Aerosol size distribution measurements at four Nordic field stations: identification, analysis and trajectory analysis of new particle formation bursts

We analyzed aerosol size distributions from the Finnish measuring stations at Hyytiälä, Värriö and Pallas and the Swedish station at Aspvreten over a period of several years. We identified occurrences of new particle formation bursts and obtained characteristics for the bursts from the size distribution data. In addition, we analyzed the directions from which air masses leading to new particle formation arrived. We found that new particle formation occurs over the whole area covered by the measurement stations. The Northern Atlantic is dominating as a source for air leading to new particle formation at all of the analyzed stations. The formation occurrence had a similar annual variation at all the stations, with peaks in springtime and autumn and minima in winter and summer. The ratio of event days to non-event days had a North-South dependence, with northern stations having lower event ratios. Particle growth rates ranged from 0.5 to 15 nm/h, with the mean growth rate being slightly higher at the southern stations. Southern stations also had a stronger particle source, on average 0.5 cm^-3 s^-1, compared to the northern stations (0.1 cm^-3 s^-1). Based on our analysis, it is evident that new particle formation occurs often in whole Nordic boreal forest area when air is transported from the North Atlantic, and that the same process or processes are very probably responsible for the formation over the whole area.