Vertically uniform formation pathways of tropospheric sulfate aerosols in East China detected from triple stable oxygen and radiogenic sulfur isotopes

Sulfate aerosols (SO42-) in the continental outflow from East China significantly alter the atmospheric sulfur budget across the Pacific Rim, but its formation pathways, especially in the free troposphere (FT), remain poorly understood. Here we analyze stable oxygen (O-17 and O-18) and radiogenic sulfur (S-35) isotopes in SO42- collected at a mountain site in East China to investigate SO42- formation pathways at varying altitudes. We find that O-17 (=O-17-0.52x(18)O) in SO42- is not correlated with S-35 (a direct measure of high-altitude air masses). This pattern notably differs from the currently known S-35-O-17 relation. The result implies that the formation pathway of tropospheric SO42- in East China is vertically uniform, likely due to large emissions and active convection in this region. Our measurements provide unambiguous isotopic constraints for reducing uncertainties in modeling SO42- in the FT over East China, which greatly affects regional climate but current models fail to accurately estimate. Plain Language Summary Anthropogenic sulfur dioxide (a criteria pollutant) emissions lead to the formation of sulfate aerosols, which profoundly affect air quality, ecosystems, and climate. East China is one of the largest sulfur dioxide emission regions in the world, but there is little information on formation pathways of sulfate aerosols in this region. Here we use isotopic fingerprinting of the elements oxygen and sulfur (stable O-16, O-17, O-18, and radioactive S-35) in sulfate aerosols collected from a remote mountain site in East China to gain insight into the formation process. A major finding is that sulfate in the higher atmosphere over East China is produced in a way similar to that occurring at the ground level, which is never detected by any other current technique before. The identical formation pathways of sulfate aerosols at different altitudes above the ground level may result from a combination of large air pollutant emissions and active vertical mixing in the atmosphere over East China. The measurement presented in this study is useful in improving current atmospheric chemistry and climate models to evaluate the extent to which sulfur emissions in East China influence regional and global climates.