Design and validation of a volatility hygroscopic tandem differential mobility analyzer (VH-TDMA) to characterize the relationships between the thermal and hygroscopic properties of atmospheric aerosol particles

The nature of atmospheric aerosols is extremely complex and often requires advanced analytical tools for the determination of its physical and chemical properties. In particular, the interaction of particles with atmospheric water is a complex function of both particle size and composition. The ability of a particle to grow in a humid environment can be measured by humidity tandem differential mobility analyzing techniques (H-TDMA). In this article, we present a new development combining thermo-desorption and humidification aerosol conditioning in series that allows to measure changes in the hygroscopic behavior of aerosol at 90% relative humidity (RH) after conditioning of the particle by thermo-desorption to a temperature between 25 degrees C and 300 degrees C. The main feature of this system, named Volatility Hygroscopic-Tandem Differential Mobility Analyzer (VH-TDMA), is to allow for rapid (10 minutes) series of scans to control particle response to 1-thermal conditioning, 2-RH increase to 90% and 3-a combination of both thermal and RH conditioning. The VH-TDMA is, therefore, suited to investigate particle ageing through a simple coupling of H-TDMA and V-TDMA performances. The aim of the present article is to describe the instrument design and to validate its performances by focusing on the measurement of hygroscopic behavior of pure inorganic particles such as sodium chloride or ammonium sulfate, as well as internally mixed organic-inorganic particles. Based on laboratory experiments and applications to natural aerosols, we show that the VH-TDMA system can be used to investigate the hygroscopic properties of the non-volatile fraction of ambient sub-micrometer aerosols in the range of 20 to 150 nm and the influence of the more volatile fraction of the particle on hygroscopic growth.