Characteristics of the fluorescence of water-soluble organic matter in forest, remote, and urban aerosols in Japan

Excitation–emission matrix (EEM) fluorescence spectroscopy is widely used to investigate the properties, composition, and sources of chromophores in atmospheric water-soluble organic matter (WSOM). However, the fluorescence characteristics of WSOM under different environmental conditions and their implications for atmospheric processing remain unclear. In this study, EEM spectroscopy was applied to analyze the WSOM in aerosols obtained from forest, remote, and urban sites in Japan, and the collected spectra were interpreted using parallel factor analysis (PARAFAC) and fluorescence indices. The PARAFAC analysis of the EEM results revealed three components, namely, less-oxygenated humic-like species (LOS), protein-like substances (PRLIS), and highly oxygenated humic-like species (HOS). The relative contributions of PRLIS were greater in forest aerosols, whereas HOS contributed more to urban and remote aerosols than to forest aerosols, with the LOS being relatively enhanced at the forest site. Among the fluorescence indices, the humification index (HIX) and biological index (BIX) effectively differentiated the WSOM from different environments, and the HIX–BIX plot demonstrated potential for aerosol characterization. At the forest site in Wakayama, diurnal and seasonal variations in the fluorescence components were observed, attributable to biogenic secondary organic aerosol formation and biological emissions. At the urban site in Nagoya, a comparison of aerosol samples and rainwater samples showed similar proportions of fluorescence components, with a slight difference in HOS, possibly reflecting different atmospheric processing. Furthermore, seasonal changes in fluorescence imply potential roles for aqueous-phase reactions in the transformation of WSOM. These findings demonstrate that fluorescence spectroscopy coupled with PARAFAC and fluorescence indices can characterize the compositional differences of WSOM across environments.