China’s SO ₂ emission reductions enhance atmospheric ozone–driven sulfate aerosol production in East Asia

Reduction of China's SO₂ emissions has been found to nonlinearly decrease the atmospheric sulfate (SO₄^2-) aerosol concentrations in East Asia. Compared to Europe and North America, the lower effectiveness of SO₄^2- reduction in East Asia suggested much effects of "acidity-sensitive" feedback mechanisms in this high anthropogenic emission region, which have not been yet examined. In this work, we investigated these feedback mechanisms in East Asia through long-term measurements of the mass-independent oxygen-17 anomaly (Δ¹⁷O) in sulfate aerosols, machine learning and Community Multiscale Air Quality (CMAQ) model. As China's emissions reduced, the atmospheric acidity decreased, enhancing the ozone-driven oxidation of S(IV) and production efficiency of sulfate formation. This explained the weaker declining SO₄^2- concentrations than SO₂ emissions. By the evidence from observed Δ¹⁷O in non-sea-salt sulfate (Δ¹⁷O-nss-SO₄^2-) and CMAQ simulations, the highly enhanced contributions of S(IV)+O₃ to sulfate driven by reduced SO₂ emissions explained the low effectiveness of SO₄^2- reduction in East Asia. Additionally, the decreases of acidity by substantial NH₃ emissions are projected to continue until 2050, limiting SO₄^2- reduction effectiveness. Thus, we highlighted that global control of both SO₂ and NH₃ emissions are needed to efficiently mitigate the sulfate-related climate and pollution, especially in high NH₃ emission region, such as East Asia.