A major impediment to develop a better understanding of microscopic interface process (such as nucleation, aggregation, growth, dissolution and phase transformation) of mineral in environmental soil science is a lack of in situ techniques at sufficiently high spatial and temporal resolution. The recent advent of liquid cell transmission electron microscopy (LCTEM) allows directly observing important dynamic events of nanomaterials in fluid. Here,

LCTEM was utilized to directly observe aggregation and dissolution behaviours of different sized hematite nanoparticles in liquid. When mass concentrations were same, the aggregates of 9 nm nanoparticles were statistically more compact and slightly larger than those of 36 nm nanoparticles. Increasing ionic strength resulted in larger aggregates and also enhanced the attachment efficiency between small aggregates in water which directly confirms the DLVO theory that the lower energy barrier to aggregation as ionic strength increases. Dissolution behavior of isolated and aggregated hematite particles in 10, 36, and 103 nm, respectively, was also investigated using LCTEM in water under a relative high beam dose rate. The high spatial and temporal resolution of LCTEM enables us to differentiate the respective effect of primary particle size, crystal defects, and aggregation state on particle dissolution. At similar electron-beam irradiation parameters, the initial surface-area normalized dissolution rates (R_SA,Int) of isolated 10, 36, and 103 nm particles are 4.64 ± 3.60, 0.91 ± 0.44, and 0.24 ± 0.04 mg m^-2 s^-1, respectively. Interface free energy, calculated from the measured R_SA,Int, decreases with decreasing primary particle size. Furthermore, the mode of dissolution switched from surface-retreat in the relatively defect-free smaller particles to defect-based dissolution in the larger particles. In dissolution of aggregated particles, dissolution occurs more rapidly on the particles that are more accessible to bulk solution than the others in one aggregate. As dissolution proceeds, dendritic aggregates break into several smaller aggregates that respectively shrink into smaller and more compact aggregates, followed by reaggregation together. This study directly presented the influences of particle size and ionic strength on aggregation state and shows microscopic dissolution behavior of isolated and aggregated particles in different primary particle sizes, which is important to understand bioavailability, transport, and fate of nanoparticles in aquatic systems.