Natural cloud brightening enhanced by marine primary organic aerosols

Abstract Marine primary organic aerosols (mPOA) play a key role in ocean-atmosphere-climate interactions. Yet their effects on cloud formation remain uncertain because mPOA number size distribution measurements are lacking, obscuring its parameterizations for climate models. The mPOA number size distribution is resolved by combining long-term hygroscopicity, size, number and composition measurements in pristine marine air. The resulting bimodal distribution shows strong Aitken (∼60 nm) and accumulation (∼150 nm) modes, unlike the parameterization in widely used models. Total mPOA number is probably underestimated by a factor of three (observed/parameterized ≈ 3.1), doubling mPOA-originated cloud condensation nuclei (CCN) across realistic marine supersaturations. An observation-guided sensitivity analysis indicates a potential first aerosol indirect effect of –0.03 to –0.04 W/m2 over the global ocean, reaching –0.1 to –0.2 W/m2 over biologically active waters when mPOA surface tension reduction is considered, underscoring the need for trustworthy mPOA representation to reduce the uncertainties in aerosol-cloud interactions.