Dust size distribution in two-dimensional protoplanetary disks. The color map shows the logarithmic, size-dependent dust surface density σ_d(a), computed with 100 radial and dust-size bins. Left panel: reference simulation using the open-source code mcdust. Remaining panels: results obtained with the algorithm presented in this work, adopting the merging parameters X = 0.1, 0.01, and 0.001 (see Sect. 2.3). For X = 0.01 and 0.001, the small-size range is more accurately resolved, whereas larger X values reduce the resolution in this regime. The difference between X = 0.01 and 0.001 is minor, since a larger number of particles is required to fully benefit from values of X < 0.01.