Organic Component Vapor Pressures and Hygroscopicities of Aqueous Aerosol Measured by Optical Tweezers † ‡ ‡ ‡ † § § Chen Cai, , David J

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article pubs.acs.org/JPCA Organic Component Vapor Pressures and Hygroscopicities of Aqueous Aerosol Measured by Optical Tweezers † ‡ ‡ ‡ † § § Chen Cai, , David J. Stewart, Jonathan P. Reid,*, Yun-hong Zhang, Peter Ohm, Cari S. Dutcher, ∥ and Simon L. Clegg † The Institute of Chemical Physics, Key Laboratory of Cluster Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ‡ School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K. § Department of Mechanical Engineering, University of Minnesota, 111 Church Street SE, Minneapolis, Minnesota 55455, United States ∥ School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. *S Supporting Information ABSTRACT: Measurements of the hygroscopic response of aerosol and the particle-to-gas partitioning of semivolatile organic compounds are crucial for providing more accurate descriptions of the compositional and size distributions of atmospheric aerosol. Concurrent measurements of particle size and composition (inferred from refractive index) are reported here using optical tweezers to isolate and probe individual aerosol droplets over extended timeframes. The measurements are shown to allow accurate retrievals of component vapor pressures and hygroscopic response through examining corre- lated variations in size and composition for binary droplets containing water and a single organic component. Measurements are reported for a homologous series of dicarboxylic acids, maleic acid, citric acid, glycerol, or 1,2,6-hexanetriol. An assessment of the inherent uncertainties in such measurements when measuring only particle size is provided to confirm the value of such a correlational approach. We also show that the method of molar refraction provides an accurate characterization of the compositional dependence of the refractive index of the solutions. In this method, the density of the pure liquid solute is the largest uncertainty and must be either known or inferred from subsaturated measurements with an error of <±2.5% to discriminate between different thermodynamic treatments. 1. INTRODUCTION for S-VOCs and water is approached on considerably different 10 The equilibrium partitioning of water and organic components time scales. However, recent simulations of the activation of between the gas and condensed phases is critical in determining cloud condensation nuclei (CCN) have reminded us that the particle size distributions and compositions in atmospheric two are intimately linked and the cocondensation of both water and water-soluble S-VOCs must be considered to accurately aerosol impacting on the tropospheric particulate mass burden, 11,12 cloud droplet formation, heterogeneous chemistry, and the predict cloud droplet number. − direct radiative forcing of aerosol.1 3 Many techniques have Hygroscopic growth measurements are frequently reported been developed to measure changes in the mass of water for submicrometer diameter particles as a change in the wet size partitioned to the liquid phase with change in water activity/gas of a particle relative to the dry size with variation in RH using a ff 4 phase relative humidity (RH, see for example refs 4−7). The hygroscopic tandem di erential mobility analyzer (HTDMA ). partitioning of semivolatile organic components between the Alternatively, the relative change in mass or radius of a single gas and condensed phases can be inferred from equilibrium particle captured in either an electrodynamic or optical trap can state partitioning models, provided values of the pure be measured in the laboratory with variation in RH for aerosols fi component vapor pressures of the organic components are of speci c composition to provide robust data for equilibrium 13 known or can be estimated, and subject to the availability of state models. Pure component vapor pressures of S-VOCs models of the water activity dependent activity coefficients of can be measured from bulk phase samples, from ensembles of the organic species.8,9 Laboratory and field measurements often seek to isolate the changes in gas−particle partitioning of water Received: October 20, 2014 and (semi-) volatile organic compounds, collectively referred to Revised: December 15, 2014 as S-VOCs below, by assuming that the equilibrium partitioning Published: December 18, 2014 © 2014 American Chemical Society 704 DOI: 10.1021/jp510525r J. Phys. Chem. A 2015, 119, 704−718 The Journal of Physical Chemistry A Article − particles, or from single trapped particles.13 15 Knudsen cell instead rely simply on measurements of changes in particle size effusion methods are usually restricted to measuring the vapor to