Dust and smoke layers over the Atlantic Ocean weaken the underlying low-level cloud-top radiative cooling through different pathways

Aerosol semi-direct effects represent one of the least understood yet important pathways of aerosol interactions. These effects occur when absorbing aerosols rapidly adjust Earth’s radiative budget through modifications of thermodynamic structures that influence cloud cover. Over the Atlantic Ocean, where two primary radiation-absorbing aerosols (smoke and dust) dominate above clouds, the mechanisms by which aerosol-layer properties affect underlying low-level cloud-top radiative cooling — a critical parameter controlling cloudiness—remain unclear. Using ten years of satellite-derived aerosol, cloud, and radiative flux observations, combined with radiative-transfer simulations, we find that dust and smoke layers induce longwave-dominated warming responses that weaken the mean radiative cooling at low-level cloud tops. However, the pathways of this warming response differ, resulting in dust layers impacting cloud-top cooling about ten times more than smoke layers. Whereas dust properties dominate dust-induced warming responses through direct interactions in longwave, smoke-induced warming responses involve enhanced smoke-layer moisture that induces longwave radiation, opposing the impacts of smoke properties at cloud tops. This weakened cloud-top cooling response reduces low-level cloudiness by approximately 1.21% and 0.28% for dust and smoke, respectively. Our findings demonstrate the importance of accounting for longwave-mediated processes beyond traditional shortwave-dominated mechanisms in estimates of aerosol semi-direct effects.