We developed a real-time aerosolization method combined with membrane filtration technique to determine size and number of dissolved and suspended colloid nanoparticles in water. Feed water before and after being treated by MF, UF or NF membranes was in-situ aerosolized and dried, and size and number of particles were determined in real time. Humic acid, CaCO3, and NaCl nanoparticles with varying solute concentrations, and standard nanoparticles with known size and number, were used to evaluate performance of the current method. We also measured size and number of colloidal nanoparticles in wastewater effluent, river water, and seawater. We found that there exists a significant amount of nanoparticles remaining even after UF and NF filtration, suggesting that dissolved nanoparticles that survived by UF and NF filtration may affect the fouling in RO membranes in the desalination process. The dissolved nanoparticles are defined here as those that are originally not in particle phase in feed water but that are able to precipitate near the membranes by crystallization or concentration polarization in the form of particle phase. Separating, of dissolved and suspended nanoparticles in water was achieved with the current method, leading to determine their relative contribution in the feed water. The fraction of dissolved nanoparticles in seawater that survived through NF filtration in total particle count is 66% while that in river water is 44%. TEM/EDS analysis verified that a significant amount of dissolved nanoparticles survived through UF and NF filtration, and that distinct types of dissolved nanoparticles exist in river water and seawater. Dissolved nanoparticles in seawater were classified into four types: carbon-rich spherical particles, carbon-mineral rich (Ca-O-C-Mg-S-Cl-Si) irregular particles, carbon-mineral rich spherical particles (C-Cu-Ca-O-Si-Ma), and Na-containing cubic particles.