Secondary organic aerosol formation vs primary organic aerosol emission: In situ evidence for the chemical coupling between monoterpene acidic photooxidation products and new particle formation over forests

In a Eucalyptus forest in Portugal were investigated (a) the formation of secondary organic aerosol formed through the condensation of low vapor pressure products of monoterpenes (alpha- and beta-pinene) photooxidation and (b) the chemical structure of these products related to their ability to form new particles. Two isomers of pinonic acid (cis- and rrans-2,2-dimethyl-3-acetylcyclobutylethanoic acid) and norpinonic acid (cis- and trans-2,2-dimethyl-3acetylcyclobutylmethanoic acid), pinic acid (cis-2,2-dimethyl-3-carboxycyclobutylethanoic acid), pinonaldehyde (2,2dimethyl-3-acetylcyclobutylethanal), and nopinone (6,6-dimethylbicyclo[3.1.1]heptan-2-one) were detected in all forest aerosol samples. By considering the diurnal concentration pattern of the acidic products and Aitken nuclei observed during the same periods, our results indicated that cis- and trans-pinonic, cis- and trans- norpinonic and pinic acids are photooxidation products of a-pinene chemically coupled with new particles formed. Lipids such as n-alkanes, n-alkanols, n-alkanals, and n-alkanoic acids determined in the forest aerosol were associated with primary biogenic emissions from Eucalyptus trees.