Hygroscopic behavior of individual NaNO₃ particles

Abstract. Previous controversial studies on the hygroscopic behavior of NaNO₃ aerosols and our frequent observation of crystalline NaNO₃-containing ambient aerosol particles prompted this extensive hygroscopic study on NaNO₃ aerosol particles. In this work, the hygroscopic behavior of individual NaNO₃ particles of 2.5–4.0 μm in diameter is investigated on a single-particle basis using an optical microscopy technique. Quite different hygroscopic behaviors between particles generated by the nebulization of NaNO₃ solution and powdery particles were observed; i.e., most of generated particles continuously grew and shrank during humidifying and dehydration processes, respectively, and yet all the individual powdery particles had reproducible deliquescence and efflorescence relative humidities (DRHs and ERHs). The different behaviors of the two NaNO₃ systems are due to the different nucleation mechanisms. Our hygroscopic studies of NaNO₃ particles generated from aqueous NaNO₃ solutions indicate that they nucleate via homogeneous nucleation, but the time scale for the nucleation to occur is too long to be atmospherically relevant. And thus no efflorescence of the particles has been observed in the laboratory measurements. However, when chemical species acting as heterogeneous nuclei are present, then efflorescence occurs which can explain the observation of ambient crystalline NaNO₃ particles. It is imperative to work with heterogeneous nucleation systems which are more relevant to the real world.