Biogenic acids were measured in aerosols at the SMEAR II (Station for Measuring Forest Ecosystem-Atmosphere Relations II) station in Finland from June 2010 until October 2011. The analysed organic acids were pinic, pinonic, caric, limonic and caryophyllinic acids from oxidation of α-pinene, β-pinene, limonene, Δ³-carene and β-caryophyllene, respectively. Due to a lack of authentic standards, the caric, limonic and caryophyllinic acids were synthesised for this study. The mean, median, maximum and minimum concentrations (ng m⁻³) were as follows: limonic acid (1.26, 0.80, 16.5, below detection limit (< LOD)), pinic acid (5.53, 3.25, 31.4, 0.15), pinonic acid (9.87, 5.07, 80.1, < LOD), caric acid (5.52, 3.58, 49.8, < LOD), and caryophyllinic acid (7.87, 6.07, 86.1, < LOD).

The highest terpenoic acid concentrations were measured during the summer. Of the acids, β-caryophyllinic acid showed the highest concentrations in summer, but during other times of the year pinonic acid was the most abundant. The β-caryophyllinic acid contribution was higher than expected, based on the emission calculations of the precursor compounds and yields from oxidation experiments in smog chambers, implying that the β-caryophyllene emissions or β-caryophyllinic acid yields were underestimated. The concentration ratios between terpenoic acids and their precursors were clearly lower in summer than in winter, indicating stronger partitioning to the aerosol phase during the cold winter season. The β-caryophyllinic and caric acids were weakly correlated with the accumulation-mode particle number concentrations.