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

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. The low sensitivity of particle number concentrations to the nucleation mechanisms used in this study, may not exist in other regions outside Europe. 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. For N10, the ternary ammonia and ELVOC-based parameterizations were in better agreement with the field data compared to the other tested mechanisms. In the case of N100, all used parameterizations resulted in predictions that were consistent with the available field measurements.