Abstract: In this paper, a hygroscopic tandem differential mobility analyzer (HTDMA) system was used to study the mechanism involving the variation of particle size of sodium fluorescein aerosol under different relative humidities (RHs) and its influence on in-place filter efficiency, and then the result was compared with that of NaCl aerosol. The results indicate that no obvious abrupt deliquescence is observed in sodium fluorescein aerosol and the particle size of sodium fluorescein aerosol increases with RH. However, for NaCl aerosol, the obvious deliquescence point is determined and the particle size remains constant within the range of RH at 0%-75%. Meanwhile, for sodium fluorescein aerosol of diameter with 100 nm, the filter efficiency decreases with the increase of RH. For NaCl aerosol of diameter with 100 nm, the filter efficiency remains unchanged when RH is below 75%, but decreases dramatically when RH is over 75%. Therefore, from the view of aerosol hygroscopic growth, when RH is below 75%, both NaCl and sodium fluorescein aerosols can be used for in-place testing, and NaCl aerosol is more superior. When RH is over 75%, NaCl aerosol is not recommended to be used for in-place testing.