Investigating an indirect aviation effect on mid-latitude cirrus clouds – linking lidar derived optical properties to in-situ measurements

Aviation has a large impact on the Earth’s atmosphere and climate by various processes. Line shaped contrails and contrail cirrus clouds lead to changes in the natural cirrus cloud cover, and have a major contribution to the effective radiative forcing from aviation. In addition, aviation emitted aerosols may also change the microphysical properties and, in particular, the optical properties of naturally formed cirrus clouds. Latter aerosol-cloud interactions show large differences in the estimated resulting effective radiative forcing and our understanding on how aviation induced aerosols affect cirrus cloud properties is still poor. Up to now, observations of this aviation induced aerosol effect are rare. In this study, we use combined airborne lidar and in-situ ice cloud measurements to investigate differences in the microphysical and optical properties of naturally formed cirrus clouds, which either formed under influences of aviation induced aerosol emissions or which formed under rather pristine conditions. We relate collocated lidar measurements performed aboard HALO during the ML-CIRRUS mission of the particle linear depolarization ratio with in-situ cloud probe measurements of the number and effective diameter of the ice particles. We find that those clouds, which are more affected by aviation induced aerosol emission, are characterized by larger values of the particle linear depolarization ratio. These aviation-affected cirrus clouds exhibit larger mean effective ice particle diameters connected to decreased ice particle number concentrations, than the cirrus clouds, which evolved in more pristine regions. With this study, we provide new observations of aerosol-cloud interactions, that will help to quantify related changes in the atmospheric energy budget