Optical Properties of Laboratory-Synthesized Dark Brown Carbon Tar Balls

SHU-WEN YOU, Prabhav Upadhyay, Zezhen Cheng, Guodong Ren, Taveen Kapoor, Joseph V. Puthussery, Benjamin Sumlin, Swarup China, Rohan Mishra, Rajan K. Chakrabarty, Washington University in St. Louis

     Abstract Number: 256
     Working Group: Carbonaceous Aerosols

Abstract
Tar balls (TBs) are a subset of brown carbon (BrC) aerosol generated from combustion sources and contribute to approximately 30% of atmospheric biomass burning (BB) aerosol mass. Understanding the interplay between TB’s formation mechanisms, physicochemical properties, and optical properties is crucial to predicting their climatic effects. Dark BrC (d-BrC) TBs, which were reported during FIREX-AQ, have properties similar to black carbon, i.e., they strongly absorb light, are insoluble in organic solvents, and are resistant to photobleaching. However, few studies have systematically investigated the fundamental properties of d-BrC TBs, partially due to their unclear formation mechanisms. In this study, we generated d-BrC TBs using tar-containing condensate from the dry distillation of apple wood. The condensate was filtered, diluted with pure methanol, aerosolized, and then passed through a thermodenuder (for a heat shock environment similar to that in wildfires) at temperatures of 250, 300, 350, and 400 °C to generate TBs with a range of optical properties. D-BrC TBs were generated at higher heat shock temperatures, with the complex refractive indexes obtained from scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS). The imaginary refractive index ranged from 0.24 to 0.32 at 550 nm, similar to those observed in the FIREX-AQ field campaign. The aerosol ensemble single scattering albedo of d-BrC TBs, as reported by custom-built integrated photoacoustic nephelometers (IPNs), ranged from 0.37 to 0.42 at 375 nm and from 0.48 to 0.50 at 721 nm. In addition, the d-BrC TBs were collected for further analyses, including morphology, volatility distribution, and functional group characterization with scanning transmission X-ray microscopy (STXM). We report the fundamental properties of laboratory-synthesized d-BrC TBs, which not only advance our understanding of their potential formation mechanisms but also our capability to produce d-BrC TBs for future studies.