The Impact of Aging on the Ice-Nucleating Ability of Soot Particles

Out of all atmospheric aerosol particles, only a very minor fraction can act as ice nucleating particles (INP). These INPs impact cloud microphysical properties, life cycles and ultimately climate. Freshly emitted soot particles are generally found to be poor INPs, due to their limited wettability. However, particle hydrophilicity can be increased during atmospheric aging (Zhang et al. 2008). In general, water uptake on soot aggregates is largely determined through an interplay of particle morphology and chemistry, in particular surface functional groups, and thus depends on the combustion conditions (source). Through affecting these physicochemical properties, atmospheric aging can significantly influence the potential of soot particles to act as INP (Yun et al. 2013). In this study, we investigated the ice nucleation ability of fresh and aged soot particles, combined with particle chemical characterization using spectroscopic techniques and dynamic vapor sorption (DVS) measurements.

Different propane flame soots with high and low organic carbon (OC) content, respectively, were generated from a miniCAST burner. Aging of soot particles was achieved in water and aqueous solutions of sulfuric acid, in order to address the acidic environment commonly found in cloud droplets and haze droplets in polluted urban environments. The ice nucleation activity of size selected fresh and aged soot particles was investigated by means of the horizontal ice nucleation chamber (Lacher et al. 2017; Mahrt et al. 2018) at cirrus temperatures. Scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) was used to map chemical functional groups and particle morphology on a single particle level (Moffet et al. 2010). These measurements are complemented by quantification of particle hydrophilicity based on water sorption isotherms.

The ice nucleation activity of aged soot was observed to be significantly enhanced with respect to non-aged soot. On distinct finding of our work is the similarity in ice nucleation activity after aging, independent of aging and soot particle type. Aging extended the range over which soot particles can nucleate ice to lower ice supersaturations at a given temperature. Specifically, our results reveal that ice nucleation activities of aged soot particles are comparable to that of mineral dust. For soot with higher OC fraction aging even caused a change in ice nucleation mechanism. These results are corroborated by our DVS results, revealing an increase in particle hydrophilicity upon aging. Moreover, we find the soot water interaction to differ between the soot with low and high OC fraction, respectively. Particle hydrophilicity is connected with our NEXAFS analysis, revealing distinct spectral features for the individual soot types. For example, fresh, low OC soot had ketone functionalities indicated by an absorption peak at 287 eV which was mostly absent for aged soot. Aged soot was generally more oxidized and had a greater absorption peak signature at 285.4 eV attributable to carbon double bonding.

Overall, we find soot ice nucleation to be closely related to particle physicochemical properties, which are strongly affected by aging in aqueous solutions. Such aging effects should be considered when assessing the ice nucleation activity of combustion particles.

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Mahrt, F., et al. (2018), 'Ice nucleation abilities of soot particles determined with the Horizontal Ice Nucleation Chamber', Atmospheric Chemistry and Physics, 18 (18), 13363-92.

Moffet, R. C., Tivanski, A. V., and Gilles, M. K. (2010), 'Scanning Transmission X-ray Microscopy: Applications in Atmospheric Aerosol Research', in Ruth Signorell and J. P. Reid (eds.), Fundamentals and Applications in Aerosol Spectroscopy (Taylor and Francis Books Inc. ), 419-62.

Yun, Y., Penner, J. E., and Popovicheva, O. (2013), 'The effects of hygroscopicity on ice nucleation of fossil fuel combustion aerosols in mixed-phase clouds', Atmos. Chem. Phys., 13 (8), 4339-48.

Zhang, Renyi, et al. (2008), 'Variability in morphology, hygroscopicity, and optical properties of soot aerosols during atmospheric processing', Proceedings of the National Academy of Sciences of the United States of America, 105 (30), 10291-96.