Article pubs.acs.org/JPCA
Phase State and Saturation Vapor Pressure of Submicron Particles of meso-Erythritol at Ambient Conditions Eva U. Emanuelsson, Morten Tschiskale, and Merete Bilde* Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus, Denmark S Supporting Information *
ABSTRACT: meso-Erythritol is a sugar alcohol identified in atmospheric aerosol particles. In this work, evaporation of submicron-sized particles of meso-erythritol was studied in a TDMA system including a laminar flow tube under dry conditions at five temperatures (278−308 K) and ambient pressure. A complex behavior was observed and attributed to the formation of particles of three different phase states: (1) crystalline, (2) subcooled liquid or amorphous, and (3) mixed. With respect to saturation vapor pressure, the subcooled liquid and amorphous states are treated to be the same. The particle phase state was linked to initial particle size and flow tube temperature. Saturation vapor pressures of two phase states attributed to the crystalline and subcooled liquid state respectively are reported. Our results suggest a mass accommodation coefficient close to one for both states.
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INTRODUCTION
as accommodation coefficients and surface energies. This presents a problem for testing and development of models.14 One class of organic aerosol species is the alcohol-sugars (polyols), of which erythritol (C4H10O4, 1,2,3,4,-Butanetetrol, Smiles OCC(O)C(O)CO) is an example, identified in atmospheric organic aerosol particles.15−19 It has been attributed as tracer for fungal spores16 and being strongly associated with biomass burning.17 Smog chamber experiments have identified erythritol from photodecomposition of the conjugated diene 1,3-butadiene,20,18 which is an anthropogenic volatile organic compound. Erythritol has been used as a polyol alcohol surrogate standard,21 in kinetic studies,22 and in the development for highly specialized atmospheric gas and particle composition instrumentation23 of organic aerosol. Besides its occurrence in the atmosphere, erythritol is found in vegetables and fruit, and is used as an excipient in pharmaceutical formulations and as a low calorie food sweetener.24,25 Polyalcohols such as erythritol with relatively high melting points and high latent heats are also of interest as phase change materials (PCM) for heat storage.26 This work focuses on properties of the meso- form of erythritol (Table 1). With a melting point of almost 400 K, bulk meso-erythritol is a solid at room temperature. This work contributes experimentally derived saturation vapor pressures from submicron aerosol particles of mesoerythritol at ambient conditions. Furthermore, we discuss how temperature and particle size may affect particle phase state.
Aerosol particles in the atmosphere have a complex chemical composition with a significant fraction being organic.1 This organic fraction is of anthropogenic or biogenic origin, and can be emitted as primary particles for example in connection with biomass burning2 or formed as secondary organic aerosol (SOA) in the atmosphere by oxidation of volatile organic compounds.3−5 The quantity and properties of organic aerosol particles in the atmosphere has implications for air-quality6 and global climate,7 and correct representation of the distribution of organic compounds between the gas phase and the condensed phase is essential in atmospheric models. The apportionment of an organic compound between gas and condensed phase in the atmosphere depends on the saturation vapor pressure of the compound, the availability and chemical composition of preexisting particles, and the phase state of these.8 Limited information is available on the phase state of SOA particles which are considered to be liquids, or amorphous solids9 including glasses.10 The physical state of the particles has a large impact on both aerosol chemistry and physics.11 For prediction of gas to particle partitioning, the condensed phase of secondary organic aerosols is often assumed to be best represented as a subcooled liquid.12,13 The saturation vapor pressures of atmospherically relevant secondary organic aerosol species are in general poorly known due to the difficulty of direct measurements at ambient conditions, difficulties in measuring vapor pressures of metastable, subcooled liquid, or amorphous solid states, and lack of knowledge about other thermodynamic properties such © XXXX American Chemical Society
Received: April 29, 2016 Revised: August 15, 2016
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DOI: 10.1021/acs.jpca.6b04349 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A Table 1. Properties of meso-Erythritol CAS: 149-32-6 at 298 Ka
Saturation vapor pressures and surface energies were derived using eq 1 from the experimental data for fixed values of αm using a minimization procedure.29 This relies on several assumptions: spherical particles, constant and isotropic surface energy, negligible partial pressure of i far from the particle surface, and negligible latent heat effects. A detailed discussion of criteria for neglecting latent heat effects can be found in Davis 198335 and Seinfeld and Pandis 2006.36 Sensitivity analysis has shown that the determined saturation vapor pressures are sensitive to the mass accommodation coefficient αm. The vapor pressure derived from measured evaporation rates will be too low if the accommodation coefficient used is too high.
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EXPERIMENTAL SECTION Flow Tube Set Up. The saturation vapor pressures for meso-erythritol were determined from measured evaporation rates of aerosol particles (Dpi < 250 nm) at five temperatures: 278, 288, 298, 303, and 308 K respectively, using a tandem differential mobility analyzer (TDMA) setup including a laminar flow tube. The setup has been used in several previous studies of atmospheric relevant organic compounds at the University of Copenhagen.29,32,37−39 In 2013, the setup was moved to Aarhus University, where it was upgraded with, e.g., continuous logging of pressure inside the flow tube and temperature at several points around and along the flow tube. Thus, the setup used in this present study is an updated version of the flow tube, now named ARAGORN (AaRhus Aerosol Gas evapORatioN flow tube). The principle of the experimental system is to generate aerosol particles of known chemical composition, size select to a mono disperse distribution using a differential mobility analyzer (DMA), bring the monodisperse aerosol particles out of equilibrium by dilution using a makeup flow, and measure the particle size before and after evaporation in a temperature controlled laminar flow tube using a scanning mobility particle sizer (SMPS) system. A stable aerosol of meso-erythritol (Sigma >99%) was generated from an aqueous solution of meso-erythritol (0.1 g in 500 mL Milli-Q) using an atomizer (TSI3076) and a mass flow controlled (MFC Brooks) flow of purified air (TSI3074B). In the present work, the aerosol entered a 5 l glass volume before passing through two silica gel diffusion dryers. After the dryers, a pump where the flow was controlled by a second MFC, removed the majority of the aerosol flow. A stable residual flow (typically 300 mL min−1) was lead through a Nafion dryer (PermaPure MD070-24S-4) before relative humidity (RH) and temperature were monitored (Rotronic Hygroflex). The dry (typically 7% RH at room temperature) polydisperse aerosol particles were then passed through an aerosol neutralizer (TSI 3087) before being size selected using a DMA (Hauke Wienna short type, Negative power supply FUG HCE). The initial aerosol particle size was adjusted by altering the voltage of the DMA. The DMA had a recirculated sheath flow of 6 l min−1 that was passed through a charcoal unit, silica dryer, and a HEPA filter. The monodisperse aerosol entered the center of the flow tube, where it was surrounded by a clean and dry (
