Seasonal differences in observed versus modelled new particle formation at two European boreal stations

Abstract. Realistic representation of atmospheric aerosol size distribution dynamics in large-scale climate models is important for developing accurate descriptions of aerosol–cloud interactions. Despite the dynamic nature of the distributions, which have large seasonal and diurnal changes, model evaluations often focus on the annual median size distribution. Using more comprehensive monthly and diurnal model illustrations can be crucial for evaluating model performance and potential aerosol effects for short-term variations. In this study, we assess the impact of a molecular model scheme for NH3–H2SO4 nucleation integrated in the Earth System Model (ESM) EC-Earth3, across different seasons, months, and days within the boreal climate during the year 2018. Measured number size distributions from two in situ boreal stations are used to evaluate and study particle formation and growth representation in EC-Earth3 over 2018. Additionally, we utilize results from the ADCHEM model, a state-of-the-art 1-D Lagrangian aerosol chemistry model. This allows us to compare EC-Earth3 against results from highly detailed model descriptions of aerosol formation and growth at the boreal stations. When comparing diurnal EC-Earth3 model results with in situ observations at an hourly temporal resolution, we establish that using solely organic–H2SO4 nucleation parameterization will underestimate the aerosol number concentrations. The newly added NH3–H2SO4 nucleation parameterization in this study improves the resulting aerosol number concentrations and the reproduction of particle formation events with EC-Earth3. However, from March to October, EC-Earth3 still underestimates particle formation and growth.