Abstract
Reduction of China's SO(2) emissions has been found to nonlinearly decrease the atmospheric sulfate (SO(4)(2-)) aerosol concentrations in East Asia. Compared to Europe and North America, the lower effectiveness of SO(4)(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 (Δ(17)O) 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 SO(4)(2-) concentrations than SO(2) emissions. By the evidence from observed Δ(17)O in non-sea-salt sulfate (Δ(17)O-nss-SO(4)(2-)) and CMAQ simulations, the highly enhanced contributions of S(IV)+O(3) to sulfate driven by reduced SO(2) emissions explained the low effectiveness of SO(4)(2-) reduction in East Asia. Additionally, the decreases of acidity by substantial NH(3) emissions are projected to continue until 2050, limiting SO(4)(2-) reduction effectiveness. Thus, we highlighted that global control of both SO(2) and NH(3) emissions are needed to efficiently mitigate the sulfate-related climate and pollution, especially in high NH(3) emission region, such as East Asia.