Marine stratocumulus clouds cover nearly one-quarter of the ocean surface and thus play an extremely important role in determining the global radiative balance. The semipermanent marine stratocumulus deck over the southeastern Atlantic Ocean is of particular interest, because of its interactions with seasonal biomass burning aerosols that are emitted in southern Africa. Understanding the impacts of biomass burning aerosols on stratocumulus clouds and the implications for regional and global radiative balance is still very limited. Previous studies have focused on assessing the magnitude of the warming caused by solar scattering and absorption by biomass burning aerosols over stratocumulus (the direct radiative effect) or cloud adjustments to the direct radiative effect (the semidirect effect). Here, using a nested modeling approach in conjunction with observations from multiple satellites, we demonstrate that cloud condensation nuclei activated from biomass burning aerosols entrained into the stratocumulus (the microphysical effect) can play a dominant role in determining the total radiative forcing at the top of the atmosphere, compared with their direct and semidirect radiative effects. Biomass burning aerosols over the region and period with heavy loadings can cause a substantial cooling (daily mean −8.05 W m⁻²), primarily as a result of clouds brightening by reducing the cloud droplet size (the Twomey effect) and secondarily through modulating the diurnal cycle of cloud liquid water path and coverage (the cloud lifetime effect). Our results highlight the importance of realistically representing the interactions of stratocumulus with biomass burning aerosols in global climate models in this region.

海洋层积云覆盖了近四分之一的海洋表面，因此在确定全球辐射平衡中发挥着极其重要的作用。东南大西洋上的半永久性平流层积云层特别受关注，因为它与南部非洲排放的季节性生物质燃烧气溶胶相互作用。了解生物质燃烧气溶胶对平积云的影响以及对区域和全球辐射平衡的影响仍然非常有限。以前的研究集中在评估由平流积云上的生物量燃烧气溶胶对太阳的散射和吸收（直接辐射效应）或对直接辐射效应的云调整（半直接效应）引起的升温幅度。这里，使用嵌套建模方法结合多颗卫星的观测结果，我们证明了从夹带到平积层中的生物质燃烧气溶胶激活的云凝结核（微物理效应）可以在确定大气层顶部的总辐射强迫中起主导作用，与它们的直接和半直接辐射效应相比。在重负荷的区域和期间，生物质燃烧的气溶胶会引起大量的降温（每天平均−8.05 W m 与它们的直接和半直接辐射效应相比。在重负荷的区域和期间，生物质燃烧的气溶胶会引起大量的降温（每天平均−8.05 W m 与它们的直接和半直接辐射效应相比。在重负荷的区域和期间，生物质燃烧的气溶胶会引起大量的降温（每天平均−8.05 W m^-2），主要是由于通过减小云滴尺寸（Twomey效应）使云增亮，其次是通过调节云液态水路径和覆盖率的昼夜周期（云寿命效应）。我们的结果强调了在该地区的全球气候模型中现实地表示平流层积云与燃烧生物质气溶胶的相互作用的重要性。