Molecular characterization of aerosol-derived water soluble organic carbon using ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry
Despite the acknowledged relevance of aerosol water-soluble organic carbon (WSOC) to climate and biogeochemical cycling, characterization of aerosol WSOC has been limited. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) is utilized in the present study to provide detailed molecular-level characterization of the high molecular weight (HMW; m/z>223) component of aerosol-derived WSOC collected from rural sites in Virginia and New York, USA. More than 3000 organic compounds were detected by ESI FT-ICR MS within a m/z range of 223-600 for each sample. Approximately 86% (Virginia) and 78% (New York) of these peaks were assigned molecular formulas using only carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S) as elemental constituents. H/C and O/C molar ratios were plotted on van Krevelen diagrams and indicated a strong contribution of lignin-like and lipid-like compounds to the aerosol-derived WSOC samples. Double bond equivalents were calculated from the molecular formulas and used to identify black carbon (BC) compounds present in aerosol WSOC. BC compounds were found to comprise only 1-4% of the identified compounds in the aerosol-derived WSOC. Several high magnitude peaks in the mass spectra of both samples corresponded to molecular formulas consistent with molecular formulas proposed in previous secondary organic aerosol (SOA) laboratory investigations indicating that SOAs are important constituents of the WSOC. Overall, ESI FT-ICR MS provides the level of molecular characterization needed for detailed compositional and source information of the high molecular weight constituents of aerosol-derived WSOC.