Primary and Secondary Emissions of Carboxylic Acids from Solid Fuel Combustion: Insight into the Source Markers and Secondary Formation Mechanism

Carboxylic acids play an important role in atmospheric photochemical reactions, aerosol nuclei, and climate change. Primary and secondary carboxylic acid emissions from various combustion scenarios were quantified. Coal combustion emitted more low-molecular weight (MW) monocarboxylic acids (<C9) (12.7–16.4%), while biomass burning released more ultrahigh-MW monocarboxylic acids (>C29) (15.5–32.3%). Ultrahigh-MW monocarboxylic acid abundance and hexadecanoic acid (C16) versus nonadecanoic acid (C19) remained stable between the primary emission and aging processes, suggesting that they could be ideal markers for source characterization. Significant correlations were observed between the decreasing of toluene and benzene and the increasing of oxalic acid (C2), malonic acid (C3), fumaric acid (C4), suberic acid (C8), azelaic acid (C9), and sebacic acid (C10) (p < 0.05) during coal combustion, suggesting that oxidation of toluene and benzene lead to the formation of dicarboxylic acids during photochemical aging. On the other hand, the oxidation of monocarboxylic acids occurs on carbons farther away from the −COOH group, leading to the formation of dicarboxylic acids. The secondary formation mechanism of dicarboxylic acids from biomass burning differed from that of coal because of the abundance of low-chemical reactivity, ultrahigh-MW monocarboxylic acids; further study is required.