Implementing detailed nucleation predictions in the Earth system model EC-Earth3.3.4: sulfuric acid-ammonia nucleation

Representing detailed atmospheric aerosol processes in global Earth system models (ESMs) has proven challenging both from a computational and a parameterization perspective. The representation of secondary organic aerosol (SOA) formation and new particle formation (NPF) in large ESMs are generally constructed with low detail to save computational costs. The simplification could result in losing the representation of some processes. In this study, we test and evaluate a new approach for improving the description of NPF processes in the ESM EC-Earth3 (ECE3) without loss of significant computational time. The current NPF scheme in EC-Earth3 is derived from the nucleation of low-volatility organic vapors and sulfuric acid (H ₂SO ₄) together with a homogeneous water-H ₂SO ₄ nucleation scheme. We expand the existing schemes and introduce a new look-up table approach that incorporates detailed formation rate predictions by molecular modeling of sulfuric acid-ammonia nucleation (H ₂SO ₄-NH ₃). We apply tables of particle formation rates for H ₂SO ₄-NH ₃ nucleation, including dependence on temperature, atmospheric ion production rate, and molecular cluster scavenging sink. The resulting differences between using the H ₂SO ₄-NH ₃ nucleation in ECE3 and the original default ECE3 scheme are evaluated and compared with a focus on changes in aerosol composition, cloud properties, and radiation balance. From this new nucleation scheme, EC-Earth3’s global average aerosol concentrations in the sub-100 nm sizes increased by 12–28 %. Aerosol concentrations above 100 nm and the direct radiative effect (in Wm ⁻²) from the changed nucleation only resulted in minor changes. However, the radiative effect from clouds affected by aerosols from the new nucleation scheme resulted in a global decrease (cooling effect) by 0.28–1 Wm ⁻². Additionally, several stations with observed aerosol size number distribution measurements are compared with the model results to examine the performance of the NPF schemes.