Aging Induced Changes in Hygroscopicity and CCN Activity of Black Carbon Particles from Biomass Burning

OGOCHUKWU ENEKWIZU, Arthur J. Sedlacek, Ernie R. Lewis, Brookhaven National Laboratory

     Abstract Number: 277
     Working Group: Aerosols, Clouds and Climate

Abstract
Biomass burning (BB) is a major source of atmospheric black carbon (BC), which affects climate both directly through absorption of solar radiation and indirectly by changing cloud properties such as cloud amount and lifetime. BB events also concurrently release an abundance of condensable organic vapors that form very thick coatings that encapsulate the BC. These coatings increase the hydrophilicity of the otherwise hydrophobic BC particles while at the same time increase the particle size, both of which enhance the ability of these particles to act as cloud condensation nuclei (CCN). Due to the complex interplay between hydrophilic properties of the coating and the hydrophobic properties of the BC “core”, along with the particle size, better quantification of the hygroscopic properties for this important class of particle needs to be achieved if we are to improve our understanding of the direct and indirect climatic forcing contributions by these particles. To this end, we are conducting laboratory measurements of the CCN activity of pure and thickly coated BC surrogate particles under typical atmospheric supersaturations using levoglucosan, a known tracer for BB, as a proxy for fresh organic coating and oleic acid as a surrogate for aged organic coating, as a function of coating thickness for different core (i.e., BC) diameters, and we compare our experimental results with theoretical calculations derived from κ-Kohler theory. The limiting cases of fresh and aged organic coatings, which span the range of coating properties, will be used to investigate the activation behavior of an ensemble BC particle population and how this, in turn, affects its optical properties. Our findings will a provide critical fundamental-level understanding of aerosol-cloud interactions for this important class of particle.