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 C_20-33H_hO_o/C_18,30H_hO_oN_n 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 PM_2.5 than inland urban PM_2.5 in the same region.