Dust suppresses aerosol first indirect effects in marine warm clouds over the North Atlantic Ocean

Mineral dust is one of the most abundant atmospheric aerosols, yet its role in aerosol-cloud interactions remains poorly constrained. Although aging during transport enables weakly hygroscopic dust to act as cloud condensation nuclei and modify cloud droplet size, how dust affects the first indirect effect remains unclear. Using satellite observations and reanalysis (2003-2024), we quantify how Saharan dust modulates cloud droplet size and the first indirect effect for single-layer marine warm clouds. We find that in dusty environments, droplet size increases by 9%, and the first indirect effect weakens by 1.25 times, shifting from positive to negative for thin clouds. This response likely arises through two mechanisms: a microphysical pathway in which coarse dust acts as giant cloud condensation nuclei and a radiative pathway in which dust-induced warming suppresses supersaturation and enhances evaporation of smaller droplets, both producing fewer but larger droplets. This results in a positive top-of-atmosphere forcing of 0.38 watts per square meter.