Abstract
Abstract. Soot and black carbon (BC) particles are generated in the incomplete combustion of fossil fuels, biomass, and biofuels. These airborne particles affect air quality, human health, aerosol–cloud interactions, precipitation formation, 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. Ice nucleation in field studies is often difficult to interpret. Nonetheless, most field studies seem to suggest that BC particles are not efficient ice-nucleating particles (INPs). On the other hand, laboratory measurements show that in some cases, BC particles can be highly active INPs under certain conditions. By working with well-characterized BC particles, our aim is to systematically establish the factors that govern the ice nucleation activity of BC. The current study focuses on laboratory measurements of the effectiveness of BC-containing aerosol in the formation of ice crystals in temperature and ice supersaturation conditions relevant to cirrus clouds. We examine ice nucleation on BC particles under water-subsaturated cirrus cloud conditions, commonly understood as 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 temperature and saturation conditions in the ranges of 217≤T≤235 K and 1.0≤Sice≤1.5 and 0.59≤Swater≤0.98, respectively, 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 INPs, 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 particles. The measured ice nucleation activity dependencies on temperature, supersaturation conditions, and the physicochemical properties of the BC particles are consistent with an ice nucleation mechanism of pore condensation followed by freezing. Coatings and surface oxidation modify the initial formation efficiency of pristine ice crystals on BC-containing aerosol. Depending on the BC material and the coating, both inhibition and enhancement in INP activity were observed. Our measurements at low temperatures complement published data and highlight the capability of some BC particles to nucleate ice under low ice supersaturation conditions. These results are expected to help refine theories relating to soot INP activation in the atmosphere.
Highlights
Ice-nucleating particle (INP) types in the atmosphere vary widely across the globe
This figure shows a plot of the water vapor supersaturation ratio over ice versus the temperature at which (1 %) ice onset occurs. This time-dependent approach is based on kinetics in the ∼ 10 s period of particles passing through the chamber and the set supersaturation conditions
Our findings show that large agglomerates, such as those observed in wildfires (Chakrabarty et al, 2014), could be a potential source of efficient heterogeneous INPs via the pore condensation and freezing (PCF) mechanism in cirrus cloud conditions in the troposphere
Summary
Ice-nucleating particle (INP) types in the atmosphere vary widely across the globe. Their number concentrations are typically low, their atmospheric impact, governing ice cloud formation and properties, is significant. Bond et al (2013) modeled the contribution of soot to clouds and climate and distinguished between the homogeneous and heterogeneous freezing mechanisms. They showed that in the case of ice clouds when homogeneous nucleation dominates, coverage of high clouds is reduced and cooling prevails, while when heterogeneous nucleation of BC prevails, more high clouds are formed that in turn contribute indirectly to the warming effect. The increasing number concentration of emitted soot particles since the preindustrial times (Bond et al, 2013; Lavanchy et al, 1999), the emissions of soot particles in the upper troposphere from aviation (Lee et al, 2009), and estimates that the concentration of soot will remain high in the near future (Gasser et al, 2017) underscore the importance of understanding the efficiency of soot particles acting as INPs
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