Molecular Insights into Forest Aerosols: Diacids, Oxoacids, and Fatty Acids over a Year in Hokkaido

To better understand the seasonal changes, sources, and transformation pathways of water-soluble organics in a boreal forest, aerosol samples were collected from the deciduous forest in Hokkaido, Japan during January to December 2010. Here, we present the seasonal distributions of diacids (C₂–C₁₀), oxoacids (C₂–C₉), α-dicarbonyls, and fatty acids (FAs) in the forest aerosols. Oxalic acid is the most abundant diacid species followed by malonic and succinic acids, whereas glyoxylic acid (ωC₂) is the dominant ω-oxoacid. The maximum concentrations of short chain diacids (C₂–C₄) in the summer explain an enhanced emission of biogenic volatile organic compounds (VOCs). In contrast, higher concentrations of phthalic acid (Ph), ωC₂, pyruvic acid (Pyr), glyoxal (Gly), and methylglyoxal (MeGly) in the winter suggest enhanced anthropogenic emission of polynuclear aromatic hydrocarbons followed by photochemical degradation in the cold season. Azelaic acid (C₉), a tracer for the oxidation of biogenic unsaturated fatty acids, showed higher concentrations during the summer. Concentration ratios of Ph/C₉ peaked in February and gradually decreased until May, showing lower concentrations until October. Higher concentrations of glutaric (C₅), adipic (C₆), and FAs (except for C_14:0) in autumn suggest the emission from microbial activities in deciduous forests. Water-soluble organic carbon (WSOC) strongly correlated with aerosol liquid water content (LWC) (r = 0.74) in summer, suggesting WSOC is directly involved in the formation of cloud condensation nuclei (CCN) activation. A strong correlation between C₅ and C₆ in autumn suggests a common source, likely VOC emissions, and subsequent degradation. Their correlation with LWC indicates the role of heterogeneous aqueous-phase reactions, suggesting that C₅ and C₆ may be generated from cycloalkenes through such processes.