Size separation method for absorption characterization in brown carbon: Application to an aged biomass burning sample

The majority of brown carbon (BrC) in atmospheric aerosols is derived from biomass burning (BB) and is primarily composed of extremely low volatility organic carbons. We use two chromatographic methods to compare the contribution of large and small light‐absorbing BrC components in aged BB aerosols with UV‐vis absorbance detection: (1) size exclusion chromatography (SEC) and (2) reverse phase high‐performance liquid chromatography. We observe no evidence of small molecule absorbers. Most BrC absorption arises from large molecular weight components (>1000 amu). This suggests that although small molecules may contribute to BrC absorption near the BB source, analyses of aerosol extracts should use methods selective to large molecular weight compounds because these species may be responsible for long‐term BrC absorption. Further characterization with electrospray ionization mass spectrometry (MS) coupled to SEC demonstrates an underestimation of the molecular size determined through MS as compared to SEC. Key Points Brown carbon in aged biomass burning aerosols is dominated by large molecules Size exclusion chromatography demonstrates absorbing molecules > 1000 amu Electrospray mass spectrometry does not accurately characterize brown carbon