Abstract
The immersion freezing ability of soot particles has in previous studies been reported in the range of low/insignificant to very high. The aims of this study were to: (i) perform detailed physico-chemical characterisation of freshly produced soot particles with very different properties, (ii) investigate the immersion freezing ability of the same particles, and (iii) investigate the potential links between physico-chemical particle properties and ice-activity. A miniCAST soot generator was used to produce eight different soot samples representing a wide range of physico-chemical properties. A continuous flow diffusion chamber was used to study each sample online in immersion mode over the temperature (T) range from −41 to −32 °C, at a supersaturation of about 10% with respect to liquid water. All samples exhibited low to no heterogeneous immersion freezing. The most active sample reached ice-activated fractions (AF) of 10−3 and 10−4 at temperatures of 1.7 and 1.9 K , respectively, above the homogeneous freezing temperature. The samples were characterized online with respect to a wide range of physico-chemical properties including effective particle density, optical properties, particle surface oxidation and soot maturity. We did observe indications of increasing immersion freezing ice-activity with increasing effective particle density and increasing particulate PAH fraction. Hence, those properties, or other properties co-varying with those, could potentially enhance the immersion freezing ice-activity of the studied soot particle types. However, we found no significant correlation between the physico-chemical properties and the observed ice-nucleating ability when the particle ensemble was extended to include previously published results including more ice-active biomass combustion soot particles. We conclude that it does not appear possible in general and in any straightforward way to link observed soot particle physico-chemical properties to the ice-nucleating ability using the online instrumentation included in this study. Furthermore, our observations support that freshly produced soot particles with a wide range of physico-chemical properties have low to insignificant immersion freezing ice-nucleating ability.
Highlights
Soot particles are known to influence climate significantly through direct interaction with radiation [1], and indirectly via acting as cloud condensation nuclei (CCN) [2]
Mobility size-selected soot samples generated with a miniCAST burner at eight different settings representing a large matrix of soot maturity and particle properties were studied
These samples were characterized with respect to their effective density, optical properties, soot surface oxidation, polycyclic aromatic hydrocarbons (PAHs) levels and fragmentation related to particle nanostructure
Summary
Soot particles are known to influence climate significantly through direct interaction with radiation [1], and indirectly via acting as cloud condensation nuclei (CCN) [2]. It is highly uncertain to which extent soot particles may affect cloud properties and climate indirectly through facilitating heterogeneous freezing of cloud droplets. Yun et al [3] estimated that the radiative forcing due to heterogeneous ice nucleation caused by anthropogenic soot particles in mixed-phase clouds could potentially be as high as ∼1 W/m2. A significant reason for the pronounced uncertainties in such modelling studies is the high uncertainty of the ice-nucleating ability of ambient soot particles. Soot is formed through the incomplete combustion of hydrocarbons, for example biomass and fossil fuels, and properties such as soot maturity and primary particle diameter depend on fuel composition and combustion conditions. Major sources of soot particles include combustion of fossil fuels in transportation, open burning of biomass and combustion of solid fuels for industrial and residential use [9]. There have been several studies performed in recent years examining the immersion freezing ability of soot particles [10,11,12,13,14,15,16]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
