A statistical analysis of in situ observations of aerosols in the midlatitude free troposphere and tropopause region in summer is presented. Vertical profiles of the number density and the associated variability of nuclei mode and larger aerosol particles are discussed. These data confirm that the upper troposphere is an efficient net source region of new particles. Aerosols are studied by analyzing size-segregated data, and it is suggested that observed extreme cases of aerosol evolution include a nucleation state, an accumulation state, and a cloud-processed state. This interpretation implies that aerosol sources (nucleation from the gas phase) are closely tied to aerosol sinks (cloud scavenging of aerosol particles). Processes that could modify the proposed aerosol life cycle are discussed. Number size and surface area distributions typical for the various aerosol states are examined. The dry aerosol surface area ranges from 1 to 20 ¿m2 cm-3 in the free troposphere and from 2 to 13 ¿m2 cm-3 in the tropopause region. Parameterizations of the aerosol size distributions and total surface area concentrations are provided to facilitate the use of the data in atmospheric models.