In situ detection of stratosphere‐troposphere exchange of cirrus particles in the midlatitudes

Airborne trace gas, microphysical, and radiation measurements were performed during the AIRcraft TOwed Sensor Shuttle ‐ Inhomogeneous Cirrus Experiment over northern Germany in 2013. Based on high‐precision nitrous oxide (N 2 O) and carbon monoxide (CO) in situ data, stratospheric air could be identified, which contained cirrus cloud particles. Consistent with the stratospheric N 2 O data, backward trajectories indicate that the sampled air masses crossed the dynamical tropopause in the last 3 h before the measurement. These air masses contained cirrus particles, which were formed during slow ascent in the troposphere and subsequently mixed with stratospheric air. From the CO‐N 2 O correlation the irreversibility of this transport is deduced. To our knowledge, this is the first in situ detection of cirrus particles mixed with stratospheric air in the midlatitudes.