Investigating the molecular weight distribution of atmospheric water-soluble brown carbon using high-performance size exclusion chromatography coupled with diode array and fluorescence detectors

Atmospheric brown carbon (BrC) contain amounts of organic species, but their molecular weight (MW) distributions is still poorly understood. This study applied high-performance size exclusion chromatography (HPSEC) coupled with a diode array detector (DAD) and fluorescence detector (FLD) to characterize the MW distributions of typical chromophores and fluorophores within water-soluble BrC. The investigation focused on the spring season, encompassing both typical urban and rural aerosols. Our results showed that chromophores (at 254 and 365 nm), and humic-like and protein-like fluorophores identified by excitation-emission matrix parallel factor analysis (EEM-PARAFAC) within BrC were broadly distributed along the MW continuum (∼50–20,000 Da). This suggests that BrC mainly comprises complex chromophores and fluorophores with heterogeneous molecular sizes. High-MW (HMW, >1 kDa) species (66%–74%) dominated the chromophores at 254 and 365 nm. However, the latter chromophores were enriched with more HMW species. This result suggested that the HMW chromophores might contribute more to BrC absorption at longer wavelengths. The PARAFAC-derived fluorescent components also exhibited different MW distributions. Three humic-like substances (HULIS) were all dominated by HMW fractions (51%–74%), but protein-like fluorescent component (PLOM) enriched low-MW (LMW, <1 kDa) species (60%–66%). Furthermore, the molecular size (i.e., weight-averaged and number-averaged MW) and the ratios between HMW and LMW species decreased in the order highly-oxygenated HULIS > less-oxygenated HULIS > PLOM, indicating that the fluorophores with longer Em were generally related to larger MW. To our knowledge, this is the first report on the molecular size of individual fluorescent components within aerosol BrC. The results obtained here enhanced our knowledge of heterogeneous composition, complex physicochemical properties, and potential atmospheric fates of aerosol BrC. © 2023