Molecular Signatures and Sources of Fluorescent Components in Atmospheric Organic Matter in South China

Excitation emission matrix (EEM) spectra coupled with parallel factor analysis (PARAFAC) have been used to characterize brown carbon (BrC). The molecular composition of PARAFAC components is not well understood in atmospheric science, which has impeded the accurate interpretation of the chemical changes and source apportionment of BrC using EEM-PARAFAC methods. We assigned numerous formulas identified by Fourier transform ion cyclotron resonance mass spectrometry (both negative and positive electrospray ionization modes) to each PARAFAC-derived fluorescent component (FC) in atmospheric PM2.5 samples. Obvious differences in molecular characteristics and sources were observed between atmospheric and hydrospheric FCs, indicating the need for caution in explaining the composition and source of atmospheric FC based on hydrospheric FC. Previously assigned protein-like components (C4) and less-oxidized humic-like FC (C1) were associated mainly with highly saturated photoresistant species and less-polar anthropogenic emissions. Highly oxidized humic-like FCs were associated with aromatic and highly unsaturated/phenolic compounds (high oxygen), which were potentially influenced by biomass burning and related secondary processes. The seasonal variations in light absorption were similar to those of fluorescence intensity and the degree of humification, which were influenced by highly unsaturated/phenolic compounds (high oxygen). These linkages indicated the potential of EEM-PARAFAC for investigating the changes of light absorption, molecular composition, and source of BrC.

Automated summary

Excitation emission matrix (EEM) spectra coupled with parallel factor analysis (PARAFAC) have been used to characterize brown carbon (BrC). The molecular composition of PARAFAC components is not well understood in atmospheric science, which has impeded the accurate interpretation of the chemical changes and source apportionment of BrC using EEM-PARAFAC methods. Through Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), numerous formulas were assigned to each PARAFAC-derived fluorescent component (FC) in atmospheric PM2.5 samples, revealing differences in molecular characteristics and sources between atmospheric and hydrospheric FCs.