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@ARTICLE{Schneider:901822,
      author       = {Schneider, Julia and Höhler, Kristina and Wagner, Robert
                      and Saathoff, Harald and Schnaiter, Martin and Schorr,
                      Tobias and Steinke, Isabelle and Benz, Stefan and
                      Baumgartner, Manuel and Rolf, Christian and Krämer, Martina
                      and Leisner, Thomas and Möhler, Ottmar},
      title        = {{H}igh homogeneous freezing onsets of sulfuric acid aerosol
                      at cirrus temperatures},
      journal      = {Atmospheric chemistry and physics},
      volume       = {21},
      number       = {18},
      issn         = {1680-7324},
      address      = {Katlenburg-Lindau},
      publisher    = {EGU},
      reportid     = {FZJ-2021-03845},
      pages        = {14403 - 14425},
      year         = {2021},
      abstract     = {Homogeneous freezing of aqueous solution aerosol particles
                      is an important process for cloud ice formation in the upper
                      troposphere. There the air temperature is low, the ice
                      supersaturation can be high and the concentration of
                      ice-nucleating particles is too low to initiate and dominate
                      cirrus cloud formation by heterogeneous ice nucleation
                      processes. The most common description to quantify
                      homogeneous freezing processes is based on the water
                      activity criterion (WAC) as proposed by Koop et al. (2000).
                      The WAC describes the homogeneous nucleation rate
                      coefficients only as a function of the water activity, which
                      makes this approach well applicable in numerical models. In
                      this study, we investigate the homogeneous freezing behavior
                      of aqueous sulfuric acid aerosol particles by means of a
                      comprehensive collection of laboratory-based homogeneous
                      freezing experiments conducted at the AIDA (Aerosol
                      Interaction and Dynamics in the Atmosphere) cloud simulation
                      chamber, which were conducted as part of 17 measurement
                      campaigns since 2007. The most recent experiments were
                      conducted during October 2020 with special emphasis on
                      temperatures below 200 K. Aqueous sulfuric acid aerosol
                      particles of high purity were generated by particle
                      nucleation in a gas flow composed of clean synthetic air and
                      sulfuric acid vapor, which was added to the AIDA chamber.
                      The resulting chamber aerosol had number concentrations from
                      30 cm−3 up to several thousand per cubic centimeter with
                      particle diameters ranging from about 30 nm to 1.1 µm.
                      Homogeneous freezing of the aerosol particles was measured
                      at simulated cirrus formation conditions in a wide range of
                      temperatures between 185 and 230 K with a steady increase
                      of relative humidity during each experiment. At temperatures
                      between about 205 K and about 230 K, the AIDA results
                      agree well with the WAC-based predictions of homogeneous
                      freezing onsets. At lower temperatures, however, the AIDA
                      results show an increasing deviation from the WAC-based
                      predictions towards higher freezing onsets. For temperatures
                      between 185 and 205 K, the WAC-based ice saturation ratios
                      for homogeneous freezing onsets increase from about 1.6 to
                      1.7, whereas the AIDA measurements show an increase from
                      about 1.7 to 2.0 in the same temperature range. Based on the
                      experimental results of our direct measurements, we suggest
                      a new fit line to formulate the onset conditions of
                      homogeneous freezing of sulfuric acid aerosol particles as
                      an isoline for nucleation rate coefficients between 5×108
                      and 1013 cm−3 s−1. The potential significant impacts
                      of the higher homogeneous freezing thresholds, as directly
                      observed in the AIDA experiments under simulated cirrus
                      formation conditions, on the model prediction of cirrus
                      cloud occurrence and related cloud radiative effects are
                      discussed.},
      cin          = {IEK-7},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IEK-7-20101013},
      pnm          = {2112 - Climate Feedbacks (POF4-211) / DFG project 392369854
                      - Erforschung tropischer Zirruswolken durch die Kombination
                      von Labor- und Feldexperimenten mit Prozess- und
                      Zirkulationsmodellen},
      pid          = {G:(DE-HGF)POF4-2112 / G:(GEPRIS)392369854},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000703046400001},
      doi          = {10.5194/acp-21-14403-2021},
      url          = {https://juser.fz-juelich.de/record/901822},
}