The files in Data Repository describe the fitting results of Raman spectroscopy for sulfate and water. The Table S includes the information of the fitting peak area, which is used to calculate the peak area ratio of ν1(SO42−) and νS(O-H). The error of each fitted component given by PeakFit is in Table E. The total error of the peak area ratio, ν1(SO42−)/νS(O-H), was then calculated based on the errors of each fitted component and the rules of error propagation, and the results are shown in Table E.