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.

This paper reports on the successful first deployment of a new, autonomously operating measurement system on a Grob G 520 Egrett aircraft, which was used as a chase aircraft to perform in-flight aerosol and trace gas measurements of engine exhaust from other aircraft. A suite of custom-built and commercially available instruments was selected, modified, and adapted to operate in the unpressurized compartment of the Egrett over a wide range of ambient temperatures and pressures. We performed these first in-flight emission measurements at cruise altitudes between 7.6 and 10.4 km (FL250 and FL340) behind a Piper Cheyenne, a twin-turboprop aircraft powered by Garrett/Honeywell TPE 331-14 engines over Texas in April 2022. The instrumentation and inlets on the Egrett were designed to measure non-volatile particulate matter (nvPMDp>10), total particulate matter (tPMDp>10), nitrogen oxides (NO and NO2), water vapor (H2O), carbon dioxide (CO2), and contrail ice particles. All instruments were operated in relevant plume conditions at cruise altitudes and distances ranging from 100 to 1200 m between the two aircraft. The instruments proved to have high reliability, a large dynamic range, and sufficient accuracy for measuring the emissions of the turboprop engine. We derived the emission indices (EI) for tPM, nvPM, and NOx at cruise. The particulate emission indices range from 9.6 to 16.2 × 1014 kg-1 (particles per kg fuel burned) for EItPM and from 8.1 to 12.4 × 1014 kg-1 for EInvPM (medians). For NOx we find rather low EINOx between 7.3 and 7.7 g kg-1 for EINOx (medians). Furthermore, the tPM aerosol size distributions have been measured in the exhaust plume, taking into account the size-resolved sampling efficiency of the instrument. The analysis of the size-resolved emission index indicates a log-normal distribution with geometric mean and standard deviation at Dg=27.5±2.0 nm. This geometric diameter value is in the range of jet engine soot emissions previously measured in flight. The measurements help to constrain the climate impact of small-class turboprop engines and need to be compared to larger turboprop aircraft in the future. The current work provides a benchmark for future alternative H2 propulsion systems, such as fuel cells and direct combustion engines.

The HALO-(AC)³ aircraft campaign was carried out in March and April 2022 over the Norwegian and Greenland seas, the Fram Strait, and the central Arctic Ocean. Three research aircraft - the High Altitude and Long Range Research Aircraft (HALO), Polar 5, and Polar 6 - performed 54 partly coordinated research flights on 23 flight days over areas of open ocean, the marginal sea ice zone (MIZ), and the central Arctic sea ice. The general objective of the research flights was to quantify the evolution of air mass properties during moist and warm-air intrusions (WAIs) and cold-air outbreaks (CAOs). To obtain a comprehensive data set, the three aircraft operated following different strategies. HALO was equipped with active and passive remote sensing instruments and dropsondes to cover the regional evolution of cloud and thermodynamic processes. Polar 5 carried a similar remote sensing payload to HALO, and Polar 6 was instrumented with in situ cloud, aerosol, and trace gas instruments focusing on the initial air mass transformation close to the MIZ. The processed, calibrated, and validated data are published in the World Data Center PANGAEA as instrument-separated data subsets and listed in aircraft-separated collections for HALO (, https://doi.org/10.1594/PANGAEA.968885), Polar 5 (, https://doi.org/10.1594/PANGAEA.968883), and Polar 6 (, https://doi.org/10.1594/PANGAEA.968884). A detailed overview of the available data sets is provided here. Furthermore, the campaign-specific instrument setup, the data processing, and quality are summarized. Based on measurements conducted during a specific CAO, it is shown that the scientific analysis of the HALO-(AC)³ data benefits from the coordinated operation of the three aircraft.

Cirrus clouds play a crucial role in the Earth’s energy budget. They reflect incoming sunlight and absorb and re-emit terrestrial infrarred radiation. The magnitude of these components depends on the cirrus micro physical properties (e.g., ice crystal number and effective diameter), which can result in either net warming or cooling effects. This thesis investigates the differences in those properties of high- and mid-latitude cirrus, as well as their interactions with atmospheric aerosol and aviation-induced contrail cirrus. To explore these differences, cirrus clouds were measured with the German research aircraft HALO (High Altitude and LOng Range) during the CIRRUS-HL (CIRRUS at High Latitudes) campaign in June and July 2021. A total of 24 flights and 35 hours of in situ measurements of cirrus particle data were collected with the Cloud Droplet Probe (CDP), the Cloud Imaging Probe Grayscale (CIPG) and the Precipitation Imaging Probe (PIP). A comprehensive intercomparison among the instruments, along with a detailed uncertainty analysis, was performed, resulting in an accurate and quality-controlled data set. The results of these measurements show that high-latitude cirrus, compared to mid latitude cirrus, have lower median ice number concentration (0.001 and 0.0086 cm−3), higher median effective diameter (210 and 165 μm), and lower median extinction coefficient (0.042 and 0.072 m−1, respectively). In addition, high relative humidity over ice is observed, particularly at high latitudes, with median values around 125%. The influence of the formation region on cirrus properties was assessed by combining measurements with weather model data and backward trajectories starting at the flight tracks. The results show that a large part of the high-latitude cirrus were formed at mid-latitudes, leading to different properties compared to cirrus formed directly at high latitudes. Simulations from an aerosol-chemistry-climate model were combined with the backward trajectories and a strong contribution of heterogeneous nucleation was identified in the measured cirrus. Thus, the low concentrations of ice nucleating particles at high latitudes (from the model) combined with high ice supersaturation levels might explain the lower ice number concentration and larger effective diameter of cirrus measurements at high latitudes compared to mid-latitudes. Aviation emissions have a large local impact on the cirrus microphysical properties through contrail formation and their evolution to contrail cirrus. The CIRRUS-HL data set shows a higher occurrence of contrail cirrus at mid-latitudes, and a potential impact on natural cloudiness by reducing supersaturation levels at cirrus altitudes. The effect of contrails from future propulsion technologies may depend on background aerosol concentrations. Comparisons of measurements and model data for total aerosol number concentrations show good agreement for the larger particle size mode (> 250nm), but likely an underestimation above 300 hPa in the Aitken mode (> 12nm). By combining observations with model data, this study contributes to enhancing the understanding of the variability in cirrus properties due to different formation mechanisms and aerosol influences, as well as the interaction of natural and contrail cirrus.

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.