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Laboratory study of the heterogeneous ice nucleation on black carbon containing aerosol

This paper is available in a repository.
This paper is available in a repository.

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Black carbon (BC) particles are generated in the incomplete combustion of fossil fuels, biomass, and biofuels. These airborne particles affect air quality, human health, and climate. At present, the climate effects of BC particles are not well understood. Their role in cloud formation is obscured by their chemical and physical variability, and by the internal mixing states of these particles with other compounds. The current study focuses on laboratory measurements of the effectiveness of BC-containing aerosol in the formation of ice crystals in cirrus clouds. Ice nucleation in field studies is often difficult to interpret. Nonetheless, most field studies seem to suggest that BC particles are not efficient ice nuclei (IN). On the other hand, laboratory measurements show that in some cases, BC particles can be highly active IN. By working with well-characterized BC-containing particles, our aim is to systematically establish the factors that govern the IN activity of BC. We examine ice nucleation on BC-containing particles under cirrus cloud conditions, commonly understood to be deposition mode ice nucleation. We study a series of well-characterized commercial carbon black particles with varying morphologies and surface chemistries, as well as ethylene flame-generated combustion soot. The carbon black particles used in this study are proxies for atmospherically relevant BC aerosols. These samples were characterized by electron microscopy, mass spectrometry, and optical scattering measurements. Ice nucleation activity was systematically examined in the temperature range from 217–235 K, using a SPectrometer for Ice Nuclei (SPIN) instrument, which is a continuous flow diffusion chamber coupled with instrumentation to measure light scattering and polarization. To study the effect of coatings on IN, the BC-containing particles were coated with organic acids found in the atmosphere, namely, stearic acid, cis-pinonic acid, and oxalic acid. The results show significant variations in ice nucleation activity as a function of size, morphology and surface chemistry of the BC-containing particles. The measured IN activity dependence on temperature and the physicochemical properties of the BC-containing particles are consistent with an ice nucleation mechanism of pore condensation followed by freezing. Coatings and surface oxidation modify the initial ice nucleation ability of BC-containing aerosol. Depending on the BC material and the coating, both inhibition and enhancement in IN activity were observed. Our measurements at low temperatures complement published data, and highlight the capability of some BC particles to nucleate ice under low supersaturation conditions. These results are expected to help refine theories relating to soot IN activation in the atmosphere.

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