Aside from the instrument error of ±2%, temperature fluctuations and the imaging electron beam were the main potential sources of error in the RH measurement. The room temperature was used for RH measurement, which could vary slightly from the sample temperature. However, the TEM instrument room has strict temperature control and the sample was allowed to thermally equilibrate for ~10 minutes after placing in the microscope so this is unlikely to lead to significant error. The previous study by Wang et al. showed that the electron beam was able to prompt water vapor condensation by decreasing the local water vapor pressure (increasing the local RH). In this prior study the authors showed that a broad, stationary TEM beam with an electron flux of 0.226 e -/Å 2 s induced water condensation from saturated water vapor (RH = 100%). Here we used STEM mode with a smaller electron flux of < 0.1 e -/Å 2 s and subsaturated vapor. Taken together, this suggests the electron beam will not promote water condensation. As shown below, several measurements showed that NaCl deliquesces under the electron beam near the accepted value of 75%, suggesting temperature difference and the electron beam did not significantly modify the RH.