measure either the evaporation of an S-VOC or the pressures of bulk phase solid samples.14 The value for a evaporation/condensation of water.13 Indeed, rather that subcooled liquid, typically required for atmospheric aerosol, attempting to maintain steady environmental conditions (e.g., must then be inferred using the enthalpy of fusion, the change RH) during measurements of the vapor pressure of an S-VOC, in heat capacity on melting, and the melting temperature.14,16,17 fluctuations in RH are central to the technique, allowing parallel Aerosol ensemble and single particle techniques can allow and self-consistent measurements of the equilibrium vapor direct access to subcooled liquids and can even permit pressures of S-VOCs and the hygroscopicity of the aerosol. measurements of vapor pressures on supersaturated aqueous Having previously outlined the framework for performing these solutions. However, a restricted sensitivity to changes in size measurements, we report new measurements of the hygro- over a fixed period of time can often be limiting when scopic growth and vapor pressures of a number of organic performing a measurement, and inferred vapor pressures can be components in this publication and compare them with susceptible to fluctuations in environmental conditions predictions from thermodynamic models. Section 2 provides (temperature and RH).18 Indeed, the impact of fluctuations/ an overview of the equilibrium state models used to compare changes in environmental conditions should be recognized as with the experimental data. In section 3, we consider some of driving concomitant changes in the gas−particle partitioning of the inherent uncertainties arising in measurements of the both water and any S-VOC often leading to ambiguity in equilibrium behavior of aerosol and consider the sensitivities to measurements.10 key experimental variables, and we briefly summarize the Although measurements of aerosol hygroscopicity and framework used to interpret single particle measurements in volatility are standard aerosol analyses that are commonly section 4. Finally, in section 5 we report equilibrium state reported, a number of important challenges remain both in measurements for a variety of binary aqueous/organic aerosol performing measurements and in their interpretation. Ensemble droplets. measurements of aerosol volatility now commonly represent the volatility of organic components using a volatility basis set 2. THERMODYNAMIC MODELS derived from measurements at elevated temperatures under dry Water and solute activities in liquid mixtures containing water 19−25 conditions. Reported vapor pressures of even individual and organic compounds such as the dicarboxylic acids of semivolatile components measured by different laboratory interest here can be calculated using the universal quasichemical 26 techniques can vary by more than 4 orders of magnitude. functional-group activity coefficient (UNIFAC) group-contri- The scarcity of accurate pure component vapor pressures for 35 − bution method. UNIFAC treats solutions as a mixture of organic compounds with the low vapor pressures (<10 2 Pa) interacting molecular structural groups. The parameters that and multifunctionalities representative of atmospheric compo- describe these interactions are determined by fitting to a wide nents limits the validation and improvement of vapor pressure range of vapor−liquid equilibrium and activity data (e.g., Wittig 10,27,28 estimation techniques. Barley et al. have shown that the et al., 2003,36 and references therein). Peng et al.37 have ambient condensed mass of secondary organic aerosol (SOA) optimized six of the ai,j UNIFAC energy parameters, using their can vary by orders of magnitude dependent on the vapor own measurements of water activities of solutions of seven pressure estimation method used. Indeed, they have also dicarboxylic acids at room temperature to improve the examined the sensitivity of the ambient SOA mass to the degree representation of solvent and solute activities for this class of of nonideality in the solution behavior of organic components compounds (see their Figures 1 to 7). In the UNIFAC when in an aqueous solution droplet. Drying SOA prior to approach all compounds are assumed to be nondissociating performing a growth measurement using an HTDMA can lead (i.e., there is no treatment of ions). The UNIFAC-Peng to significant changes in the gas−particle partitioning of S- equations (i.e., the UNIFAC model with Peng parameters) are VOCs and, thus, RH dependent growth curves that can be used frequently within atmospheric chemical models to substantially in error. Indeed, many of the thermodynamic estimate the contributions of dicarboxylic acids and other treatments

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