the numbers of particles whose sizes lie within a few fixed intervals. Additionally, we show that a proper choice of such intervals can eliminate the harmful influence of the non-monotonic calibration curve of the aerosol counter. The algorithms and procedures are illustrated by numerical examples with a set of polystyrene particles whose sizes are distributed log-normally within the range 0.3 mm to 5 mm, analyzed by the aerosol counter that employs a semiconductor-laser beam of the wavelength 0.85 mm and radius 0.1 mm crossing the 1-mm air flow channel.