Collaborative Research – BC5 laboratory studies of the optical properties and ice nuclei activity of carbonaceous particles as a function of mixing state and phase state

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Description

We held our fifth Boston College – Aerodyne Black Carbon (BC5) study in May, 2018. This laboratory intensive focused on the wavelength-dependent optical properties of absorbing aerosols, specifically black carbon (BC) containing particles, including combustion soot. We conducted an optical closure study of nascent, thinly coated, and thickly coated BC particles to tease out the varying effects of intra-particle (i.e., morphology) and inter-particle (i.e., population) mixing states on observed absorption enhancement effects. We also investigated the effects of highly absorbing, non-refractory particles on laser absorption-based Single Particle Soot Photometer (SP2) and Soot Particle Aerosol Mass Spectrometry (SP-AMS) measurements. This project was done in collaboration with Professors Chris Cappa from UC-Davis and Geoffrey Smith from University of Georgia, and Dr. Arthur Sedlacek and Ernie Lewis from Brookhaven National Laboratory. In addition, we are combining our method of measuring secondary organic aerosol (SOA) glass transitions with measurements of ice nuclei (IN), in collaboration with Professor Daniel Cziczo from MIT. Our laboratory results indicate that IN activity is enhanced for glassy SOA. Thus, the phase state of SOA particles from organic aerosols including biogenic sources may influence ice (e.g., cirrus) and mixed phase cloud formation. Given that a significant fraction of global particulate matter is composed of SOA material, which may become glassy and exhibit efficient ice nucleation in the free troposphere, this class of aerosol may contribute significantly to the global IN budget. In a related study, we are collaborating with Prof. Cziczo to systematically explore the effectiveness of BC-containing aerosol in the formation of ice crystals in cirrus clouds. We have tested the IN activity, in the cold temperature deposition mode regime, of various types of black carbon particles before and after they undergo oxidation and organic coating to better understand how the surface properties and morphologies of BC particles govern IN activity. Our measurements at low temperatures 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.