Acidic reaction products of mono- and sesquiterpenes in atmospheric fine particles in a boreal forest
Biogenic acids were measured from PM2.5 aerosols at SMEAR II station (Station For Measuring Forest Ecosystem-Atmosphere Relations) in Finland from June 2010 until October 2011. The measured organic acids were pinic, pinonic, caric, limonic and caryophyllinic acids from oxidation of α-pinene, β-pinene, limonene, ∆3-carene and β-caryophyllene. Due to lack of authentic standards caric, limonic and caryophyllinic acids were synthesized at the Laboratory of Organic Chemistry, University of Helsinki.
The highest terpenoic acid concentrations were measured during summer concomitant with the precursor mono- and sesquiterpenes. Of the acids β-caryophyllinic acid had 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 on the basis of emission calculations of precursor compounds and yields in oxidation experiments in smog chambers implicating that β-caryophyllene emissions or β-caryophyllinic acid yields are underestimated. Concentration ratios between terpenoic acids and their precursor were clearly lower in summer than in winter indicating stronger partitioning to the aerosol phase during cold winter season. The β-caryophyllinic and caric acids were correlated with the accumulation mode particle number concentrations.