As part of the project INCA (Interhemispheric Differences in Cirrus Properties from Anthropogenic Emissions) two aircraft field campaigns have been performed to study aerosol and cirrus cloud properties of the upper troposphere (UT) in the midlatitudes of the southern hemisphere (SH) and the northern hemisphere (NH). This paper focuses on the measurements of UT number concentrations and tropospheric vertical profiles of volatile and refractory Aitken as well as total accumulation mode particles. The results are discussed with respect to interhemispheric differences. Aerosol number concentrations in the NH are more variable and typically a factor of 2--3 higher. The SH data considerably extend the few existing UT in situ aerosol measurements in the SH midlatitudes.