Iodine-initiated new particle formation (I-NPF) has long been recognized in coastal hotspot regions. However, no prior work has studied the exact chemical composition of organic compounds and their role in the coastal I-NPF. Here we present an important complementary study to the ongoing laboratory and field researches of iodine nucleation in coastal atmosphere. Oxidation and NPF experiments with vapor emissions from real-world coastal macroalgae were simulated in a bag reactor. On the basis of comprehensive mass spectrometry measurements, we reported for the first time a series of volatile precursors and their oxidation products in gas and particle phases in such a highly complex system. Organic compounds overwhelmingly dominated over iodine in the new particle growth initiated by iodine species. The identity and transformation mechanisms of organic compounds were identified in this study to provide a more complete story of coastal NPF from low-tide macroalgal emission.

We investigated size-dependent molecular characteristics of coastal organic aerosols from < 0.032 μm to 3.2 μm at a new particle formation (NPF) hotspot of east China by using Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR-MS). Strong connection between C20-33HhOo/C18,30HhOoNn compounds in particles smaller than 0.10 μm and the VOCs emitted from local intertidal macroalgae suggests that the organic compounds (OC) in ultrafine particles are formed probably via the gas-phase oxidation of long-chain fatty aldehydes or acids, followed by particle-phase accretion reactions or imine formation during the coastal NPF events. In 0.18-0.56 μm particles, dominant C8-C20 CHO, CHON, CHOS and CHONS compounds (maximum: C10 or C15) are suggested most likely to be terpene oxidation products. Highly oxygenated compounds with 0.6 ≤ H/C ≤ 1.5 and 0.67 ≤ O/C ≤ 1.2 reside mostly in 0.18-0.56 μm particles, accounting for 5% of the OC formulas in this size range. Iodinated OC are subsequently formed via electrophilic substitution of non-iodinated OC by iodine cations in iodine-rich particles. CHN and Cl/Br-containing OC account altogether for only 1-4% of total OC intensity. As a result of the above compound distribution, the intensity weighted unsaturation degree and carbon oxidation state of OC increase with particle size. The distribution of aromatic compounds (i.e. Aromaticity Index > 0.5) is bimodal with peaks in 0.056-0.18 μm and 1.0-3.2 μm. In addition, our study observed higher unsaturation degree, carbon oxidation state and aromaticity of OC in coastal PM2.5 than inland urban PM2.5 in the same region.