Nano-aerosols, less than 100 nm, are present in high concentration in air in the millions per cubic meter both indoor and outdoor. By virtue of their small size, they are easily inhaled into our body and can lead to chronic diseases. Nanofiber filter is effective in capturing these nano-aerosols suspended in air. Understanding the pressure drop behavior during aerosol loading of nanofiber filter is important for the operation of these filters. Given these filters are highly efficient for capturing nano-aerosols, over aerosol loading they can change from depth filtration, for which aerosols are captured inside the filter, to surface filtration, for which a cake forms on the filter surface that ultimately becomes the effective filter media. This transition, which is important in operation of nanofiber filters, is largely not understood. This study fulfills such unmet needs by investigating experimentally and theoretically the transition from depth-to surface-filtration of nanofiber filter challenged by nano-aerosols. Initially, filtration occurs across the filter that quickly leads to most filtration taking place in the upstream thin skin layer facing the incoming flow. As flowable pores in the skin layer get blocked, aerosols "bridge" across the pore openings and subsequently build-up above the filter surface. A new bridging model is developed that characterize this behavior. The bridge of aerosols among adjacent pores starts to interact at the filter surface that eventually leads to a continuous aerosol cake layer forming across the filter. Experiments are conducted using 50-400 nm sodium chloride aerosols challenging high-efficiency nanofiber filters (>50% for 300 nm sodium chloride challenging aerosol) that exhibit transition change from depth-to surface-filtration under accelerated loading condition. The new model not only matches well the test data in the depth-surface transition regime, it also predicts correctly the concave downward behavior of the pressure-drop test data. It also provides a smooth transition to the cake filtration for which the pressure drop varies linearly with deposited aerosol mass. (C) 2018 Published by Elsevier Ltd.