Abstract
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.
Highlights
Many studies of cirrus and orographic cloud formation have focussed on the homogeneous freezing of supercooled sulphuric acid droplets as the major mechanism (Archuleta et al, 2005; Field et al, 2001; Haag et al, 2003; Heymsfield and Miloshevich, 1993; Heymsfield and Sabin, 1989; Jensen et al, 1998; Marti and Mauersberger, 1993; Sassen and Dodd, 1988; Tabazadeh et al, 1997)
Aerosol residuals were analysed using a number of instruments but for this work we focus on measurements of rBC core mass and associated coatings made by a Single Particle Soot Photometer (SP2; DMT, Boulder, Colorado, USA) (Baumgardner et al, 2004; Schwarz et al, 2006; Stephens et al, 2003)
Artefacts from incomplete evaporation of cloud particles in the pumped counterflow virtual impactor (PCVI) are minimal for the OC5 soot; if the coating thickness was increased by incomplete evaporation it would be expected that there would be a significant shift in the modal value of τd between the pre-expansion and residual particles
Summary
Many studies of cirrus and orographic cloud formation have focussed on the homogeneous freezing of supercooled sulphuric acid droplets as the major mechanism (Archuleta et al, 2005; Field et al, 2001; Haag et al, 2003; Heymsfield and Miloshevich, 1993; Heymsfield and Sabin, 1989; Jensen et al, 1998; Marti and Mauersberger, 1993; Sassen and Dodd, 1988; Tabazadeh et al, 1997). Soot may offer significant concentrations of active IN in the lower troposphere where an increase in ice crystal number concentration may result in more rapid and frequent glaciation of mixed phase clouds by the Bergeron-Findeisen process (Lohmann and Feichter, 2005; Rogers and Yau, 1996; Schwarzenbock et al, 2001). This would likely act to reduce cloud top albedo and increase ice phase precipitation, reducing cloud lifetime. Since the SP2 measures both elastic scatter and incandescence it is possible to determine whether the sample aerosol contains incandescent material, allowing the fraction of the aerosol population containing refractory black carbon to be estimated
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