Methoxyphenols are widely acknowledged tracers of biomass burning emissions, undergo complex chemical transformations in atmospheric aqueous environments that significantly modulate aerosol properties. The role of sulfate radicals (SO₄^{&bull;&ndash;}), the highly potent electrophiles prevalent in cloud and fog waters, remains poorly constrained. Herein, we investigated the aqueous-phase kinetics and aqueous secondary organic aerosol (aqSOA) formation of three representative methoxyphenols<span> </span>containing different substituents, namely vanillic acid (VAA), vanillin (VAL), and coniferyl aldehyde (CFA), upon<span> </span>oxidation by SO₄^{&bull;&ndash;}. The determined second-order rate constants for VAA, VAL, and CFA with SO₄^{&bull;&ndash;} were in the range of (2.40-3.37) &times; 10⁹ M^-1s^-1, revealing that side-chain substituents critically govern reactivity through &pi;-electron density modulation. These reactions efficiently generated aqSOA with maximum mass yields of 63.12%-70.49%, characterized by high oxidation degree comparable to that of atmospheric low-volatility oxygenated organic aerosols (LV-OOA). Notably, the oxidation process driven the generation of humic-like substances (HULIS) with pronounced light-absorbing<span> </span>capabilities in the near-ultraviolet and visible regions, thereby contributing to atmospheric brown carbon. Furthermore, the oxidation products exhibited significantly elevated oxidative potentials compared to their precursors, posing enhanced health risk. Our findings identify SO₄^{&bull;&ndash;}-initiated aqueous chemistry as a critical yet previously overlooked pathway that transforms biomass burning emissions into toxic and light-absorbing secondary aerosols. Therefore, it is necessary to incorporate SO₄^{&bull;&ndash;}-related processes into atmospheric models to accurately predict air quality and climate forcing.