Field Evidence of Nocturnal Multiphase Production of Iodic Acid

Iodic acid (HIO₃) is ubiquitously present in the atmosphere and has garnered extensive attention in recent years for its contribution to particle formation and growth. The understanding of its underlying formation mechanisms, especially in inland urban areas, remains severely limited. In this study, through concurrent measurements of gas-phase iodic acid and particulate iodine in the Yangtze River Delta region, we observed continuous nighttime production of iodic acid. We found that elevated concentrations of particulate iodine and ozone (O₃) are required to effectively form the nocturnal iodic acid, with the production rate of which being proportional to the product of the concentration of aerosol iodine components and ozone concentration. Furthermore, the observed particulate iodine was significantly lower than the accumulated amount of gaseous iodic acid condensation. These findings suggest that the particulate iodine species, such as those deriving from the condensation of gaseous iodic acid, do not act as the terminal products in the atmospheric iodine cycle. Instead, they can rapidly revert to the gas phase and form iodic acid through multiphase reactions. This process may explain why, in the absence of significant iodine sources on land, iodic acid can maintain relatively high concentrations and significantly contribute to particle growth.