Experiments probing the phase and behavior of NaNO3 particles at different relative humidities, important for elucidating the role these particles play in the chemistry and radiative proper-ties of marine regions, are presented. Changes in NaNO3 particles during hydration were studied using environmental scanning electron microscopy (ESEM) and conventional SEM coupled with energy dispersive X-ray analysis (SEM/EDX). Mixtures of NaNO3 and NaCl, which are typical of partially processed sea salt particles, were also studied. Complementary studies using long path FTIR were carried out to determine the extent of water association with NaNO3 aerosols, and for comparison, NaCl, MgCl2, and NH4NO3, as a function of relative humidity. The combination of these techniques shows that NaNO3 particles exist as unusual metastable, amorphous solids at low relative humidity that undergo continuous hygroscopic growth with increasing relative humidity. While other evidence for this phenomenon has been reported, this is the first direct observation using ESEM. (C) 2004 Elsevier Ltd. All rights reserved.