Characteristics of the Fluorescence of Water-Soluble Organic Matter in Atmospheric Aerosols under Different Environments in Japan

Abstract

    The fluorescence characteristics of atmospheric aerosols are related to their chemical characteristics including the oxygenation state; hence they show differences based on their types and sources. Excitation-emission matrix (EEM) fluorescence spectroscopy is becoming an important method for analyzing the chromophores of water-soluble organic aerosols. Although a number of studies have reported the fluorescence characteristics of chromophores in atmospheric aerosols, the relationship betweentheir fluorescence characteristics and types is still unclear. In this study, the fluorescence characteristics of water-soluble components of atmospheric aerosols in different environments in Japan were studied as a means to understand their changes in characteristics and their relationship with aerosol types. The fluorescence was also analyzed for urban rainwater for comparison with aerosol.     Atmospheric aerosol samples collected at urban (Nagoya), forest (Wakayama), remote (Okinawa)sites and rainwater samples collected at the urban site were subjected to the analysis of EEM for water-soluble extracts. The EEM of water-soluble organic matter from forest aerosol samples showed that the relative contribution of protein-like substances (PRLIS) to total analyzed fluorescence was on average higher than that of other samples. For marine aerosol samples, the intensity of fluorescence originated from humic-like substances (HULIS) containing highly oxygenated compounds (HOS) was on average 4.5 times higher than that originated from HULIS containing less oxygenated compounds (LOS), according to our definition of the quantification of the fluorescence intensity. In the case of forest aerosol samples, the difference in the intensity was smaller (3.4 times on average). This result suggests that the studiedremote aerosols were more aged, while forest aerosols were fresher. For forest aerosol samples, the temporal variation of the fluorescence within a day was obtained. The fluorescence index (FI), humidity index (HIX), and biological index (BIX) were also compared. The HIX-BIX plots showed different patterns for forest, remote and urban aerosol samples, which suggests that they had different degrees of aging and/ordifferent source types, and that this method is useful for the analysis of the characteristics of atmospheric organic aerosols. A parallel factor (PARAFAC) analysis was applied to EEMs, and it identified three different components including two different types of HULIS and one PRLIS. In Nagoya,the contribution by HOS was on average largest among three components in the case of both rainwater and aerosol samples, and the proportion of HOS component for aerosol samples was on average slightly higherthan that for rainwater samples. Although the difference may be affected by the presence ofcoexisting inorganic substances, further comparison may provide a clue to understand the relationship between aerosol and rain chromophores.