1Centre for Atmospheric Science, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, M13 9PL, Manchester, UK
2Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, \newline Postfach 3640, 76021 Karlsruhe, Germany
*now at: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA
Abstract. The ice nucleation efficiency of propane flame soot particles with and without a sulphuric acid coating was investigated using the aerosol and cloud chamber facility AIDA (Aerosol Interaction and Dynamics in the Atmosphere). The test soot for cloud formation simulations was produced using a propane flame Combustion Aerosol Standard generator (CAST, Jing-CAST Technologies). The organic carbon content (OC) of the test soot was altered in a reproducible fashion by changing the fuel/air mixture of the generator. The soot content of ice nuclei was subsequently investigated using a combination of a pumped counterflow virtual impactor (PCVI) to separate and evaporate the ice crystals, and a DMT single particle soot photometer (SP2) to examine the mixing state of the BC containing ice residuals.
Ice nucleation was found to be most efficient for uncoated soot of low organic carbon content (~5% organic carbon content) where deposition freezing occurred at an ice saturation ratio Sice~1.22 at a temperature T = 226.6 K with 25% of the test soot becoming active as ice nuclei. Propane flame soot of higher organic carbon content (~30% and ~70% organic carbon content) showed significantly lower ice nucleation efficiency (an activated fraction of the order of a few percent in the experiments) than the low organic carbon content soot, with water saturation being required for freezing to occur. Ice nucleation occurred over the range Sice = 1.22–1.70, and T = 223.2–226.6 K. Analysis of the SP2 data showed that the 5% organic carbon content soot had an undetectable OC coating whereas the 30% organic carbon content soot had a thicker or less volatile OC coating.
The application of a sulphuric acid coating to the flame soot shifted the threshold of the onset of freezing towards that of the homogeneous freezing of sulphuric acid; for the minimum OC flame soot this inhibited nucleation since the onset of freezing occurred at colder temperatures and required a greater ice saturation ratio; for the medium and maximum OC flame soot, the addition of a sulphuric acid significantly reduced the freezing threshold.