Development and Characterization of a Fast-Stepping/Scanning Thermodenuder for Chemically-Resolved Aerosol Volatility Measurements

A thermodenuder (TD) system, based on the design of Wehner et al. (2002), was designed, constructed, and characterized in the laboratory. The TD consists of a heated tube (2.5 cm ID, 55 cm long) held at a constant temperature by a 3-zone controller, followed by a cooling zone with a diffusion tube lined with activated charcoal for adsorption of evaporated gases. An important improvement over previous designs is the ability to step through TD temperatures in ~10 min. per step by the reduction of thermal inertia, and the addition of two cooling fans. The TD was characterized in the laboratory, showing that temperature profiles inside are relatively uniform and for response to standard generated particle species. Losses at ambient temperature are close to diffusion losses estimated with literature techniques and to those experimentally measured by Wehner et al. Particle number losses are observed to increase for volatile species upon heating due to particles shrinking to sizes where diffusion and thermophoresis are more efficient. The thermodenuder was placed upstream of an Aerodyne Aerosol Mass Spectrometer (AMS) during several field experiments. An automated valve system was designed and built to allow rapidly alternating data points between thermodenuder-processed aerosol and un-processed aerosol. This system has enabled the rapid (1–3 h) collection of chemically-resolved volatility over the range of 54–230°C in the field for the first time. Examples of field data are shown where the species volatilities vary as expected.