Number size distribution of ambient submicron and ultrafine aerosol particles have been measured on a continuous basis (every 10 minutes) for three quarters of the year 1996, at a forest site in Southern Finland. Continuous monitoring offers additional insight over the diurnal dynamics of the submicron size distribution, including existence of clearly separate size modes as well as events of new particle formation. Selected examples of the measured size distributions are presented, including the particle formation events observed at the measurement site. Typical characteristics of days with particle formation events versus days of no events are discussed.¿ 1997 American Geophysical Union