The researchers studied the differences in air composition between the upper troposphere and lower stratosphere. They used airborne measurements to analyze aerosols at the tropopause level. They found that stratospheric particles were rich in Si, while tropospheric ones contained Fe or Al. Sulfates were the main component in stratospheric aerosols, while tropospheric ones had a mix of ions. The size distribution of particles was different in the two layers, with larger particles in the stratosphere indicating their age. Nucleation mode particles were also detected in the lower stratosphere during some flights.