MSA speeds up cold-region aerosol formation, reshaping climate projections

A CERN CLOUD chamber study shows methanesulfonic acid (MSA) nucleates with ammonia below −10 °C at rates comparable to sulfuric acid, and forms synergistic multi-acid clusters with SA. Even with very low ammonia, MSA drives particle growth near the kinetic limit below 9 °C at relative humidity above 40%. Since MSA and SA co-occur in cool marine regions, nucleation rates could be up to ten times faster and growth up to twofold compared with SA–NH3 alone, potentially boosting CCN concentrations, especially in polar regions. Global modeling indicates MSA is an important, currently underrepresented driver of biogenic particles in cool marine atmospheres, affecting present and pre-industrial climate.