Accurately determining and reducing the climate impact of aviation and its uncertainties is one of the pressing challenges of our times. Contrail cirrus are estimated to contribute more than half of the total effective radiative forcing from aviation, yet the uncertainties in their optical and radiative properties are large. In contrast to midlatitude cirrus, high-latitude cirrus are less anthropogenically influenced; thus, they are more pristine. However, little is known about Arctic cirrus properties and their role in the amplified warming of this region. The Cirrus in High Latitudes (CIRRUS-HL) mission using the High Altitude and Long Range Research Aircraft (HALO) provides measurements in mid-and high latitudes during summer (June/July) 2021, exploiting HALO’s capabilities and a comprehensive cloud–aerosol–trace gas and radiation instrumentation. The results of 24 HALO flights provide new insights into both natural cirrus and contrail cirrus properties in high (60°–76°N) and midlatitudes (38°–60°N). In particular, we find lower ice water content (−42%) and lower number concentrations (−88%) of cirrus particles with larger mean diameters (+22%) in high latitudes. Ice supersaturated regions were frequently observed in midand high latitudes, with median in-cloud relative humidity over ice between 105% and 122%. Mean aerosol number concentrations in the midlatitudes were reduced by up to 80% compared to pre-COVID-19 times. Less air traffic during the COVID-19 lockdowns, reduced contrail cirrus coverage, and lower ice nucleating particle concentrations in high latitudes help to explain the observed differences in cirrus properties. The extensive dataset will be used to improve weather and climate models.

Despite their proven importance for the atmospheric radiative energy budget, the effect of cirrus on climate and the magnitude of their modification by human activity is not well quantified. Besides anthropogenic pollution sources on the ground, aviation has a large local effect on cirrus microphysical and radiative properties via the formation of contrails and their transition to contrail cirrus. To investigate the anthropogenic influence on natural cirrus, we compare the microphysical properties of cirrus measured at mid-latitude (ML) regions (<60 N) that are often affected by aviation and pollution with cirrus measured in the same season in comparatively pristine high latitudes (HLs; ≥60 N). The number concentration, effective diameter, and ice water content of the observed cirrus are derived from in situ measurements covering ice crystal sizes between 2 and 6400 μm collected during the CIRRUS-HL campaign (Cirrus in High Latitudes) in June and July 2021. We analyse the dependence of cirrus microphysical properties on altitude and latitude and demonstrate that the median ice number concentration is an order of magnitude larger in the measured mid-latitude cirrus, with 0.0086 cm^-3, compared to the high-latitude cirrus, with 0.001 cm^-3. Ice crystals in mid-latitude cirrus are on average smaller than in high-latitude cirrus, with a median effective diameter of 165 μm compared to 210 μm, and the median ice water content in mid-latitude cirrus is higher (0.0033 gm^-3) than in high-latitude cirrus (0.0019 gm^-3). In order to investigate the cirrus properties in relation to the region of formation, we combine the airborne observations with 10 d backward trajectories to identify the location of cirrus formation and the cirrus type, i.e. in situ or liquid origin cirrus, depending on whether there is only ice or also liquid water present in the cirrus history, respectively. The cirrus formed and measured at mid-latitudes (M-M) have a particularly high ice number concentration and low effective diameter. This is very likely a signature of contrails and contrail cirrus, which is often observed in the in situ origin cirrus type. In contrast, the largest effective diameter and lowest number concentration were found in the cirrus formed and measured at high latitudes (H-H) along with the highest relative humidity over ice (RHi). On average, in-cloud RHi was above saturation in all cirrus. While most of the H-H cirrus were of an in situ origin, the cirrus formed at mid-latitudes and measured at high latitudes (M-H) were mainly of liquid origin. A pristine Arctic background atmosphere with relatively low ice nuclei availability and the extended growth of few nucleated ice crystals may explain the observed RHi and size distributions. The M-H cirrus are a mixture of the properties of M-M and H-H cirrus (preserving some of the initial properties acquired at mid-latitudes and transforming under Arctic atmospheric conditions). Our analyses indicate that part of the cirrus found at high latitudes is actually formed at mid-latitudes and therefore affected by mid-latitude air masses, which have a greater anthropogenic influence.