Is nucleation capable of explaining observed aerosol integral number increase during southerly transport over Scandinavia?

Using a pseudo-Lagrangian approach, changes in aerosol size distribution was investigated during southerly transport under clear sky conditions from Finnish Lapland to Hyytiälä. Seventy-nine individual transport cases were considered. The mean transport distance was 700 km and mean transport time 66 h. On average, a sevenfold increase in Aitken mode number concentration could be observed. An increase in number concentration was observed in virtually all the cases. Several of the studied cases were associated with indications of nucleation at the receptor site. Six of the cases were simulated in detail utilizing a box-model approach. Aerosol dynamics was evaluated using the University of Helsinki Multi-component Aerosol model. Particle formation was assumed to be controlled by a kinetic nucleation mechanism. Growth of particles was suggested to be controlled by, except water and ammonia, sulphuric acid, and some unknown species with saturation vapour pressure of 3 × 10⁶ cm^-3. This product was supposed to derive from terpene oxidation by hydroxyl radical, ozone, and nitrate radical.

The investigation strongly suggests nucleation events occurring over large scales to be responsible for the observed number increase during transport under modelled conditions.

Using a simplified two layer structure of the lowermost troposphere, we highlight the role of vertical exchange.

Modelled growth rates were found to be in agreement with observational data, in the order of 1–2 nm h^–1. In order to reproduce the observed growth rates, a molar yield of condensable products from terpene oxidation of 10% was required. Concentration of sulphuric acid and condensable organic vapours were on average 3 × 10⁶ and 1.5 × 10⁷ cm^-3, respectively.