Iodine oxoacids enhance nucleation of sulfuric acid particles in the atmosphere.

The main nucleating vapor in the atmosphere is thought to be sulfuric acid (H2 SO4 ), stabilized by ammonia (NH3 ). However, in marine and polar regions, NH3 is generally low, and H2 SO4 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 H2 SO4 and HIO x during atmospheric particle nucleation. We found that HIO x greatly enhances H2 SO4 (-NH3 ) nucleation through two different interactions. First, HIO3 strongly binds with H2 SO4 in charged clusters so they drive particle nucleation synergistically. Second, HIO2 substitutes for NH3 , forming strongly bound H2 SO4 -HIO2 acid-base pairs in molecular clusters. Global observations imply that HIO x is enhancing H2 SO4 (-NH3 ) nucleation rates 10- to 10,000-fold in marine and polar regions.