Comparative study of nucleation mode aerosol particles and intermediate air ions formation events at three sites

Abstract

Quantitative characteristics of the formation and growth events of nucleation mode aerosol particles occurred at three stations (Hyytiälä and Värriö in Finland and Tahkuse in Estonia) have been compared. Aerosol size distributions were measured on all locations and, in addition, air ion mobility distribution at Tahkuse. Among 157 measuring days, nucleation events were identified on 62, 50, and 45 days at Hyytiälä, Värriö, and Tahkuse, respectively. Eighteen days were found when nucleation events occurred on all three locations at the same day. The concentration of condensable vapor, its source strength, the growth rate, the total condensation sink, the formation rate of 3 nm particles, the formation rate, and concentration of 1 nm particles were estimated using measured aerosol number size distributions. The estimated growth rates were in the range of 2–9 nm hr−1. The growth rates found from the measurements of air ion mobility distributions were comparable with that of nanometer particles. The condensable vapor concentrations were 1–12 × 107 cm−3, which is smaller at Hyytiälä than at Tahkuse and Värriö. At Tahkuse, source rates 6–55 × 104 cm−3s−1 were estimated, whereas at Hyytiälä and Värriö the rates were 2–14 × 104 cm−3s−1 and 1–20 × 104 cm−3s−1, respectively. The condensation sink values were usually substantially higher at Tahkuse than on the other locations. Tahkuse size distribution data showed 5–10% underestimates in the total condensation sink considering particles in the diameter range <500 nm. At Tahkuse the charged fraction of nucleation mode particles estimated through comparison of aerosol size distribution and air ion mobility distribution turned to be much higher than steady state charging probability of a single charge on the particle. Analysis of air mass trajectories and meteorological data indicated that the nucleation events are synoptic‐scale phenomenon occurring in horizontal extent more than 1000 km.