Ice nucleating ability of black carbon in cirrus regime: effects of morphology, mobility size, mixing state, SOA coating and atmospheric aging

Black carbon (BC), as potential ice nucleating particles (INPs) in troposphere, can affect cirrus formation and hence global climate. Yet, the IN ability of BC, internally-mixed BC, and BC-seeded secondary organic aerosol (SOA) remain highly uncertain.

We have systematically examined the IN ability of (1) size-selected BC with different morphology; (2) BC internally mixed with inorganic salts; (3) BC-seeded SOA from aircraft exhaust (toluene and n-dodecane) and biogenic (β-caryophyllene) volatile organic compound in contrail-cirrus regime (-46 to -38°C). Several aerosolized BC proxies were selected to examine particle morphology at different atmospheric aging stages. Co-atomization of BC and salt solution generated internally-mixed BC particles. SOA coating was generated by a potential aerosol mass (PAM) oxidation flow reactor. IN activity was determined with the Spectrometer for Ice Nuclei (SPIN). Particle number concentration and chemical composition were monitored online by a Condensation Particle Counter (CPC) and Particle Analysis by Laser Mass Spectrometry (PALMS), respectively. SOA composition was also monitored by an Aerosol Mass Spectrometer (AMS).

The preliminary results suggest that: (1) the more fractal BC particles don’t necessarily act as superior deposition INP over more spherical ones. The onset of some deposition nucleation, as opposed to purely homogeneous freezing, occurs for some BC types between 100-200 nm; (2) mixing with inorganic salts may partially form hygroscopic sites or salt layers on BC particles. Such particles may deliquesce and form a liquid film, leading to homogeneous rather than heterogeneous ice freezing; (3) n-dodecane and toluene derived SOA nucleate ice through homogeneous freezing only, while β-caryophyllene derived SOA can form ice through deposition ice nucleation. When the relative humidity approaches water saturation, the hygroscopic β-caryophyllene derived SOA may deliquesce and form liquid shells. Such difference in ice nucleation properties may be due to the difference of the chemical composition in SOA coating, highlighting the importance of understanding the aging process of atmospheric BC particles in order to better predict their ice nucleation behaviour and contribution to cirrus cloud formation.