The aim of this study is to experimentally investigate pressure drop and filtration efficiency of fibrous filters operating in aerodynamic slip regime under continuous loading of sub-micron aerosols. Test aerosol is sodium chloride (NaCl) particulates with electrical mobility diameter (EMD) ranging from 41 to 514 nm generated from atomization. Two types of non-woven filters composed of fibers with diameter 1.8 mu m, (micro-fibrous) and 300 nm (nano-fibrous) are tested. Most penetrating particle size (MPPS) of clean micro- and nano-fibrous filters are 203 and 103 nm EMD, respectively. Both of them have clean filter filtration efficiencies closely agree with Payet's model. Under continuous aerosol loading, MPPS of both types of filters shift to smaller size. A semi-empirical model is developed to describe pressure drop across a fibrous filter operating in aerodynamic slip regime under continuous loading of sub-micron aerosols. Experimental results of filters with thickness ranging from 1 X 10(-5) to 1 X 10(-4) m generally agree with this model. It shows the potential of extending the model to describe pressure drop across aerosol-loaded filter of increased thickness with non-uniform particle deposition across filter depth being taken into account. Although clean micro- and nano-fibrous filters have comparable pressure drop while nano- offers higher filtration efficiency against sub-micron aerosols, which may suggest it as an "absolutely better" filter, experimental results show that the mass holding capacity of nano-fibrous filter is lower than its micro-fibrous counterpart. Hence, it is essential to analyze loading behavior through quantitative model during filter design stage. (c) 2008 Elsevier B.V. All rights reserved.