The application potential of cellulose nanofibril (CNF) aerogels has been hindered by the slow and costly freeze-or supercritical drying methods. Here, CNF aerogel membranes with attractive mechanical, optical, and gas transport properties are prepared in ambient conditions with a facile and scalable process. Aqueous CNF dispersions are vacuum-filtered and solvent exchanged to 2-propanol and further to octane, followed by ambient drying. The resulting CNF aerogel membranes are characterized by high transparency (>90% transmittance), stiffness (6 GPa Young's modulus, 10 GPa cm(3) g(-1) specific modulus), strength (97 MPa tensile strength, 161 MPa m(3) kg(-1) specific strength), mesoporosity (pore diameter 10-30 nm, 208 m(2) g(-1) specific surface area), and low density (approximate to 0.6 g cm(-3)). They are gas permeable thus enabling collection of nanoparticles (for example, single-walled carbon nanotubes, SWNT) from aerosols under pressure gradients. The membranes with deposited SWNT can be further compacted to transparent, conductive, and flexible conducting films (90% specular transmittance at 550 nm and 300 Omega square(-1) sheet resistance with AuCl3-salt doping). Overall, the developed aerogel membranes pave way toward use in gas filtration and transparent, flexible devices.