We used a commercially available MEMS based in situ TEM holder with minor modifications for all experiments (Poseidon Plus sample holder, Protochips, USA). The microfluidic lines (PEEK HPLC tubes) in the sample holder were first flushed with 200 proof ethanol, filtered compressed air, and finally dried in a vacuum pump (Pfeiffer Vacuum, USA) overnight to remove residual water and ethanol. The sample holder was then plasma cleaned with an air plasma for 15 minutes to remove residual carbon. A microfluidic gas chamber was formed by sandwiching together two silicon MEMS chips and aligning the imaging windows. The bottom chip was surrounded by a gasket and the top a metal plate, which together hermetically sealed the sample region at near atmospheric pressure (Figure ). A leak test was performed in a vacuum pump overnight to guarantee that the sample was hermetically sealed, with the sample reaching a minimum pressure of at least 8.6×10 -5 hPa within 15 minutes. The microfluidic cell initially contained room air with an RH of ~30-40%. The RH in the sample chamber was increased by flowing filtered room air through a water-filled Nafion® humidification tube (Perma Pure, MH™-Series Humidifiers) into the sample region at a rate of 0.5 mL/s (Figure ). The high RH vapor was delivered to the sample chamber via fluid tubes connecting the sample chamber to the back of the sample holder. The RH was measured continuously using an RH meter (Vaisala HMT330 with an error of ± 2%) at the outlet of the TEM sample holder. However, deliquescence of NaCl occurred at an RH of ~64% as measured at the sample holder outlet, suggesting that this RH measurement was incorrect due to water vapor adsorption in the sample holder. We performed a set of control experiments that showed the RH measured at the gas flow inlet to the sample holder was consistent with the expected deliquescence RH. We determined a correction factor for the outlet RH measurement, 1.16 ± 0.01 , which was used to correct all RH data (Figure in Supporting Information). The control experiments for correcting RH for water vapor losses in the microfluidic tubing are described in the Supporting Information. All RH data displayed in this article are expected to have an error of at least ± 2%, which is the instrument error. However, there are other potential sources of error that are difficult to quantify, which are discussed below in the results.