Peter Oelsner

Accurate and sustained monitoring of water vapor (H₂O) in the upper troposphere-lower stratosphere (UTLS) is essential for quantifying its role in the atmospheric radiative balance, chemistry, and climate variability. However, in situ measurements in this region remain challenging due to the extremely low H₂O concentrations, harsh environmental conditions, and strict payload limitations on meteorological balloon platforms. ALBATROSS is a compact (3.6 kg), balloon-borne mid-infrared laser absorption spectrometer developed to provide highly accurate, SI-traceable H₂O measurements with high vertical resolution in the UTLS. Following recent comprehensive laboratory characterization, we present the first in-flight validation of ALBATROSS, based on seven balloon flights conducted between 2022 and 2026 from the meteorological observatories of Payerne, Switzerland (six flights) and Lindenberg, Germany (one flight). All flights included simultaneous measurements by a cryogenic frostpoint hygrometer (CFH) and a meteorological radiosonde (Vaisala RS41), with CFH operating either in a tandem-flight configuration or on the same payload as ALBATROSS. In the troposphere (up to 12 km altitude), the ALBATROSS retrieval agrees with both reference instruments within a mean relative difference of –2 ± 7 % with respect to RS41 and –1 ± 8 % with respect to CFH over nearly three orders of magnitude in H₂O mixing ratio. Fine‑scale vertical structures and cirrus cloud layers are consistently resolved. In the stratosphere, following the implementation of an in-flight purging system to eliminate contamination from internal moisture sources, ALBATROSS achieves an agreement of 3 ± 10 % relative to CFH between 12 and 28 km altitude. An important advantage of laser absorption spectroscopy is demonstrated through observations of balloon-induced H₂O spikes detected simultaneously by ALBATROSS and CFH in the stratosphere. These measurements reveal fast dynamic response of the laser spectrometer of ~1 s, corresponding to an effective vertical resolution of 5 m, compared to 3–4 s (15–20 m) for frostpoint hygrometry. Overall, these results demonstrate the robust performance of ALBATROSS under real atmospheric conditions and establish mid-infrared laser absorption spectroscopy as a reference-quality technique for balloon‑borne observations and long‑term monitoring of UTLS H₂O.

Accurate and sustained monitoring of water vapor (H₂O) in the upper troposphere-lower stratosphere (UTLS) is essential for quantifying its role in the atmospheric radiative balance, chemistry, and climate variability. However, in situ measurements in this region remain challenging due to the extremely low H₂O concentrations, harsh environmental conditions, and strict payload limitations on meteorological balloon platforms. ALBATROSS is a compact (3.6 kg), balloon-borne mid-infrared laser absorption spectrometer developed to provide highly accurate, SI-traceable H₂O measurements with high vertical resolution in the UTLS. Following recent comprehensive laboratory characterization, we present the first in-flight validation of ALBATROSS, based on seven balloon flights conducted between 2022 and 2026 from the meteorological observatories of Payerne, Switzerland (six flights) and Lindenberg, Germany (one flight). All flights included simultaneous measurements by a cryogenic frostpoint hygrometer (CFH) and a meteorological radiosonde (Vaisala RS41), with CFH operating either in a tandem-flight configuration or on the same payload as ALBATROSS. In the troposphere (up to 12 km altitude), the ALBATROSS retrieval agrees with both reference instruments within a mean relative difference of –2 ± 7 % with respect to RS41 and –1 ± 8 % with respect to CFH over nearly three orders of magnitude in H₂O mixing ratio. Fine‑scale vertical structures and cirrus cloud layers are consistently resolved. In the stratosphere, following the implementation of an in-flight purging system to eliminate contamination from internal moisture sources, ALBATROSS achieves an agreement of 3 ± 10 % relative to CFH between 12 and 28 km altitude. An important advantage of laser absorption spectroscopy is demonstrated through observations of balloon-induced H₂O spikes detected simultaneously by ALBATROSS and CFH in the stratosphere. These measurements reveal fast dynamic response of the laser spectrometer of ~1 s, corresponding to an effective vertical resolution of 5 m, compared to 3–4 s (15–20 m) for frostpoint hygrometry. Overall, these results demonstrate the robust performance of ALBATROSS under real atmospheric conditions and establish mid-infrared laser absorption spectroscopy as a reference-quality technique for balloon‑borne observations and long‑term monitoring of UTLS H₂O.

