Briefly, a 0.01 g/mL aqueous NaCl (ACS, 99.0% min, Thermo Scientific Chemicals) solution was atomized and the liquid droplet stream was passed through a silica gel diffusion drier to form dry NaCl particles using a sheath flow of filtered room air. The resulting dry NaCl nanoparticles were charged with Krypton 85 and then impacted on an electrically grounded MEMS sample chip supported on a steel mesh. The particle concentration was monitored downstream using an Ultrafine Condensation Particle Counter (TSI 3756 with a detection lower limit of 2.5 nm) and was found to always be > 10 5 cm -3 . The deposition time was 4 hours and sample flow rate was 0.3 L/min. The sample chips were stored in a closed container with silica desiccant prior to use.