Characterisation of a commercially available thermodenuder and diffusion drier for ultrafine particles losses

Volatility of particles is an important physical property and it directly influences the chemical composition of aerosols and thus their reactivity and related toxicity. Thermodenuders (TD) are widely used for the volatility studies, which is primarily giving an insight into the kinetics of evaporation and condensation within the device. In addition, characterisation of particle phase component depends on the humidity of the carrier gas and the presence of semi volatile organic compounds. Diffusion dryers are most commonly used for the removal of gas phase and volatile organic compounds and water vapour. The interpretation of data when using thermodenuders and diffusion dryers often excludes the correction factors that describe particle losses inside these instruments. To address this deficiency a commercially available TD and diffusion drier were characterised in the laboratory. For the TD the temperature profiles inside the TD showed optimal results only within a very narrow flow-window of 1 L/min resulting in inhomogeneous profiles for the flow rates outside this range. Losses at ambient temperature were very high for particles smaller than 50 nm and were dependent on the particle composition with higher losses observed for sodium chloride particles (> 60%). As with TD particle losses for NaCl at room temperature, the biggest losses (15–50%) are for small particles with diameters smaller than 50 nm. The number losses for particles bigger than 65 nm are considerably smaller and they are much lower (~10%) for particles bigger than 0.1 µm. From the experimental results a logistic regression model is fitted to the size dependent loss function. This model should be used to correct for the losses especially if aerosols smaller than 50 nm are studied.