Observations of new particle formation and size distributions at two different heights and surroundings in subarctic area in northern Finland

Two years of continuous aerosol particle number size distribution measurements have been carried out in a subarctic area in northern Finland. Measurements have been made at two different heights (340 m and 560 m above sea level) and surroundings (inside a forest and at the top of a fjeld) to find out the effects of local-scale conditions on new particle formation and growth as well as on the size distribution of aerosol particles. Measured aerosol particle size range was 7-500 nm. Average total number concentration in the year 2001 was 700 cm(-3) at the higher and 870 cm(-3) at the lower site. One-day averages varied between 40 and 3500 cm(-3). Seasonal variation of total concentration was observed to be similar at both stations, high values in spring and summer and low values in winter. Also, modal concentrations were compared. Diurnal variation of total particle concentration was similar at both sites. In total, 65 new particle formation events were recorded during the measuring period. The largest number of events occurred in April and May. Particle formation events started between 0825 and 1550 (UTC plus 2 hours), and the calculated starting times of 1 nm particle formation (nucleation) varied between 0450 and 1420. The particle growth rate was found to be 1.4-8.2 nm h(-1), and the formation rate of 7 nm particles varied from 0.06 to 0.40 particles cm(-3) s(-1). Starting times of the formation events at the two stations had maximum difference of 30 min, and it could not be explained by wind direction and speed. One possible explanation for the time difference is vertical movement of air masses caused by turbulence. Solar radiation was observed to be one key factor needed for particle formation. Wind direction was mostly between west and north on formation days, indicating polar or arctic air masses.