Sensitivity of predicted ultrafine particle size distributions in Europe to different nucleation rate parameterizations using PMCAMx-UF v2.2

Abstract. The three-dimensional chemical transport model, PMCAMx-UF v2.2, designed to simulate the ultrafine particle size distribution, was used to investigate the impact of varying nucleation mechanisms on the predicted aerosol number concentration in Europe. Two basic case scenarios were examined: the original ternary H₂SO₄-NH₃-H₂O parameterization and a biogenic vapor-sulfuric acid parameterization. Using the organic-based parameterization, PMCAMx-UF predicted higher N₁₀ (particle number above 10 nm) concentrations over Europe by 40–60 % on average during the simulated period, which is a relatively small difference given the differences in the two assumed mechanisms. Adjusting the nucleation rate by an order of magnitude for both mechanisms led to an average change of ±30 % in N₁₀ for the ternary ammonia case, and ‒30 to 40 % for the biogenic vapor case. In the biogenic organic nucleation scenario, reducing the fresh nuclei diameter from 1.7 nm to 1 nm resulted in reductions in N₁₀ and N₁₀₀ by −13 % and −1 %, respectively. Incorporating extremely low-volatility organic compounds (ELVOCs) as the nucleating species resulted in predicted increase in N₁₀ concentration by 10–40 % over continental Europe compared to the ammonia parameterization. Model predictions were evaluated against field measurements from 26 stations across Europe during the summer of 2012. Among the tested scenarios, the measurements showed better agreement with the ternary ammonia and ELVOC-based parameterizations for N₁₀, whereas for N₁₀₀, all simulated cases appear to agree quite well with the field data.