Abstract
Abstract. Ice nucleation by different types of soot particles is systematically investigated over the temperature range from 218 to 253 K relevant for both mixed-phase (MPCs) and cirrus clouds. Soot types were selected to represent a range of physicochemical properties associated with combustion particles. Their ice nucleation ability was determined as a function of particle size using relative humidity (RH) scans in the Horizontal Ice Nucleation Chamber (HINC). We complement our ice nucleation results by a suite of particle characterization measurements, including determination of particle surface area, fractal dimension, temperature-dependent mass loss (ML), water vapor sorption and inferred porosity measurements. Independent of particle size, all soot types reveal absence of ice nucleation below and at water saturation in the MPC regime (T>235 K). In the cirrus regime (T≤235 K), soot types show different freezing behavior depending on particle size and soot type, but the freezing is closely linked to the soot particle properties. Specifically, our results suggest that if soot aggregates contain mesopores (pore diameters of 2–50 nm) and have sufficiently low water–soot contact angles, they show ice nucleation activity and can contribute to ice formation in the cirrus regime at RH well below homogeneous freezing of solution droplets. We attribute the observed ice nucleation to a pore condensation and freezing (PCF) mechanism. Nevertheless, soot particles without cavities of the right size and/or too-high contact angles nucleate ice only at or well above the RH required for homogeneous freezing conditions of solution droplets. Thus, our results imply that soot particles able to nucleate ice via PCF could impact the microphysical properties of ice clouds.
Highlights
Soot, mainly composed of highly agglomerated carbon spherules, is a by-product of incomplete combustion of biomass and fossil fuels and represents a major anthropogenic pollutant
The soot types investigated cover a wide range of physicochemical properties as proxies of atmospheric soot particles, including different commercially available black carbons such as gas blacks (FW200), lamp blacks (LB_RC and LB_OEC) and fullerene soots (FS), as well as propane flame soot with different OM content derived from a miniCAST burner
Ice nucleation activity was investigated in relation to particle morphology deduced from transmission electron microscopy (TEM) and coupled DMA–CPMA measurements, temperature-dependent mass loss obtained from thermogravimetric analyzer (TGA) analysis and water vapor uptake capacity derived from dynamic vapor sorption (DVS) experiments
Summary
Mainly composed of highly agglomerated carbon spherules, is a by-product of incomplete combustion of biomass and fossil fuels and represents a major anthropogenic pollutant. Emissions of soot are estimated to reach 7500 G g y−1 (uncertainty range: 2000– 29 000 G g y−1, Bond et al, 2013), with a direct source in the upper troposphere from aviation emissions, and are of high relevance for climate (Ramanathan and Carmichael, 2008). Soot aggregates usually have diameters on the nanoscale, ranging from individual primary carbonaceous spherules to large, fractal-like aggregates (Adachi et al, 2007). The primary particle diameter itself can vary from around 10 nm to several tens of nanometers, depending on the combustion source. The diverse physicochemical properties of soot aerosols make an analytical assessment of their environmental effects challenging. Soot particles can act as ice nucleating particles (INPs) in cirrus
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