Size-segregated aerosol chemical composition at a boreal site in southern Finland, during the QUEST project

Size-segregated aerosol samples were collected during the QUEST field campaign at Hyyti ¨ al¨ a, a boreal forest site in Southern Finland, during spring 2003. Aerosol sam- ples were selectively collected during both particle formation events and periods in which no particle formation occurred. A comprehensive characterisation of the aerosol chemi- cal properties (water-soluble inorganic and organic fraction) and an analysis of the relevant meteorological parameters re- vealed how aerosol chemistry and meteorology combine to determine a favorable "environment" for new particle for- mation. The results indicated that all events, typically fa- vored during northerly air mass advection, were background aerosols (total mass concentrations range between 1.97 and 4.31µg m 3 ), with an increasingly pronounced marine char- acter as the northerly air flow arrived progressively from the west and, in contrast, with a moderate SO2-pollution influ- ence as the air arrived from more easterly directions. Con- versely, the non-event aerosol, transported from the south, exhibited the chemical features of European continental sites, with a marked increase in the concentrations of all major an- thropogenic aerosol constituents. The higher non-event mass concentration (total mass concentrations range between 6.88 and 16.30µg m 3 ) and, thus, a larger surface area, tended to suppress new particle formation, more efficiently depleting potential gaseous precursors for nucleation. The analysis of water-soluble organic compounds showed that clean nucle- ation episodes were dominated by aliphatic biogenic species, while non-events were characterised by a large abundance of anthropogenic oxygenated species. Interestingly, a sig- nificant content of -pinene photo-oxidation products was observed in the events aerosol, accounting for, on average, 72% of their WSOC; while only moderate amounts of these species were found in the non-event aerosol. If the organic vapors condensing onto accumulation mode particles are re- sponsible also for the growth of newly formed thermodynam- ically stable clusters, our finding allows one to postulate that, at the site, -pinene photo-oxidation products (and probably also photo-oxidation products from other terpenes) are the most likely species to contribute to the growth of nanometer- sized particles.

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