Bursts of Aitken mode and ultrafine particles observed at the top of continental boundary layer clouds

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 cases are investigated in detail via vertical profile measurements and horizontal flights 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.