Distinct Impacts of Aerosol Size on Aerosol‐Cloud Interactions Over Ocean and Land Regions in Eastern China

Abstract Representation of aerosol‐cloud interactions (ACI) remains one of the largest uncertainties in climate models and our understanding of climate change. Using multisource cloud, aerosol, and meteorology data during summer of 2015–2024, this study investigates ACI from the perspective of aerosol size (denoted by Ångström exponent, AE) over the ocean and land in eastern China. Our findings reveal that at a fixed cloud water path, the cloud droplet effective radius (CER) increases with the aerosol index (AI) under high‐AE conditions (fine‐mode aerosols), while CER decreases with increasing AI when AE is below 1.4 (coarse‐mode aerosols) in both regions. We interpret the opposite correlations as arising from aerosol size‐dependent regulation of cloud‐nucleating ability, which leads to distinct dominant cloud microphysical processes. Over land, smaller aerosols with lower cloud‐nucleating ability lead to weaker competition for water vapor and the collision‐coalescence process becomes dominant due to the enhanced turbulence as aerosols increase. Conversely, activation efficiency is significantly stronger for coarse‐mode aerosols over the ocean and the competition effect becomes the dominant process. In addition, the dominant aerosol size decreases as cloud top pressure increases over land, leading to a transition in the CER‐AI relationships from negative to positive. The link between lower cloud tops and finer aerosols is consistent with the enhanced radiative stabilization induced by a higher proportion of fine aerosols (often light‐absorbing). In contrast, AE values over the ocean remain consistently low, resulting in persistent negative correlations. Despite variations in meteorological conditions, the opposite correlations under dominant coarse‐ and fine‐mode aerosol conditions still exist.