Nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA) for investigating hygroscopic properties of sub-10 nm aerosol nanoparticles

摘要

Interactions between water and nanoparticles are relevant foratmospheric multiphase processes, physical chemistry, and materials science. Current knowledge of the hygroscopic and related physicochemical properties of nanoparticles, however, is restricted by the limitations of the available measurement techniques. Here, we present the design and performance of a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA) apparatus that enables high accuracy and precision in hygroscopic growth measurements of aerosol nanoparticles with diameters less than 10 nm. Detailed methods of calibration and validation are provided. Besides maintaining accurate and stable sheath and aerosol flow rates (±1 %),high accuracy of the differential mobility analyzer (DMA) voltage (±0.1 %) in the range of∼0–50 V is crucial for achieving accurate sizing and small sizing offsets between the two DMAs (<1.4 %). To maintain astable relative humidity (RH), the humidification system and the second DMAare placed in a well-insulated and air conditioner housing (±0.1 K).We also tested and discussed different ways of preventing predeliquescencein the second DMA. Our measurement results for ammonium sulfatenanoparticles are in good agreement with Biskos et al. (2006b), with nosignificant size effect on the deliquescence and efflorescence relativehumidity (DRH and ERH, respectively) at diameters down to 6 nm. For sodium sulfate nanoparticles, however, we find a pronounced size dependence of DRH and ERH between 20 and 6 nm nanoparticles.