Aerosol particle size modulates cloud droplet effective radius responses over land and ocean in South Asia

Aerosol–cloud interactions (ACI) are a leading uncertainty in radiative forcing, yet aerosol particle size is poorly quantified over South Asia. Using 2.7 million satellite observations (2015–2021), we test whether the Ångström Exponent (AE) modulates cloud droplet effective radius (CER) responses to aerosol index (AI) and aerosol optical depth (AOD) over the subcontinent and adjacent ocean. Three methods target distinct estimands: AE-stratified binned regression, eXtreme Gradient Boosting (XGBoost) with SHapley Additive exPlanations (SHAP), and Double Machine Learning (DML) causal forests. Binned regressions reveal a robust land–ocean contrast. Oceanic coarse-mode regimes show negative sensitivities of − 2.6 to − 5.3 μ m per AI unit, consistent with the Twomey effect. Continental fine-mode regimes show positive sensitivities of + 0.4 to + 1.3 μ m per AI unit. This contrast persists across cloud water path (CWP) strata. XGBoost–SHAP corroborates it, ranking AE among the leading predictors. DML causal forests recover confounder-adjusted loading sensitivities ∂CER/∂AOD strengthening from polluted land to pristine ocean (+ 0.82 to + 3.99 μ m per unit AOD); distinct from the negative CER–AI Twomey slope, these sign-uniform positive values reflect a cleaner marine CCN-loading response. Aerosol size thus shapes ACI regionally, with robust oceanic Twomey-like signals but specification-dependent continental sensitivities reversing under alternative AE proxies, precluding a continental anti-Twomey interpretation.