The CIRRUS-HL mission is a joint research project conducted by German research centers and universities as part of the HALO Priority Program (SPP-1294) of the German Research Foundation (DFG) bay means of the HALO research aircraft. One of the main objectives of the airborne mission is to gain new insights into the formation, properties, and climatic influence of ice clouds at high latitudes, in the region of the world with the strongest anthropogenic increase in surface temperature. A second focus is the investigation of the effects of air traffic on cirrus clouds, which was also carried out in flights over Central Europe and the North Atlantic flight corridor. The HALO cloud mission (22 research flights) took place in June and July 2021, combining in-situ and remote sensing instrumentation with state-of-the-art cloud sampling and new ice residue, aerosol particle, trace gas, and radiation measuring devices. More details on the scientific questions, research objectives, instrumentation, flight strategies, and participants are available on the CIRRUS-HL website.

The HALO-CVI (HALO-Counterflow Virtual Impactor) cloud inlet of TROPOS was operated as a key instrument in CIRRUS-HL. This special inlet collects liquid droplets and/or ice particles within clouds and makes their residues (CDR: cloud drop residuals; IPR: ice particle residuals) available for particle analysis. More details about the CVI working principle can be found here (Virtual Counterflow Impactor).

Using our own measuring instruments behind the HALO-CVI inlet, the size distribution and absorption coefficient (as a measure of black carbon, BC) of cloud particle residues for different cloud types was determined. Obviously, the highest concentration is found due to the presence of droplets in low clouds, but after that, most IPRs in the various ice cloud types were observed in contrails. After normalizing the size distributions by the respective residue number concentration, all IPR size distributions up to a particle size of 300 nm look very similar. However, there are differences above 600 nm (interestingly, no IPR in contrails for this particle diameters), which suggests a different nature of IPR in this size range. Looking at the amount of BC (represented by the absorption coefficient) in the IPR, the highest values were again found in contrails. Indeed, when the absorption coefficient is normalized to the IPR number, the values are still slightly higher than for liquid origin cirrus clouds, but lower than for in-situ cirrus clouds. However, this result refers to the BC mass, i.e., on one hand, the BC number concentration in contrails may be higher due to small, freshly emitted soot particles from aircraft emissions, and on the other hand, aged BC particles from air traffic appear to play a greater role in the formation of especially in situ cirrus clouds.