Molecular Insights into the Black Carbon Aggregation Process and Phase-mixing State in Aerosol Droplets

XIAOHAN LI, Ian Bourg, Princeton University

     Abstract Number: 471
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
Black carbon (BC) particles have been considered the second largest anthropogenic contributor to global warming after CO2, but remain one of the largest uncertainties in current global radiative forcing estimates, partly because of the poor quantification of BC phase-mixing state (i.e., coating structure) in aerosol particles. There are three factors that govern the phase-mixing state of BC particles: (a) their oxidation degree, which is the indicator of the particle’s hydrophilicity and promotes the particle distribution from the surface to the center of the droplet; (b) their surface charge, which mainly originates from the deprotonation of carboxylic groups at the edge and promotes electrostatic repulsion of BC-BC and BC-interface; (c) their relative size compared with aerosol droplets, which reflects the intensity of BC’s distortion to the hydrogen bonding structure in aerosol droplets and thus the possibility of forming partly-encapsulated coating structure. However, the collective impacts of those three factors on the aggregation behavior and morphology of BC particles are poorly understood due to limitations in both experimental characterization and thermodynamic modeling. Here, we present molecular dynamics (MD) simulation results to characterize the mechanisms governing BC phase-mixing states. Specifically, we characterized BC-BC interaction in the bulk liquid phase, BC interaction with the water-air interface, and the geometric restrain of the droplet shape to the BC distribution in the droplet. Meanwhile, we constructed the configuration phase diagram of the BC coating structure as a function of BC O/C ratio, surface charge, and droplet size. Finally, we developed a BC-ensemble model, taking our configuration phase diagram as the input, to predict the light absorption properties of BC aerosols, which shows nice consistency with previous field measurements.