Morphology and Mixing State of Atmospheric Black Carbon Particles Collected in Central California during CARES

Black carbon aerosol absorption depends on the mixing state and morphology of black carbon inclusions and other non-absorbing species. Here, microscopic methods are applied to particles collected in the California central valley during the CARES 2010 field campaign. Selected samples were from a period of high photochemical activity and pollution build-up. Particle mixing state and morphology were characterized using Scanning Transmission X-ray Microscopy (STXM) at the carbon K-edge. Observations of compacted black carbon core morphologies and thick organic coatings from both urban and rural sites demonstrate that the majority of the particles were highly aged. Based on the observation of thick coatings and more convex black carbon inclusion morphology, the contribution of fresh black carbon emissions at the urban site was relatively small. These measurements of black carbon morphology and mixing state provide important constraints for the morphological effects of black carbon on aerosol optical properties expected in aged urban plumes.