Hygroscopic behavior of wet dispersed and dry deposited NaNO3 particles

Previous controversial studies on the hygroscopic behavior of NaNO3 aerosol particles and our frequent observation of crystalline NaNO3-containing ambient particles prompted this hygroscopic study on NaNO3 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 microscopic technique. Quite different hygroscopic behavior between particles wet dispersed by the nebulization of aqueous NaNO3 solutions and dry deposited powdery particles was observed; i.e., most of wet dispersed particles continuously grew and shrank during humidifying and dehydration processes, respectively, and yet all the dry deposited particles had reproducible deliquescence and efflorescence relative humidities (DRHs and ERHs). The different behavior of the two NaNO3 systems is due to the different nucleation mechanisms. Our hygroscopic study of wet deposited NaNO3 particles indicates 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.