The relative abundance of nitrate over phosphate in the marginal seas downstream of the East Asian continent increased steadily after 1980, driven largely by atmospheric deposition of pollutant nitrogen. Anthropogenic aerosol emissions over the region have since declined sharply, yet the SSP scenarios used in current Earth system projections prescribe continued increases, so the oceanic response to this aerosol demasking effect remains unquantified. We defined the effect as the difference between the SSP3-7.0-lowNTCF and SSP3-7.0 experiments of CESM2-WACCM and compared the simulated response with in situ nutrient observations collected in the East Asian marginal seas from 1987 to 2023. In the demasked field, the reduction in dissolved inorganic nitrogen deposition produced a corresponding decrease in surface excess nitrate (N*), and the two covaried closely (R² = 0.62, p < 10⁻⁷). The trends in nitrogen deposition were of opposite sign in the two scenarios, yet N* increased in both, indicating attenuation rather than reversal of the upward trend. The observations were consistent with this behaviour: after the 2010s, the positive N* trends in the Yellow Sea and the East Sea (Sea of Japan) weakened and in some regions turned negative.