Airborne in situ measurements of "bursts" (high number concentrations) of aerosol particles in the Aitken (10-100 nm in diameter) and ultrafine (3-10 nm) modes are described. The bursts were observed at the top of stratiform boundary layer clouds over Germany in winters 1996/1997 and 1997/1998 as well as in summer 1998. The pronounced bursts reached particle number concentrations of several thousand particles cm(-3) of air in the Aitken and several tens of thousand particles cm(-3) in the ultrafine mode. Two casts are investigated in detail via vertical profile measurements and horizontal nights at cloud top. These measurements reveal that the particle bursts occur in the cloud-free air just between the cloud parcels at cloud top. In one case, a correlation of a distinct particle burst with low pre-existing particle surface area was observed.