The 2D area ratio increased monotonically up to an RH of 74.5%, at which point the NaCl dissolved completely and the liquid droplet grew larger than the image area and could not be measured any longer. We take this point as the deliquescence RH. The particle size at t = 0 s was 880 nm × 680 nm. The error of each length measurement was ± 10 nm and the error of the area measurement was 0.011 µm .