The particle size distribution and composition of nanosecond laser-generated aerosols from brass samples in atmospheric argon has been measured by low-pressure impaction and subsequent quantitative analysis of the aerosols by total reflection X-ray fluorescence. Ablation was performed applying a Nd:YAG laser at 1.06 microm both without and with a prepulse plasma breakdown generated by a second Nd:YAG laser at 2-60 micros prior to the ablation pulse. The beam of the prepulse laser had orthogonal direction to the ablation laser beam, and the breakdown was produced 2.5 mm above the ablation spot. Ultrafine aerosol particles (<50 nm) were generated in the double-pulse experiment representing practically the total mass impacted, while in single-pulse ablation the proportion of large particles (>0.1 microm) was dominating. The predominance of ultrafine aerosols in the prepulse experiment indicates that particle formation from vapor-phase condensation is the major mechanism, while the appearance of large particles in single-pulse ablation points at fragmentary evaporation in the laser-produced plasma. It was also shown that the total mass impacted in double-pulse ablation increases almost linearly with the power of the prepulse laser. The better atomization and the larger sample mass ablated can be assumed to be the main reasons for the increase of the line intensities in double-pulse laser-induced breakdown spectrometry with orthogonal prepulses reported by several research groups.