Iodine oxoacids enhance nucleation of sulfuric acid particles in the atmosphere

The main nucleating vapor in the atmosphere is thought to be sulfuric acid (H₂SO₄), stabilized by ammonia (NH₃). However, in marine and polar regions, NH₃ is generally low, and H₂SO₄ is frequently found together with iodine oxoacids [HIO_x, i.e., iodic acid (HIO₃) and iodous acid (HIO₂)]. In experiments performed with the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we investigated the interplay of H₂SO₄ and HIO_x during atmospheric particle nucleation. We found that HIO_x greatly enhances H₂SO₄(-NH₃) nucleation through two different interactions. First, HIO₃ strongly binds with H₂SO₄ in charged clusters so they drive particle nucleation synergistically. Second, HIO₂ substitutes for NH₃, forming strongly bound H₂SO₄-HIO₂ acid-base pairs in molecular clusters. Global observations imply that HIO_x is enhancing H₂SO₄(-NH₃) nucleation rates 10- to 10,000-fold in marine and polar regions.