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Classical theory of binary homogeneous nucleation is extended to the ternary system H2SO4-NH3-H2O. For NH3 mixing ratios exceeding about 1 ppt, the presence of NH3 enhances the binary H2SO4-H2O nucleation rate by several orders of magnitude. The Gibbs free energies of formation of the critical H2SO4-NH3-H2O cluster, as calculated by two independent approaches, are in substantial agreement. The finding that the H2SO4-NH3-H2O ternary nucleation rate is independent of relative humidity over a large range of H2SO4 concentrations has wide atmospheric consequences. The limiting component for ternary H2SO4-NH3-H2O nucleation is, as in the binary H2SO4-H2O case, H2SO4; however, the H2SO4 concentration needed to achieve significant nucleation rates is several orders of magnitude below that required in the binary case.

[1] During a 33-month measurement period at a high-latitude background site in northern Finland, a total of 103 new particle formation events were observed. In 19 cases, movement of the air masses allowed the observation of particle formation and growth over a sufficiently long time to investigate the production of “potential” cloud condensation nuclei (CCN) resulting from new-particle formation. All the CCN formation events took place in relatively clean air that had traversed the Northern Atlantic/Arctic Ocean prior to arrival at our measurement site. The number of formed new, “potential” CCN varied significantly between the different events and correlated weakly with the number of new-particles formed during the same events. This is consistent with recent theoretical suggestions of some sort of decoupling between atmospheric new-particle formation and growth. The vapours responsible for the “potential” CCN production could not be identified but were mostly something else than sulfuric acid resulting from the oxidation of SO2 in the gas phase. Although atmospheric new-particle formation is likely to give only a minor contribution to the overall CCN budget at our measurement site, the situation may be different over areas where new-particle formation events are more common. At the very least, our results demonstrate that because of atmospheric new-particle formation, it may not be possible to find any universal relation between the aerosol mass and cloud droplet number concentration that is applicable to clean conditions.

[1] 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.