Ricardo UK Ltd. 1 article published in JoVE Engineering Measuring Sub-23 Nanometer Real Driving Particle Number Emissions Using the Portable DownToTen Sampling System Markus Bainschab1, Lukas Landl2, Jon Andersson3, Athanasios Mamakos4, Stefan Hausberger2, Alexander Bergmann1 1Institute of Electrical Measurement and Sensor Systems, Graz University of Technology, 2Institute of Internal Combustion Engines and Thermodynamic, Graz University of Technology, 3Ricardo UK Ltd., 4AVL List GmbH Presented here is the DownToTen (DTT) portable emission measurement system to assess real driving automotive emissions of sub-23 nm particles.