Accurate and sustained monitoring of water vapor (H2O) in the upper troposphere-lower stratosphere (UTLS) is essential for quantifying its role in the atmospheric radiative balance, chemistry, and climate variability. However, in situ measurements in this region remain challenging due to the extremely low H2O concentrations, harsh environmental conditions, and strict payload limitations on meteorological balloon platforms. ALBATROSS is a compact (3.6 kg), balloon-borne mid-infrared laser absorption spectrometer developed to provide highly accurate, SI-traceable H2O measurements with high vertical resolution in the UTLS. Following recent comprehensive laboratory characterization, we present the first in-flight validation of ALBATROSS, based on seven balloon flights conducted between 2022 and 2026 from the meteorological observatories of Payerne, Switzerland (six flights) and Lindenberg, Germany (one flight). All flights included simultaneous measurements by a cryogenic frostpoint hygrometer (CFH) and a meteorological radiosonde (Vaisala RS41), with CFH operating either in a tandem-flight configuration or on the same payload as ALBATROSS. In the troposphere (up to 12 km altitude), the ALBATROSS retrieval agrees with both reference instruments within a mean relative difference of –2 ± 7 % with respect to RS41 and –1 ± 8 % with respect to CFH over nearly three orders of magnitude in H2O mixing ratio. Fine‑scale vertical structures and cirrus cloud layers are consistently resolved. In the stratosphere, following the implementation of an in-flight purging system to eliminate contamination from internal moisture sources, ALBATROSS achieves an agreement of 3 ± 10 % relative to CFH between 12 and 28 km altitude. An important advantage of laser absorption spectroscopy is demonstrated through observations of balloon-induced H2O spikes detected simultaneously by ALBATROSS and CFH in the stratosphere. These measurements reveal fast dynamic response of the laser spectrometer of ~1 s, corresponding to an effective vertical resolution of 5 m, compared to 3–4 s (15–20 m) for frostpoint hygrometry. Overall, these results demonstrate the robust performance of ALBATROSS under real atmospheric conditions and establish mid-infrared laser absorption spectroscopy as a reference-quality technique for balloon‑borne observations and long‑term monitoring of UTLS H2O.

Routine ground‐based measurements of total ozone column (TOC), as well as ozone profile soundings started in the late 1960s in Germany. The resulting ozone and temperature records at Hohenpeissenberg and Berlin/Potsdam/Lindenberg show long‐term changes similar to other stations in Central Europe, and to the changes seen globally. Following the increase of ozone depleting substances (ODS), stratospheric ozone has declined from the 1960s until the 1990s. Since about 2000, ozone has leveled or slightly increased, consistent with declining amounts of ODS. The stratosphere has been cooling and the troposphere has been warming, in agreement with general expectations due to increasing greenhouse gas concentrations. The clearest signs of recovering ozone are seen around 40 km altitude. Two factors contribute to this increase: the decrease of stratospheric chlorine loading and cooling of the upper stratosphere, which slows gas‐phase ozone destruction cycles, and enhances the ter‐molecular reaction producing ozone. Tropospheric ozone has increased substantially from the 1960s to the early 1990s. Since then, it has remained more or less constant, on a level higher compared to the 1960 and 1970s. Particularly low tropospheric ozone was observed in 2020, due to reduced precursor emissions during the COVID‐19 related lockdowns. The atmospheric concentrations of greenhouse gases will likely continue to rise, while the concentrations of ozone depleting substances are expected to slowly decline. To see how the atmosphere responds, and to help understand future changes, continued monitoring will be required for many years to come, both over Germany and worldwide.

