Simulating Ultrafine Particle Formation in Europe Using a Regional CTM: Contribution of Primary Emissions Versus Secondary Formation to Aerosol Number Concentrations

A three-dimensional regional chemical transport model (CTM) with detailed aerosol microphysics, PMCAMx-UF, was applied to the European domain to simulate the contribution of direct emissions and secondary formation to total particle number concentrations. The model predicts nucleation events that occur over scales of hundreds up to thousands of kilometers especially in the Balkans and Southeast Europe. The model reproduces more than 70% of the hourly concentrations of particles larger than 10 nm (N₁₀) within a factor of 2. About half of these particles are predicted to originate from nucleation in the lower troposphere. Regional nucleation is predicted to increase the total particle number concentration by approximately a factor of 3. For particles larger than 100 nm the effect varies from an increase of 20% in the eastern Mediterranean to a decrease of 20% in southern Spain and Portugal resulting in a small average increase of around 1% over the whole domain. Nucleation has a significant effect in the predicted N₅₀ levels mainly in areas where there are condensable vapors to grow the particles to larger sizes. A semi-empirical ternary sulfuric acid-ammonia-water parameterization performs better than the activation or the kinetic parameterizations in reproducing the observations.