Chemical Structural Characteristics of HULIS and Other Fractionated Organic Matter in Urban Aerosols: Results from Mass Spectral and FT-IR Analysis

The chemical characteristics of complex organic matter in atmospheric aerosols remain poorly understood. Water-insoluble organic matter (WISOM) and water-soluble organic matter (WSOM) in the total suspended particulates collected in the city of Nagoya in summer/early autumn and winter were extracted using multiple solvents. Two fractions of humic-like substances, showing neutral and acidic behavior (HULIS-n and HULIS-a, respectively), and the remaining highly polar part (HP-WSOM) were fractionated from WSOM using solid phase extraction. The chemical structural characteristics and concentrations of the organic matter were investigated using mass spectrometry and Fourier transform infrared (FT-IR) spectroscopy. WISOM and HULIS-n had low O/C ratios (0.1 and 0.4, respectively) and accounted for a large fraction of the organics in aerosols (70%). HULIS-a and HP-WSOM had higher O/C ratios (0.7 and 1.0, respectively), and their concentrations in summer and early autumn were on average ∼2 times higher than those in winter. The mass spectrum and FT-IR analyses suggest the following: (1) WISOM were high-molecular-weight aliphatics (primarily C₂₇–C₃₂) with small proportions of −CH₃, −OH, and CO groups; (2) HULIS-n was abundant in aliphatic structures and hydroxyl groups (primarily C₉–C₁₈) and by branched structures; (3) HULIS-a and HP-WSOM contained relatively large amounts of low-molecular-weight carboxylic acids and alcohols (primarily C₄–C₁₀); and (4) WISOM and HULIS-n were relatively abundant in amines and organic nitrates