Technical note: Identification of chemical composition and source of fluorescent components in atmospheric water-soluble brown carbon by excitation-emission matrix with parallel factor analysis: Potential limitation and application
Tao Cao, Meiju Li, Cuncun Xu, Jianzhong Song, Xingjun Fan, Jun Li, Wanglu Jia, Ping’an Peng
Abstract
Abstract. Three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy is an important method for identification of occurrences, chemical composition, and sources of atmospheric chromophores. However, current knowledge on identification and interpretation of fluorescent components is mainly based on aquatic dissolved organic matter and might not be applicable to atmospheric samples. Therefore, this study comprehensively investigated EEM data of different types of strong light-absorbing organic compounds, water-soluble organic matter (WSOM) in different aerosol samples (combustion source samples and ambient aerosols), soil dust, and purified fulvic and humic acids by an EEM-parallel factor method. The results demonstrated that organic compounds with high aromaticity and strong electron-donating groups generally present strong fluorescence spectra at longer emission wavelength, whereas organic compounds substituted with electron-withdrawing groups have relatively weaker fluorescence intensity. In particular, aromatic compounds containing nitro groups (i.e., nitrophenols), which show strong absorption and are the major component of atmospheric brown carbon, exhibited no significant fluorescence. Although fluorescent component 1 (235, 270/330 nm) in ambient WSOM is generally considered as protein-like groups, our findings suggested that it is mainly composed of aromatic acids, phenolic compounds, and their derivatives, with only traces of amino acids. Principal component analysis and Pearson correlation coefficients between mass absorption efficiency at 365 nm (MAE365) and humification index (HIX), C1, C2, and C3 indicated that the highly aromatic and oxidized fluorescent component 3 may be an important contributor to the light-absorption capacity of ambient WSOM. These findings provide new insights for the analysis of chemical properties and sources of atmospheric fluorophores using the EEM method.