A New Fit Line to Homogeneous Freezing Onsets of Aqueous H2SO4 Aerosol Particles at Cirrus Conditions

Abstract:

Cirrus and mixed phase clouds strongly influence the weather and climate on our planet. Due to their microphysical and optical properties, they contribute to the global energy budget of the Earth’s atmosphere and impact the hydrological cycle. In the lower troposphere, the formation of the ice phase is only triggered by solid aerosol particles, whereas in the colder upper troposphere and lower stratosphere, homogeneous nucleation of supercooled liquid aerosol particles is also of importance. Therefore, the knowledge about homogeneous freezing processes are essential to understand and formulate cirrus cloud processes.

During several laboratory measurement campaigns in the past years, the homogeneous freezing of sulphuric acid solution droplets has been investigated in the cloud and aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) at the Karlsruhe Institute of Technology. Inside the AIDA chamber, conditions of atmospheric cirrus with temperatures down to 190 K and high ice supersaturations can be realized. The instrumentation at the AIDA provides continuous records of thermodynamic quantities and aerosol properties and allows for a sensitive detection of ice crystal formation and growth. We summarize a comprehensive collection of AIDA experiments with aqueous sulphuric acid aerosols to review homogeneous freezing processes at cirrus formation conditions simulated in the cloud chamber. At temperatures below about 210 K, we observe homogeneous nucleation to occur at higher supersaturations than predicted by the water-activity-based parameterization suggested by Koop et al., 2000. The systematic discrepancy increases with decreasing temperature, and is observed in the more recent data sets of AIDA homogeneous freezing experiments as well as in the older measurements. Based on these observations, we review the description of homogeneous ice nucleation in the cirrus temperature regime and discuss a new fit line to the onset of homogeneous freezing to be applied in atmospheric models.

Koop, T., Luo, B. P., Tsias, A., and Peter, T.: Water activity as the determinant for homogeneous ice nucleation in aqueous solutions, Nature, 406, 611–614, 2000.