21 Apr 2021

21 Apr 2021

Review status: this preprint is currently under review for the journal ACP.

High Homogeneous Freezing Onsets of Sulfuric Acid Aerosol at Cirrus Temperatures

Julia Schneider1, Kristina Höhler1, Robert Wagner1, Harald Saathoff1, Martin Schnaiter1, Tobias Schorr1, Isabelle Steinke2, Stefan Benz3, Manuel Baumgartner4,a, Christian Rolf5, Martina Krämer5,4, Thomas Leisner1, and Ottmar Möhler1 Julia Schneider et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 2Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, USA
  • 3Landesanstalt für Umwelt Baden-Württemberg, Karlsruhe, Germany
  • 4Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany
  • 5Institute for Energy and Climate Research, IEK-7: Stratosphere, Research Center Jülich, Jülich, Germany
  • anow at: Deutscher Wetterdienst, Offenbach, Germany

Abstract. Homogeneous freezing of aqueous solution aerosol particles is an important process for cloud ice formation in theupper 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, temperature and size of the aqueous aerosol particles, which makes this approach well applicable in numerical models. In this study, we investigate the homogeneous freezing behaviour of aqueous sulfuric acid aerosol particles by means of a comprehensive collection of laboratory homogeneous freezing experiments conducted at the AIDA (Aerosol Interaction and Dynamicsin 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 were generated at high purity 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 1000 cm−3 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 K 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 K 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 our experimental results, we suggest a new fit line as a parameterization for the onset conditions of homogeneous freezing of sulfuric acid aerosol particles. As a next step, we propose the new parameterization to be implemented in atmospheric models as an improved version of the WAC-based parameterization from Koop et al. (2000). The new homogeneous freezing thresholds may have significant impacts on the prediction of cirrus cloud occurrence and related cloud radiative effects.

Julia Schneider et al.

Status: open (until 16 Jun 2021)

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Julia Schneider et al.

Data sets

Datasets to: High Homogeneous Freezing Onsets of Sulfuric Acid Aerosol at Cirrus Temperatures Schneider, Julia; Höhler, Kristina; Wagner, Robert; Saathoff, Harald; Schnaiter, Martin; Schorr, Tobias; Steinke, Isabelle; Benz, Stefan; Baumgartner, Manuel; Rolf, Christian; Krämer, Martina; Leisner, Thomas; Möhler, Ottmar

Julia Schneider et al.


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Short summary
Homogeneous freezing is a relevant mechanism for the formation of cirrus clouds in the upper troposphere. Based on an extensive set of homogeneous freezing experiments at the AIDA cloud chamber with aqueous sulfuric acid aerosol, we provide a new empirical parameterization for homogeneous freezing onset conditions. The implementation of the new line in atmospheric models may have important implications on the cirrus cloud occurrence and related cloud radiative effects.