Molecular compositions and optical properties of dissolved brown carbon in smoke particles illuminated by excitation-emission matrix spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis

Abstract. We investigated the fluorescence and chemical-structural characteristics of dissolved brown carbon (BrC) in smoke particulates emitted from the combustion of biomass and fossil fuels (coal and vehicle exhaust) by excitation-emission matrix (EEM) spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with electrospray ionization (ESI). Six components were resolved by parallel factor analysis (PARAFAC) of the water-soluble and methanol-soluble organic carbon (MSOC) fractions, respectively. These fluorescent components varied among sources. Combined with FT-ICR MS ion groups, we found that the fluorescent components agreed well with the functional groups, particularly with nitrogen (N)- and sulfur (S)-containing groups. Among the six PARAFAC components (P1–6) retrieved from the water-soluble organic carbon (WSOC) fraction, except for the P3 component, the other components exhibited different values among the three types of emission sources tested. Vehicle exhaust was characterized by high P1 and P6 components, which are mainly associated with aromatic organosulfate compounds, and a high P5 component, mainly associated with sulfonates; coal combustion was characterized by a high P4 component, which is associated with nitrooxy-organosulfate (nitrooxy-OS) compounds; and biomass burning was characterized by the P2 component. Similar results were observed in the case of the MSOC fraction. This study reveals the source contribution and possible structures of previously unclear excitation-emission matrix (EEM) fluorescent components in combustion-derived aerosols. These are the first findings of this type and are potentially applicable to further studies on EEM-based source apportionment of dissolved BrC in aerosols.