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 a erosol 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 we ll as in summer 1998. The pronounced bursts reached particle number concent rations of several thousand particles cm(-3) of air in the Aitken and sever al tens of thousand particles cm(-3) in the ultrafine mode. Two casts are i nvestigated in detail via vertical profile measurements and horizontal nigh ts at cloud top. These measurements reveal that the particle bursts occur i n the cloud-free air just between the cloud parcels at cloud top. In one ca se, a correlation of a distinct particle burst with low pre-existing partic le surface area was observed. (C) 2001 Elsevier Science Ltd. All rights res erved.