title: IMS-MS and IMS-IMS investigation of the structure and stability of dimethylamine-sulfuric acid nanoclusters.
abstract: Recent studies of new particle formation events in the atmosphere suggest that nanoclusters (i.e, the species formed during the early stages of particle growth which are composed of 10(1)-10(3) molecules) may consist of amines and sulfuric acid. The physicochemical properties of sub-10 nm amine-sulfuric acid clusters are hence of interest. In this work, we measure the density, thermostability, and extent of water uptake of <8.5 nm effective diameter dimethylamine-sulfuric (DMAS) nanoclusters in the gas phase, produced via positive electrospray ionization. Specifically, we employ three systems to investigate DMAS properties: ion mobility spectrometry (IMS, with a parallel-plate differential mobility analyzer) is coupled with mass spectrometry to measure masses and collision cross sections for <100 kDa positively charged nanoclusters, two differential mobility analyzers in series (IMS-IMS) are used to examine thermostability, and finally a differential mobility analyzer coupled to an atmospheric pressure drift tube ion mobility spectrometer (also IMS-IMS) is used for water uptake measurements. IMS-MS measurements reveal that dry DMAS nanoclusters have densities of ∼1567 kg/m(3) near 300 K, independent of the ratio of dimethylamine to sulfuric acid originally present in the electrospray solution. IMS-IMS thermostability studies reveal that partial pressures of DMAS nanoclusters are dependent upon the electrospray solution concentration ratio, R = [H2SO4]/[(CH3)2NH]. Extrapolating measurements, we estimate that dry DMAS nanoclusters have surface vapor pressures of order 10(-4) Pa near 300 K, with the surface vapor pressure increasing with increasing values of R through most of the probed concentration range. This suggests that nanocluster surface vapor pressures are substantially enhanced by capillarity effects (the Kelvin effect). Meanwhile, IMS-IMS water uptake measurements show clearly that DMAS nanoclusters uptake water at relative humidities beyond 10% near 300 K, and that larger clusters uptake water to a larger extent. In total, our results suggest that dry DMAS nanoclusters (in the 5-8.5 nm size range in diameter) would not be stable under ambient conditions; however, DMAS nanoclusters would likely be hydrated in the ambient (in some cases above 20% water by mass), which could serve to reduce surface vapor pressures and stabilize them from dissociation.
authors.affiliation: Department of Mechanical Engineering, University of Minnesota , Minneapolis, Minnesota United States.
authors.forename: Hui
authors.initials: H
authors.lastname: Ouyang
authors.forename: Siqin
authors.initials: S
authors.lastname: He
authors.forename: Carlos
authors.initials: C
authors.lastname: Larriba-Andaluz
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Hogan