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.

Automated summary

This study investigated the MW-dependent characteristics of WSOM and found that different MW fractions of WSOM exhibited distinct chemical properties and compositions, with variations in the distribution ratios of organic carbon and light-absorbing chromophores. The findings provide valuable insights into the complexity and heterogeneity of WSOM, enhancing the understanding of its chemical properties and structures.