Marine upper-tropospheric rapid particle formation dominated by methanesulfonic acid.

New particle formation (NPF) in the marine upper troposphere sustains one of the largest global aerosol reservoirs that seeds cloud condensation nuclei in the lower troposphere, with far-reaching implications for Earth's radiative balance and climate. However, the underlying NPF mechanisms remain elusive, constituting a major uncertainty in climate projections. Here, we show that methanesulfonic acid (MSA), long considered only as a key boundary-layer precursor, dominates upper-tropospheric NPF across major oceans. Quantum-chemical and cluster dynamics simulations reveal that MSA enhances sulfuric acid (HSO)-ammonia (NH) nucleation rates by 1 to 3 orders of magnitude, far surpassing the well-established nitric acid (HNO)-HSO-NH mechanism, owing to stronger intracluster hydrogen bonds and low temperatures that stabilize clusters and render nucleation nearly barrierless. Further global three-dimensional modeling constrained by field measurements confirms that the proposed HSO-MSA-NH nucleation pathway dominates the upper-tropospheric NPF over the Pacific, Atlantic, and Indian Oceans. Notably, this pathway contributes 40% of global nucleation-induced Aitken- and accumulation-mode aerosols at 0.5 to 4 km altitudes, where most cloud water resides, and yields a net top-of-atmosphere radiation forcing of -1.75 W m ( 68% of the nucleation-induced response). This study offers a detailed mechanistic insight into marine upper-tropospheric NPF and improves representation of aerosol-cloud interactions, thereby reducing uncertainties in global climate projections.