Our ability to understand and predict future climate scenarios remains limited by significant uncertainties in climate modelling, particularly those related to aerosol-cloud interactions (ACI). The Holuhraun eruption (2014–2015) provides an ideal opportunity to investigate ACI, with peak daily sulphur dioxide (SO<sub>2</sub>) emission rates exceeding that of all anthropogenic sources in Europe. In this study we perform the first Lagrangian evaluation of aerosol processes associated with an effusive volcanic perturbation that combines in-situ aerosol particle number size distribution (PNSD) from rural in-situ sites with air mass back-trajectories to understand differences in general circulation model (GCM) representations. Holuhraun significantly impacted the observed PNSD at the three sites considered, showing a consistent increase in the accumulation modal diameter and evidence of sustained growth during transport from new particle formation (NPF) in the plume. ECHAM6.3-HAM2.3 did not replicate the observed sustained growth from NPF events, instead the volcanic perturbation was associated with growth of pre-existing particles, contributing to the mass of aerosol. Contrastingly, UKESM1.0 demonstrated no increase in the modal diameter during the eruption period. The inclusion of organic-mediated boundary layer nucleation (BLN) into UKESM1.0 (UKESM-BLN), enabled a considerably better representation of PNSD changes. UKESM-BLN replicated the increase in accumulation mode diameter, as well as sustained NPF events, although it considerably overestimated number concentrations of Aitken mode particles. Investigating the perturbation in cloud condensation nuclei during the eruption year demonstrated that UKESM-BLN better replicated the perturbation at the boreal sites, highlighting the importance of BLN processes in accurate representation of ACI in GCMs.