LAUSR

Significance Unique in the marine boundary layer, new particle formation involves iodine-oxide nucleation; however, challenges remain in explaining the growth. Condensation of organic vapors has been suggested as the most probable mechanism, but condensation growth requires condensing organic molecules of low effective volatilities. Here, we show, through a combination of laboratory experiments, ambient field measurements, and model studies, that exposure of iodine-oxide nanoparticles to organic vapors may lead to accelerated particle growth through heterogeneous reactions forming low-volatility organic acids and alkylaminium salts in the particle phase. Moreover, we show that a notable fraction of iodine is recycled back into the gas phase during particle growth, providing insights into iodine-organic multiphase reactions for marine new particle formation.