Characteristics and origin of the aerosol in the lowermost stratosphere at northern midlatitudes were studied using measurements from a passenger aircraft (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container, or CARIBIC). Aerosol samples were collected during 60 intercontinental flights during 1999--2002 and analyzed for elemental composition with particle-induced X-ray emission (PIXE). Concurrent measurements of trace gases were used to interpret the aerosol measurements. It was found that particulate sulfur concentration increased steadily in the potential vorticity (PV) region of 2--7 PVU, whereas particulate potassium and iron showed no such dependence. The variability in concentration of the latter two elements was mainly connected with season, similar to their variation in the upper troposphere, whereas PV dominated the particulate sulfur variability. An ozone-based model was developed to quantitatively determine the mixing of stratospheric and tropospheric air masses. A significant dependence on PV was found, and the stratospheric fraction of the air peaked during spring. It was found that the particulate sulfur concentration was strongly dependent on the origin of the air masses. The concentration increased by a factor of 3 over the lowermost stratosphere. A discontinuity in the concentration over the tropopause indicated particle formation from sulfur dioxide transported across the tropopause. The concentration at the top of the lowermost stratosphere was used to estimate that the particulate sulfur production in the stratosphere is 0.066 Tg S/yr with approximately half of the amount transported across the top of the lowermost stratosphere originating in carbonyl sulfide.