AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Evaporation of and Water Uptake by Sub-10 Nano-meter Dimethylamine-Sulfuric Acid Nanoparticles
HUI OUYANG, Siqin He, Christopher Hogan Jr., University of Minnesota
Abstract Number: 577 Working Group: Aerosol Chemistry
Abstract The physical and chemical properties of dimethylamine-sulfuric acid (DMAS) nanoparticles are needed to better understand whether such particles would remain stable, dissociate, or grow under atmospherically relevant conditions. Particularly important are the properties of sub 10 nano-meter particles, as newly formed particles in many environments may be composed primarily of sulfuric acid and amines. In this study, the densities, vapor pressure, and extent of water uptake by DMAS nanoparticles were studied using combinations of differential mobility analysis, mass spectrometry, and atmospheric pressure drift tube mobility spectrometry. Specifically we used a parallel plate differential mobility analyzer coupled to a time flight mass spectrometer (DMA-MS) to determine the collision cross section for mass identified, sub 4 nm clusters, generated via an electrospray. The mass-collision cross section relationship was used to infer the density of DMAS nanoparticles with variable percent dimethylamine and variable percent sulfuric acid. The average density of DMAS is found to be ~1574kg/m$^3 and roughly independent of nanoparticle size. Subsequently, tandem differential mobility analysis (with two modest to high resolution half-mini DMAs) with a furnace between the DMAs was used to monitor the size change of nanoparticles due to evaporation at four different temperatures. Size-dependent partial pressure-temperature values were then inverted from results. At room temperature, the partial pressure differs from the vapor pressure reported normally for bulk DMAS, and is of the order 10$^(-5) Pa near room temperature, indicative that the Kelvin effect enhances DMAS nanoparticle vapor pressure. Finally, a new developed drift-tube mobility analyzer coupled with a half-mini DMA upstream was used to examine water absorption by DMAS in size range of 2.8~7.5nm. Cases with different relative humidity up to 50% were examined. Significant growth due to water uptake is observed for RH > 20%, though less growth for smaller sized particles.