Effects of air masses and synoptic weather on aerosol formation in the continental boundary layer

Nucleation of near nm sized aerosol particles and subsequent growth to simi lar to 100 nm in 1-2 days has in recent years been frequently observed in t he continental boundary layer at several European locations. In 1998-99, th is was the focus of the BIOFOR experiment in Hyytiala in the boreal Finnish forest. Nucleation occurred in arctic and to some extent in polar air mass es, with a preference for maritime air in transition to continental air mas ses, and never in subtropical air. The air masses originated north of the B IOFOR experiment by paths from the southwest to northeast sector. The nucle ation was also associated with cold air advection behind cold fronts, never warm air advection, This may relate to low pre-existing aerosol concentrat ion, low cloudiness and large diurnal amplitudes in the continental boundar y layer associated with cold air advection and clear skies. Arctic and pola r air together with cold air advection did not always lead to nucleation. T he most important limiting meteorological factors were cold front passages and high cloudiness, probably through reduced photochemistry and wet scaven ging of precursor gases and new aerosol particles. The preference for nucle ation to occur in arctic air masses, which seldom form in the summer, sugge sts a meteorological explanation for the annual cycle of nucleation, which has a minimum in summer, The connection to cold-air outbreaks suggests that the maximum in nucleation events during spring and autumn may be explained by the larger latitudinal temperature gradients and higher cyclone activit y at that time of the year. Nucleation was observed on the same days over l arge parts (1000-km distance) of the same air mass. This suggests that the aerosol nucleation spans from the microphysical scale to the synoptic scale , perhaps connected through boundary layer and mesoscale processes.