Microplastics (MPs) and their derivatives have received worldwide attention owing to their adverse effects on ecosystems. However, molecular diversity and dynamic formation of dissolved organic matter (DOM) during the photoaging of MPs remain unclear. Herein, we explored a molecular‒level formation mechanism for polystyrene MP (MP_PS)‒derived DOM (PSDOM) during the photoaging of MPs to explain the evolution, heterogeneity, and sequential response of molecules to irradiation. Two‒dimensional correlation spectroscopy was applied to correlate the variations of PSDOM molecules detected by Fourier transform–ion cyclotron resonance mass spectrometry with those of MP_PS functional groups detected by Fourier transform infrared spectroscopy. Irradiation‒induced PSDOM contained the most highly unsaturated structures with oxygen, but showed fewer aromatic structures than natural aquatic DOM. Photochemical transformations occurred between saturated‒reduced and oxidized molecules during PSDOM leaching, with the low‒oxidized and high‒oxidized molecules undergoing considerable changes in the normal carbon oxidation state and molecular number, respectively. The primary sequential response of PSDOM molecules to increasing irradiation time [low‒oxidized/high‒weight (450<m/z<800) → high‒oxidized → saturated/low‒weight molecules (m/z<450)] corresponded to the response of MP_PS functional groups (aromatic C‒H → carbonyl → aromatic ring, CH₂ bend → C‒H groups), demonstrating well synergistic relationships between them. These novel findings will contribute to the understanding of underappreciated behaviors or risks of MPDOM in aquatic ecosystems.

微塑料（MPs）及其衍生物因其对生态系统的不利影响而受到全世界的关注。然而，MPs 光老化过程中溶解有机物 (DOM) 的分子多样性和动态形成仍不清楚。在此，我们探索了MP 光老化过程中聚苯乙烯 MP (MP _PS ) 衍生的 DOM (PSDOM)的分子水平形成机制，以解释分子对辐照的演化、异质性和顺序响应。应用二维相关光谱将傅里叶变换离子回旋共振质谱检测到的 PSDOM 分子的变化与傅里叶变换红外光谱检测到的MP _{PS官能团的变化关联起来。}辐照诱导的 PSDOM 含有最高程度的氧不饱和结构，但芳香族结构比天然水生 DOM 少。PSDOM浸出过程中，饱和还原分子和氧化分子之间发生光化学转化，低氧化分子和高氧化分子的正常碳氧化态和分子数分别发生了相当大的变化。PSDOM分子对增加照射时间的主要顺序响应[低氧化/高分子量(450<m/z<800)→高氧化→饱和/低分子量分子(m/z<450)]对应于MP _PS官能团（芳香族 C-H → 羰基 → 芳环、CH ₂弯曲 → C-H 基团）的响应，表明它们之间具有良好的协同关系。这些新发现将有助于了解水生生态系统中 MPDOM 的未被充分认识的行为或风险。