Chemical Characterization and Single-Particle Measurements of Soot Aerosol in Houston: Insights into Sources, Processes, and Black Carbon Mixing States

RYAN FARLEY, James E. Lee, Laura-Helena Rivellini, Alex K.Y. Lee, Rachael Dal Porto, Kyle Gorkowski, Abu Sayeed Md Shawon, Katherine Benedict, Allison Aiken, Manvendra Dubey, Christopher Cappa, Qi Zhang, University of California, Davis

     Abstract Number: 539
     Working Group: Aerosols, Clouds and Climate

Abstract
The effects of soot aerosol on the Earth's climate system are highly influenced by the mixing state of black carbon aerosol and its coating materials. To shed light on these relationships, we employed a high-resolution soot particle aerosol mass spectrometer (SP-AMS) to measure the size-resolved chemical composition and the single particle mixing state of submicron soot aerosol in Houston, TX during the TRACER campaign in the summer of 2022. The SP-AMS was operated to selectively measure only refractory black carbon (rBC) and associated coating material. The average rBC concentration was 62 ± 116 ng m-3 and the BC coatings primarily consisted of organics (63%) and sulfate (26%). Positive matrix factorization analysis (ensemble particle measurements) and K-means clustering (single particle measurements) were used to understand the sources and atmospheric processing pathways of the rBC containing aerosol. Soot aerosol during TRACER had diverse sources including an oxidized organic aerosol (OOA) factor related to processed primary organic aerosol, an inorganic sulfate factor, an oxidized rBC factor and a mixed mineral dust/biomass burning factor with significant contribution from K+. Additionally, the single particle clustering allows us to further differentiate soot aerosol populations mixed with OA of various degrees of oxidation. The single particle measurements highlight the large variation in BC coating thickness with coating mass to black carbon mass ratios ranging from 0.1 to 100 for individual particles. The average mixing state index (χ) was 47 ± 12%, indicating that BC cannot be described as either fully internally or externally mixed. Higher χ values were measured during the day than at night indicating the importance of photochemical processing or regional transport of atmospherically processed aerosol. Our study provides valuable insights into the mixing state of BC aerosol and its impact on the atmosphere.