Straight and bead-free nanofibers were synthesized via electrospinning by adding potassium iodide to a precursor containing polyacrylonitrile dissolved in dimethylformamide. Nano- and submicron-fibers with diameters of 36.5-300 nm were electrospun on the surface of a microfiber mat as the base filter to form a strong multilayer filter. The size of the fibers was controlled by adjusting the electrospinning atmosphere (i.e., temperature and humidity) and polyacrylonitrile and potassium iodide concentrations in the precursor solution. The pressure drop of the prepared multilayer filters was measured simultaneously in an aerosol filtration test using monodispersed aerosol particles of diameter 100 nm. Interestingly, the rate of pressure drop was reduced for filters composed of nanofibers with a diameter of less than 100 nm deposited on the base filter. In this case, the pressure drop was more gradual than was expected for a continuous flow pattern, mainly because of the slip flow effect. Because the thickness of the boundary layer for mass transfer is greater when nanofibers are thinner, the filtration efficiency of these filters was almost constant. Further reduction of the fiber diameter leads to the flow condition entering the free-molecular regime. (C) 2015 Elsevier Ltd. All rights reserved.