Molecular weight-dependent abundance, absorption, and fluorescence characteristics of water-soluble organic matter in atmospheric aerosols

Water-soluble organic matter (WSOM) plays a significant role in solar radiative forcing and atmospheric chemistry. Its chemical composition and properties are highly molecular weight (MW)-dependent, but are still poorly understood. In this study, 10 PM2.5 samples were collected at a rural site in China, and the WSOM therein was fractionated into three MW fractions, >5 kDa (F1), 1-5 kDa (F2), and <1 kDa (F3), using an ultrafiltration (UF) system. The abundance, optical properties, and fluorescence properties of the three MW fractions were comprehensively determined using a total organic carbon (TOC) analyzer, ultraviolet-visible (UV-vis) spectroscopy, and excitation-emission matrix (EEM) fluorescence spectroscopy combined with parallel factor (PARAFAC) modeling. The proportional TOC and Abs(365) distributions of the MW fractions were F1:F2:F3 approximate to 24:22:54 and 40:24:36, respectively. This suggested that the major OC species in bulk WSOM were partitioned into the low MW (<1 kDa) fraction, while the major light-absorbing chromophores were partitioned into the large MW (F1 + F2, >1 kDa) fraction, and were especially prevalent in the >5 kDa fraction. The optical parameters, i.e., SUVA(254) and MAE(365), generally increased with the apparent increase in MW, suggesting that the higher MW fractions might have a stronger aromaticity and light absorption capacity. The EEM-PARAFAC analysis identified three humic-like substances (HULIS) (C1-C3) and one protein-like (C4) substance for all MW fractions, of which the proportional distributions were observed as C1:C2:C3:C4 approximate to 20:24:41:15, 21:23:44:12, and 41:22:13:24 for F1, F2, and F3, respectively. This indicated that the >1 kDa fraction was enriched in highly-oxygenated HULIS (C3), whereas the <1 kDa fraction had more less-oxygenated HULIS (C1) and protein-like (C4) substances than the >1 kDa fraction. The study provided a detailed insight of the MWdependent characteristics of WSOM by reducing their heterogeneity and complexity, which is of great significance for further understanding the chemical properties and structures of WSOM.