Molecular Distributions and Sources of Water-Soluble Monocarboxylic Acids (C1–C10) in the Marine Aerosols Collected from the Western Pacific and Southern Ocean

Low molecular weight (LMW) monocarboxylic acids (monoacids) are hygroscopic in nature and thus have an impact on cloud activation ability and the global climate system. This study provides new findings on spatial distributions and chemical compositions of monoacids in marine aerosols from the Western Pacific (WP) and Southern Ocean (SO). In addition, we disclose the formation mechanism of monoacids in the WP and SO. Marine aerosol samples (n = 27) were collected over the oceans and analyzed for LMW monoacids (C₁–C₁₀). Normal and branched chains (iC₄, iC₅, and iC₆) and aromatic acids (benzoic and toluic) were detected over the WP and SO. Their averaged molecular distributions over the SO showed the predominance of acetic acid (C₂) (5.2 ng m^–3) followed by formic (C₁) (4.1 ng m^–3), nonanoic (C₉), and decanoic acid (C₁₀), whereas over the WP, the concentrations of C₁ (9.7 ng m^–3) and C₂ (9.6) are in a similar level followed by C₉ and C₁₀. The molecular distributions of branched-chain monoacids (iC₅ > iC₄) are similar to those obtained by the laboratory oxidation of isoprene (C₅), suggesting that isoprene oxidation is an important source for those acids. We found a different source of formic and acetic acid over the SO. Regression analysis (R² = 0.67, p < 0.01) of formic acid with total sulfur suggests a link to biogenic dimethyl sulfide (DMS) via photochemical oxidation, whereas C₂ is suggested to form via the degradation of iC₅, C₇, and C₈ over the SO. In contrast, over the WP, it is likely that mixed sources of biogenic and anthropogenic activities influenced the distributions of formic and acetic acids in marine aerosols.