Routine ground-based measurements of total ozone column (TOC), as well as ozone profile soundings started in the late 1960s in Germany. The resulting ozone and temperature records at Hohenpeissenberg and Berlin / Potsdam / Lindenberg show long-term changes similar to other stations in Central Europe, and to the changes seen globally. Following the increase of ozone depleting substances (ODS), stratospheric ozone has declined from the 1960s until the 1990s. Since about 2000, ozone has leveled or slightly increased, consistent with declining amounts of ODS. The stratosphere has been cooling and the troposphere has been warming, in agreement with general expectations due to increasing greenhouse gas concentrations. The clearest signs of recovering ozone are seen around 40 km altitude. Two factors contribute to this increase: the decrease of stratospheric chlorine loading and cooling of the upper stratosphere, which slows gas-phase ozone destruction cycles, and enhances the ter-molecular reaction producing ozone. Tropospheric ozone has increased substantially from the 1960s to the early 1990s. Since then, it has remained more or less constant, on a level higher compared to the 1960s and 1970s. Particularly low tropospheric ozone was observed in 2020, due to reduced precursor emissions during the COVID-19 related lockdowns. The atmospheric concentrations of greenhouse gases will likely continue to rise, while the concentrations of ozone depleting substances are expected to slowly decline. To see how the atmosphere responds, and to help understand future changes, continued monitoring will be required for many years to come, both over Germany and worldwide.

Throughout spring and summer 2020, ozone stations in the northern extratropics recorded unusually low ozone in the free troposphere. From April to August, and from 1 to 8 kilometers altitude, ozone was on average 7% (≈4 nmol/mol) below the 2000–2020 climatological mean. Such low ozone, over several months, and at so many stations, has not been observed in any previous year since at least 2000. Atmospheric composition analyses from the Copernicus Atmosphere Monitoring Service and simulations from the NASA GMI model indicate that the large 2020 springtime ozone depletion in the Arctic stratosphere contributed less than one‐quarter of the observed tropospheric anomaly. The observed anomaly is consistent with recent chemistry‐climate model simulations, which assume emissions reductions similar to those caused by the COVID‐19 crisis. COVID‐19 related emissions reductions appear to be the major cause for the observed reduced free tropospheric ozone in 2020.

Throughout spring and summer 2020, ozone stations in the northern extratropics recorded unusually low ozone in the free troposphere. From April to August, and from 1 to 8 kilometers altitude, ozone was on average 7% (~4 ppbv) below the 2000 to 2020 climatological mean. Such low ozone, over several months, and at so many stations, has not been observed in any previous year since at least 2000. Atmospheric composition re-analyses from the Copernicus Atmosphere Monitoring Service and simulations from the NASA GMI model indicate that the large 2020 springtime ozone depletion in the Arctic stratosphere has contributed less than one quarter to the observed tropospheric anomaly. The observed anomaly is consistent with two recent model simulations, which assume emission reductions similar to those caused by the COVID-19 crisis. COVID-19 related emission reductions appear to be the major cause for the observed low free tropospheric ozone in 2020.

Balloon-borne water vapour measurements in the (sub)tropical upper troposphere and lower stratosphere (UTLS) by means of frost point hygrometers provide important information on air chemistry and climate. However, the risk of contamination from sublimating hydrometeors collected by the intake tube may render these measurements difficult, particularly after crossing low clouds containing supercooled droplets. A large set of measurements during the 2016–2017 StratoClim balloon campaigns at the southern slopes of the Himalayas allows us to perform an in-depth analysis of this type of contamination. We investigate the efficiency of wall-contact and freezing of supercooled droplets in the intake tube and the subsequent sublimation in the UTLS using Computational Fluid Dynamics (CFD). We find that the airflow can enter the intake tube with impingement angles up to 60°, owing to the pendulum motion of the payload. Supercooled droplets with radii > 70 μm, as they frequently occur in mid-tropospheric clouds, typically undergo contact freezing when entering the intake tube, whereas only about 50 % of droplets with 10 μm radius freeze, and droplets < 5 μm radius mostly avoid contact. According to CFD, sublimation of water from an icy intake can account for the occasionally observed high water vapour mixing ratios (χ(H₂O) > 100 ppmv) in the stratosphere. Furthermore, we use CFD to differentiate between stratospheric water vapour contamination by an icy intake tube and contamination caused by outgassing from the balloon and payload, revealing that the latter starts playing a role only at high altitudes (p < 20 hPa).