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

Abstract. Representing detailed atmospheric aerosol processes in global Earth system models (ESMs) has proven to be challenging from both a computational and a parameterization perspective. The representation of secondary organic aerosol (SOA) formation and new particle formation (NPF) in large ESMs is 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 significant additional computational burden. The current NPF scheme in EC-Earth3.3.4 is derived from the nucleation of low-volatility organic vapors and sulfuric acid (H2SO4) together with a homogeneous water–H2SO4 nucleation scheme. We expand the existing schemes and introduce a new lookup table approach that incorporates detailed formation rate predictions through molecular modeling of sulfuric acid–ammonia nucleation (H2SO2–NH3). We apply tables of particle formation rates for H2SO2–NH3 nucleation, including dependence on temperature, atmospheric ion production rate, and molecular cluster scavenging sink. The resulting differences between using the H2SO4–NH3 nucleation in ECE3 and the original default ECE3 scheme are evaluated and compared with a focus on changes in the 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 W m−2) showed only minor differences upon changing of the nucleation scheme. 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 W m−2. The modeled aerosol concentrations were compared to observed measurements at various stations. In most cases, the new NPF predictions (H2SO2–NH3) performed better at stations where previous underestimations for aerosol concentrations occurred.