The RH in the sample cell was increased from ambient levels to the deliquescence point of NaCl by flowing humidified air into the sample chamber (Figure , Figure ). This procedure of introducing humid air slowly increased RH over several hundred seconds, which contrasts with aerosol instruments that measure steady state aerosol size/mass as a function of RH to track hygroscopic growth. The slow RH increase rate coupled with continuous movie mode imaging allowed observing intermediate states of aerosol particle hydration prior to deliquescence. We repeated the experiment on around 20 different particles over separate three experiments and observed similar phenomena each time.