Brown Carbon Optical Variability in U.S. Urban Aerosols Revealed by an Enhanced Spectral/Mass Balance Approach

Light absorption by brown carbon (BrC) represents a major uncertainty in assessing the climatic effects of carbonaceous aerosols. Using 38,622 PM2.5 samples collected from the U.S. Chemical Speciation Network (2016–2018) and analyzed by a multiwavelength thermal/optical analyzer (TOA), we applied an enhanced spectral/mass balance receptor model to quantify black carbon (BC), BrC, and nonabsorbing white carbon (WtC) while allowing BrC optical properties to vary across samples. The model achieved excellent fits (r2 > 0.98) and revealed a wide range of BrC absorption Ångström exponent (AAE405–635 nm = 2.13 ± 0.74) and mass absorption efficiency (MAE532 nm = 2.03 ± 0.35 m2 g–1). An inverse AAE–MAE relationship was found, with strongly to moderately absorbing BrC being the most prevalent BrC classes. Seasonal patterns showed higher “organic brownness” (i.e., higher BrC mass fraction in organic carbon regardless of BrC class) but lower MAE in winter and the opposite in summer, reflecting the bleaching evolution of BrC with photochemical aging. BrC abundance also influenced the reconciliation between BC- and TOA-derived elemental carbon, likely through altered thermal–optical carbon analysis splits. This study provides the first nationwide characterization of BrC optical variability from national network data, establishing a scalable framework toward long-term monitoring of organic aerosol absorption within existing regulatory programs.