Opposing effects of sea salt aerosols constrain cloud droplet responses over the Southern Ocean

Cloud droplet number concentration (N d ) is modulated by aerosols and subsequently influences low-level cloud radiative effects. Using two decades of satellite and reanalysis data, we quantified N d responses to aerosol perturbations over the Southern Ocean (south of 60°S). Significant temporal increases over 2003–2022 in sulfate (47.6%), coarse sea salt (CSS, 76.5%), and fine sea salt (FSS, 80.6%) align with shifting environmental drivers. Despite these trends, summertime N d rose by only 6.6%. This muted response is primarily associated with opposing microphysical effects: FSS enhances N d by acting as cloud condensation nuclei in low-sulfate regimes, whereas large, hygroscopic CSS tends to suppress N d through water vapor competition. A neural network attributes ~60% of the N d trend to sulfate, while the counteracting effects of FSS and CSS result in a negligible net sea-salt contribution. These results reveal a buffering mechanism, necessitating size-resolved sea-salt in climate models to better constrain cloud–radiative feedbacks.