Nanoscale Chemical Imaging of Soot and Brown Carbon Components in Individual Atmospheric Particles

Biomass-burning aerosols contain complex mixtures of soot carbon (SC), brown carbon (BrC), organic carbon (OC), and inorganic (IN) constituents whose spatial distributions within individual particles strongly influence aerosol optical properties, atmospheric chemistry, and climate impacts. Scanning transmission X-ray microscopy (STXM) coupled with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) is widely used to characterize the composition and mixing state of individual particles; however, existing approaches cannot explicitly distinguish SC from BrC because both exhibit strong aromatic π*C═C absorption at 285.4 eV. Here, a novel X-ray spectromicroscopic method at the carbon K-edge is introduced to resolve SC and BrC at the nanoscale within individual particles. The approach exploits differences between the aromatic π*C═C transition at 285.4 eV and the soot-specific σ*Graphitic-like(GL) transition at 307.0 eV. Analysis of STXM/NEXAFS spectra of biomass-burning particles, laboratory-generated BrC proxies, tar balls, and soot reference particles demonstrates that the relative intensities of these transitions provide a robust basis for differentiating SC and BrC. An integrated peak ratio and a simplified four-energy Soot Index (SI) were developed to classify particle components and generate spatially resolved maps of SC, BrC, OC, and IN at the nanoscale. The SI reproduces classifications obtained from full spectral deconvolution while enabling rapid, pixel-resolved chemical imaging. Application of this method reveals substantial variability in the internal distributions of SC and BrC within biomass-burning particles and demonstrates that conventional STXM mapping approach frequently classifies BrC-rich regions as elemental carbon. The new analytical framework expands the capabilities of X-ray spectromicroscopy for atmospheric aerosol characterization and provides improved particle-resolved constraints on aerosol composition, mixing state, and morphology.