Implications of the ISOCLOUD Campaigns at the AIDA Cloud Chamber for Ice Growth in Cold Cirrus

In-situ water vapor measurements in the UTLS have routinely observed anomalous supersaturations on the order of 10-20% in the presence of ice particles when temperatures were below 200 K, raising questions about the basic physics of how clouds form at cold temperatures.^1,2,3,4 We report on experiments from the ISOCLOUD campaigns in 2012-2013 at the AIDA Aerosol and Cloud Chamber that sought to investigate this phenomena by simulating cold clouds at temperatures and pressures characteristic of the upper troposphere. Experiments tested both homogeneous nucleation of sulfate aerosols and heterogeneous nucleation with various ice nuclei, including mineral dust and organic aerosols with and without nitric acid coatings. Optical instruments, both in-situ (TDLAS) and extractive (TDLAS and OFCEAS), measured ice particle number density, water vapor, total water, and water isotopic concentrations, with multiple instruments measuring water. In a series of cirrus formation experiments, we observed no evidence of anomalous saturation vapor pressure and no evidence of inhibition of ice growth at low temperatures for the parameter space tested during the ISOCLOUD campaigns. These experiments included ice growth deposition coefficient retrievals at lower temperatures and pressures than previously measured in AIDA, producing lower error bars on the deposition coefficient at cold temperatures.⁵ These results do not support a microphysical explanation for the observed anomalous supersaturation in the UTLS.

[1] Gao, R. et al., Science 303, no. 6567, 516-520 (2004).

[2] Jensen, E. et al., Atmos. Chem. Phys. 5, 851-862 (2005).

[3] Peter, T. et al., Science 314, no. 5804, 1399-1402 (2006).

[4] Krämer, M. et al., Atmos. Chem. Phys. 9, 3505-3522 (2009).

[5] Skrotzki, J. et al., Atmos. Chem. Phys. 12, 24351-24393 (2013).