Field Evidence of New Particle Formation Assisted by Iodic Acid in Northern Coastal China

New particle formation (NPF) is a major global aerosol source, impacting air quality, public health, and Earth's radiative balance. NPF mechanisms have been clarified in megacities and in the remote marine environment, yet little is known about the compounds and molecular steps driving NPF in coastal urban atmospheres. The role of iodic acid, a well-known marine precursor, remains unclear in coastal regions. Here, we report 1 year, molecular-level measurements of nucleating clusters in Dalian, a coastal city in northern China. We provide direct observational evidence that iodic acid (HIO₃), together with sulfuric acid (H₂SO₄) and amines, contributes to coastal NPF under real atmospheric conditions. Iodine-containing clusters account for 3.3%, 17.7%, 15.5%, and 4.5% of the dimers in spring, summer, autumn, and winter, on average, respectively, and can reach up to 50%. Iodic acid also significantly improves the early growth and survival of newly formed particles, with the survival probability of particles being increased by up to 8-fold. Our ambient observational results highlight the crucial role of iodic acid in airborne aerosol production (secondary aerosol formation) within mixed marine-continental atmospheres.