Vertical structure and controlling factors of cloud condensation nuclei activation over eastern China: Insights from aircraft measurements and interpretable machine learning

Abstract. Cloud condensation nuclei (CCN) play a critical role in cloud droplet formation and microphysical processes. Based on aircraft observations, this study investigated the factors controlling CCN number concentrations (NCCN) under different aerosol vertical structures (Decrease, Increase, and Stable) and supersaturation (SS) conditions using generalized additive models (GAM) combined with SHapley Additive exPlanations (SHAP).NCCN reached up to 10³ cm⁻³ near the surface and generally decreased with altitude, while aerosol vertical structures modulated its abundance, with the Increase structure showing higher NCCN than Stable and Decrease structures. CCN activation ratios increased with SS and exhibited a non-monotonic vertical variation, with no consistent ranking among aerosol structures, indicating that supersaturation dominates CCN activation. Activated CCN droplet spectra showed unimodal distributions, with peak diameters increasing from ~2 μm at SS = 0.2 to ~5 μm at SS = 1.0. Although spectral shapes were similar among different structures, higher small-size aerosol concentrations (SA) enhanced CCN peak concentrations. GAM results identified temperature (T), SA, relative humidity (RH), and horizontal wind speed (WS) as important explanatory variables for NCCN variations, with contributions of 20 % – 56 %, 9 % – 45 %, 9 % – 40 %, and 3 % – 19 %, respectively. SHAP analysis revealed that the contributions of T varied among different aerosol vertical structures, showing positive associations under Decrease and Increase structures but negative associations under Stable conditions, whereas SA consistently exhibited positive contributions. RH showed nonlinear relationships with NCCN, with an inflection point near 60 %.