| Size-segregated aerosol samples were collected during the QUEST field
campaign at Hyytiälä, a boreal forest site in Southern Finland,
during spring 2003. Aerosol samples were selectively collected during both
particle formation events and periods in which no particle formation
occurred.
A comprehensive characterisation of the aerosol chemical properties
(water-soluble inorganic and organic fraction) and an analysis of the
relevant meteorological parameters revealed how aerosol chemistry and
meteorology combine to determine a favorable "environment" for new
particle formation. The results indicated that all events, typically favored
during northerly air mass advection, were background aerosols (total mass
concentrations range between 1.97 and 4.31 µg m-3), with an
increasingly pronounced marine character as the northerly air flow arrived
progressively from the west and, in contrast, with a moderate
SO2-pollution influence as the air arrived from more easterly
directions. Conversely, 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 anthropogenic aerosol constituents. The
higher non-event mass concentration (total mass concentrations range between 6.88 and
16.30 µg 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 nucleation episodes were dominated by aliphatic biogenic
species, while non-events were characterised by a large abundance of anthropogenic
oxygenated species. Interestingly, a significant content of α-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 responsible also for the growth of newly
formed thermodynamically stable clusters, our finding allows one to
postulate that, at the site, α-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. |