Faster than expected: Iodine species are one of only a handful of atmospheric vapors known to make new aerosol particles, which play a central role in controlling the radiative forcing of climate. He et al. report experimental evidence from the CERN Cosmics Leaving Outdoor Droplets, or CLOUD, chamber demonstrating that iodic acid and iodous acid rapidly form new particles and can compete with sulfuric acid in pristine regions. Science , this issue p. 589

The main nucleating vapor in the atmosphere is thought to be sulfuric acid (H ₂ SO ₄ ), stabilized by ammonia (NH ₃ ). However, in marine and polar regions, NH ₃ is generally low, and H ₂ SO ₄ is frequently found together with iodine oxoacids [HIO _x , i.e., iodic acid (HIO ₃ ) and iodous acid (HIO ₂ )]. In experiments performed with the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we investigated the interplay of H ₂ SO ₄ and HIO _x during atmospheric particle nucleation. We found that HIO _x greatly enhances H ₂ SO ₄ (-NH ₃ ) nucleation through two different interactions. First, HIO ₃ strongly binds with H ₂ SO ₄ in charged clusters so they drive particle nucleation synergistically. Second, HIO ₂ substitutes for NH ₃ , forming strongly bound H ₂ SO ₄ -HIO ₂ acid-base pairs in molecular clusters. Global observations imply that HIO _x is enhancing H ₂ SO ₄ (-NH ₃ ) nucleation rates 10- to 10,000-fold in marine and polar regions.

Intense new particle formation events are regularly observed under highly polluted conditions, despite the high loss rates of nucleated clusters. Higher than expected cluster survival probability implies either ineffective scavenging by pre-existing particles or missing growth mechanisms. Here we present experiments performed in the CLOUD chamber at CERN showing particle formation from a mixture of anthropogenic vapours, under condensation sinks typical of haze conditions, up to 0.1 s⁻¹. We find that new particle formation rates substantially decrease at higher concentrations of pre-existing particles, demonstrating experimentally for the first time that molecular clusters are efficiently scavenged by larger sized particles. Additionally, we demonstrate that in the presence of supersaturated gas-phase nitric acid (HNO₃) and ammonia (NH₃), freshly nucleated particles can grow extremely rapidly, maintaining a high particle number concentration, even in the presence of a high condensation sink. Such high growth rates may explain the high survival probability of freshly formed particles under haze conditions. We identify under what typical urban conditions HNO₃ and NH₃ can be expected to contribute to particle survival during haze.

Raising the ionisation rate (IR) increases the ion concentration, but also amplifies ion losses due to ion–ion recombination, consequently diminishing the formation rate of charged clusters at low HIO₃ concentrations.

Ammonium nitrate will condense to tiny particles under sub-zero conditions, activating well below 10 nm.